Oceanography of the U.S. Paciﬁc Northwest Coastal Ocean ...coast.ocean.washington.edu/coastfiles/HickeyBanas-PNWcoast.pdf · Oceanography of the U.S. Paciﬁc Northwest Coastal

1010Q 2003 Estuarine Research Federation

Estuaries Vol. 26, No. 4B, p. 1010–1031 August 2003

Oceanography of the U.S. Pacific Northwest Coastal Ocean and

Estuaries with Application to Coastal Ecology

BARBARA M. HICKEY* and NEIL S. BANAS

School of Oceanography, Box 355351, University of Washington, Seattle, Washington 98195

ABSTRACT: Ocean processes are generally large scale on the U.S. Pacific Northwest coast; this is true of both seasonalvariations and event-scale upwelling-downwelling fluctuations, which are highly energetic. Coastal upwelling supplies mostof the macronutrients available for production, although the intensity of upwelling-favorable wind forcing increases south-ward while primary production and chlorophyll are higher in the north, off the Washington coast. This discrepancy couldbe related to several mesoscale features: the wider, more gently sloping shelf to the north, the existence of numeroussubmarine canyons to the north, the availability of Columbia River plume water and sediment north of the river mouth,and the existence of a semi-permanent eddy offshore of the Strait of Juan de Fuca. We suggest that these features haveimportant effects on the magnitude and timing of macronutrient or micronutrient delivery to the plankton. These featuresare potentially important as well to transport pathways and residence times of planktonic larvae and to the developmentof harmful algal blooms. The coastal plain estuaries, with the exception of the Columbia River, are relatively small, withlarge tidal forcing and highly seasonal direct river inputs that are low to negligible during the growing season. Primaryproduction in these estuaries is likely controlled not by river-driven stratification but by coastal upwelling and exchangewith the ocean. Both baroclinic mechanisms (the gravitational circulation) and barotropic ones (lateral stirring by tide and,possibly, wind) contribute to this exchange. Because estuarine hydrography and ecology are so dominated by ocean signals,the coastal estuaries, like the coastal ocean, are largely synchronous on seasonal and event time scales, though intrusionsof the Columbia River plume can cause strong asymmetries between Washington and Oregon estuaries especially duringspring downwelling conditions. Water property correlation increases between spring and summer as wind forcing becomesmore spatially coherent along the coast. Estuarine habitat is structured not only by large scale forcing but also by fine scaleprocesses in the extensive intertidal zone, such as by solar heating or differential advection by tidal currents.

Introduction

Ocean variability in nearshore regions of theU.S. Pacific Northwest Coast (PNW) and its coastalestuaries is distinctly different from that in estuar-ies and nearshore regions of the U.S. East Coast.Whereas the West Coast is embedded in an easternboundary current system and dominated by wind-driven coastal upwelling, the East Coast is not. Up-welling provides plentiful nutrients to the WestCoast and its estuaries, but on the East Coast nu-trients are more commonly supplied by river out-flow and by onwelling from the continental slope(Nixon et al. 1996). Unlike most East Coast estu-aries, some PNW estuaries may be considered asextensions of the coastal ocean during the growingseason because river outflow is low and flushing bythe coastal ocean is high.

Ocean variability along the PNW is generallyvery large scale (.500 km), a result of large-scaleatmospheric systems (Halliwell and Allen 1987), al-though significant along-coast gradients in forcingoccur. Several mesoscale topographic features alsooccur in the PNW and the altered currents, mixing

* Corresponding author: tele: 206/543-4737; fax: 206/685-3354; e-mail: [email protected].

regimes, and potential nutrient pathways associat-ed with these features likely play important rolesin regional ecosystem function.

We have used recent and historical informationto construct an updated synthesis of large-scale cur-rent patterns and water properties of the PNWcoastal zone and their variability, extending the syn-thesis where appropriate to suggest interactions be-tween the physical environment and the ecosystem.In particular, an along-coast gradient in coastal pro-ductivity, one that opposes the along-coast gradientin wind forcing, is examined. The effects of impor-tant mesoscale features such as shelf width, subma-rine banks, canyons, and river plumes are consid-ered, and new data from the Pacific NorthwestCoastal Ecosystems Regional Study (PNCERS) arepresented to further describe important processesand variability within the PNW coastal estuaries andlinks between the estuaries and the coastal ocean.

Large Scale Processes in the Pacific NorthwestCoastal Ocean

THE CALIFORNIA CURRENT SYSTEM

The PNW coastal zone is embedded within theCalifornia Current System (CCS), a system of cur-rents with strong interannual, seasonal and several-

Oceanography of the U.S. Pacific Northwest 1011

Fig. 1. Schematic of the California Current System. Adaptedfrom Hickey and Royer (2001).

day (event) scale variability (Fig. 1; Hickey 1998).The CCS includes the southward California Cur-rent, the wintertime northward Davidson Current,the northward California Undercurrent, whichflows over the continental slope beneath the south-ward flowing upper layers, as well as nameless shelfand slope currents with primarily shorter-than-sea-sonal time scales. The PNW includes one majorriver plume (the Columbia River), several smallerestuaries, and, primarily in the north, numeroussubmarine canyons. The dominant scales and dy-namics of the circulation over much of the CCSare set by strong alongshore winds; large along-shore scales for both the winds and the bottomtopography (Halliwell and Allen 1987); and a rel-atively narrow and deep continental shelf. Becauseof these characteristics, coastal-trapped waves areefficiently generated and propagate long distancesalong the continental margins of much of westernNorth America; much of the variability in the PNWis caused by processes originating south of the re-gion (i.e., remote forcing). Because of the gener-ally southward alongshore wind stress in springand summer, coastal upwelling is the dominantprocess controlling water property variability (seereview in Smith 1995).

The California Current flows southward year-round offshore of the U.S. West Coast from theshelf break to a distance of ;1,000 km from thecoast (Hickey 1979, 1998; Fig. 1). The current is

strongest at the sea surface, and generally extendsover the upper 500 m of the water column. Typicalmean speeds are ;10 cm s21. The California Un-dercurrent is a relatively narrow feature (10–40km) flowing northward over the continental slopeof the CCS as a nearly continuous feature at depthsof about 100–400 m, transporting warmer, saltierwater northward along the coast. The undercur-rent has a jet-like structure, with the core of thejet located just seaward of and just below the shelfbreak and with peak speeds of 30–50 cm s21. Theundercurrent provides a possible northward trans-port route for larval fish and invertebrates andeven phytoplankton seed stock. Because of itsproximity to the shelf break, the undercurrent isthe source of much of the nutrient-rich water sup-plied to the shelf during coastal upwelling. Theonshore transport of this water during upwellingoffers a mechanism for onshore transport of plank-ton entrained in the undercurrent.

In fall and winter north of Point Conception,flow over the continental shelf and slope is typi-cally northward and is usually denoted the David-son Current. The northward flow is strongest at thesea surface and is generally broader (;100 km inwidth) and sometimes stronger than the corre-sponding subsurface northward flow in other sea-sons (the California Undercurrent). A southwardundercurrent (the Washington Undercurrent) oc-curs over the continental slope in the winter sea-son in the PNW (Werner and Hickey 1983). Thisundercurrent occurs at deeper depths than thenorthward undercurrent (300–500 m). The exis-tence of this undercurrent, like that of the north-ward undercurrent, likely depends on the co-oc-currence of opposing wind stress and alongshorepressure gradient forces.

The water properties of the CCS are determinedby three water masses: Pacific Subarctic, North Pa-cific Central, and Southern (sometimes termedEquatorial; Hickey 1979). Pacific Subarctic water,characterized by low salinity and temperature andhigh oxygen and nutrients, is advected southwardin the CCS (Hickey 1979, 1998). North PacificCentral water, characterized by high salinity andtemperature and low oxygen and nutrients, entersthe CCS from the west. Southern water, character-ized by high salinity, temperature and nutrients,and low oxygen, enters the CCS from the southwith the northward undercurrent. In general, sa-linity and temperature increase southward in theCCS and salinity also increases with depth.

SEASONAL VARIABILITY

Currents and water properties of the CCS bothover the shelf and offshore of the shelf undergolarge seasonal fluctuations; the California Current

1012 B. M. Hickey and N. S. Banas

Fig. 2. Seasonal variation of temperature, salinity, and ni-trate at mid-shelf off Washington and off Oregon. Contours arebased on composite measurements 1950–1984 from areas nearCopalis Beach, Washington (478029N–478129N) and YaquinaHead, Oregon (448359N–448459N). Total water column depthcovers the region 70–130 m. Adapted from Landry et al. 1989.

and Undercurrent are strongest in summer to ear-ly fall and weakest in winter. The Davidson Currentis strongest in winter. Seasonal mean shelf currentsare generally southward in the upper water columnfrom early spring to summer and northward therest of the year. Over the shelf, the seasonal dura-tion of spring-summer southward flow usually in-creases with distance offshore and with proximityto the sea surface (Strub et al. 1987a for the entireCCS; Hickey 1989 for Washington shelf). A north-ward undercurrent is commonly observed onshelves during the summer and early fall. Off thecoast of Vancouver Island a northward flowingbuoyancy driven current, the Vancouver IslandCoastal Current, exists year-round from the coastto at least mid shelf (Thomson 1981; Hickey et al.1991). This current opposes the southward shelfbreak jet current that connects to southward flowoff the outer Washington shelf.

Seasonal upwelling brings colder, saltier, and nu-trient-rich water to the surface adjacent to thecoast all along the U.S. West Coast (Fig. 2; Huyer1983). In general, the strength and duration of up-welling (as seen at the sea surface) increases to thesouth in the PNW. Maximum upwelling occurs inspring and summer. With the exception of regionsaffected by the Columbia plume, stratification inthe CCS is remarkably similar at most locations andis largely controlled by seasonal changes in large-scale advection and upwelling of water masses asdescribed above (Huyer 1983).

In contrast to most U.S. East Coast environ-ments, the shelf is relatively narrow and the nu-tricline is fortuitously positioned so that nutrient-rich deeper water can be effectively brought to thesurface by the wind-driven upwelling. Seasonal pat-terns of all macronutrients on the continental shelfare dominated by seasonal patterns in upwelling(Fig. 2; Landry et al. 1989, Hickey 1989). Ampli-tudes of seasonal nutrient changes are remarkablysimilar over the Pacific Northwest.

Seasonal current fluctuations are continuouswith similar fluctuations in the Alaskan gyre. Themajority of seasonal change in currents has beenshown to occur within a few tens of kilometers ofthe coast (Strub and James 2002). Seasonal cur-rents are largely driven by alongshore wind stress(see review included in Batteen 1997). Satellite al-timetry data illustrate that seasonal features grad-ually migrate offshore and out into the main Cal-ifornia Current, so that alternating seasonal bandsof northward and southward flow (superimposedon the long-term mean California Current) are ob-served as far as several hundred kilometers fromthe coast (Strub and James 2002).

The transition of currents and water propertiesover the shelf and slope between winter andspring, the Spring Transition, is a sudden and dra-matic event in the CCS (Strub et al. 1987b). Alongmuch of the coast, during the transition, sea leveldrops at least 10 cm, currents reverse from north-ward to southward within a period of several days,and isopycnals slope upward toward the coast inresponse to coastal upwelling (Smith 1995). Thetransition is driven by changes in the large-scalewind field and these changes are a result of chang-es in the large-scale atmospheric pressure fieldover the CCS. A similar rapid transition betweensummer upwelling and fall downwelling oceaniccharacteristics does not occur (Strub and James1988).

SEVERAL-DAY TIME SCALES

In spite of strong seasonal variability in the PNW,the dominant variability occurs at several-day timescales (Hickey 1989); on the shelf, seasonal con-ditions as described above are often reversed forshorter periods of time. Because of bottom fric-tion, reversals occur more frequently in the near-shore (Brink et al. 1987). Fluctuations in currents,water properties, and sea level over the shelf atmost locations are dominated by wind forcing, withtypical scales of 3–10 d. A schematic of the locallywind-driven ocean surface circulation in the PNWfor winds toward the south (usually fair weather)and winds toward the north (storms) is shown inFig. 3. During periods of fair weather the stress ofthe southward winds on the sea surface accelerates


Fig. 3. Schematic of wind-driven coastal circulation in thePacific Northwest illustrating the offshore and southward-di-rected surface currents and upwelling along the coast that occurin response to an upwelling-favorable wind stress (left panel, fairweather) and onshore and northward-directed surface currentsand downwelling along the coast in response to downwelling-favorable winds (right panel, storms). Freshwater flows fromcoastal estuaries and from the Strait of Juan de Fuca are illus-trated with darker shading. The location of a persistent sum-mertime mesoscale feature (the Juan de Fuca Eddy) is alsoshown.

the coastal currents, producing offshore- andalongshore-directed currents in the surface Ekmanlayer (upper 10–30 m; Lentz 1992), alongshorecurrents elsewhere in the central water column,and onshore and alongshore currents in the bot-tom boundary layer (lower 5–15 m; Lentz andTrowbridge 1991; Allen et al. 1995). Under theseconditions, plumes of fresher water originating atcoastal estuaries tend to spread offshore and to thesouth (Garcia-Berdeal et al. 2002). Upwelling ofcolder, saltier, and nutrient-rich water occurs with-in a few kilometers of the coast (typically withinone Rossby radius, about 10 km). During stormscirculation patterns reverse and freshwater plumesmove back onshore (Hickey et al. 1998).

Wind-driven upwelling of nutrients from deeperlayers fuels coastal productivity, resulting in several-day fluctuations in productivity that follow changesin the wind direction and, hence, upwelling. Dur-ing an upwelling event, phytoplankton respond tothe infusion of nutrients near the coast and thisbloom is moved offshore, continuing to grow whiledepleting the nutrient supply. When winds reverse(as occurs during storms), the bloom moves backtoward shore where it can contact the coast or en-ter coastal estuaries (Roegner et al. 2002).

The action of the alongshore wind stress on thesea surface results in an alongshore, verticallysheared coastal jet in the direction of the windstress (see model studies in Allen et al. 1995; Allen

and Newberger 1996). The location of maximumspeed in the coastal jet moves progressively fartheroffshore as long as the wind stress continues to act.Typical cross-shelf velocity profiles for Washingtonand Oregon are shown in Hickey (1989) and Huy-er and Smith (1974). The speed maximum mosttypically occurs near mid shelf (Hickey 1989). Ve-locity can decrease by a factor of more than twofrom surface to bottom (or even reverse sign) andby a factor of more than two from the inner shelfto the mid shelf. The cross shelf and vertical struc-ture of the velocity field is important when consid-ering transport of larvae by the coastal current sys-tem (Rooper 2002).

The cartoon of shelf circulation shown in Fig. 3does not include the important effects of remoteforcing; i.e., acceleration of currents caused by sea-level disturbances originating south of the regionof interest. Alongshore gradients in alongshorecoastal wind stress are significant, with strongerwinds (typically upwelling favorable) south of thePNW in the spring and summer (Hickey 1979).The alongshore differences in wind forcing resultin the generation of coastal trapped waves, featureswith wavelengths of hundreds to thousands of ki-lometers and typical propagation speeds of 300–500 km d21. These waves travel northward with thecoast on their right hand side in the northernhemisphere, accelerating local currents as theypass by. Because the West Coast north of PointConception has no promontories sufficient to sig-nificantly disrupt wave propagation, coastaltrapped waves generated as far south as central Cal-ifornia travel to the Washington coast (Battisti andHickey 1984). These waves add to the local wind-generated alongshore currents discussed above. Atany given time and location, the ratio of remoteand local forcing varies and their relative impor-tance has significant interannual variability due tothe dependence on alongshore wind stress gradi-ents (Battisti and Hickey 1984). In summer, coastaltrapped waves are usually important in the PNW,particularly at more northern latitudes such as theBritish Columbia coast (Hickey et al. 1991). In win-ter, local wind forcing dominates in the PNW, es-pecially in regions where winter storms are accom-panied by strong northward winds whose strengthincreases in the direction of propagating waves.

Fluctuations in cross-shelf velocity are not as wellunderstood as those in alongshelf velocity. Al-though model results show onshore and offshoreflow in the surface and bottom boundary layersafter sufficient adjustment of the system to an ap-plied wind stress (e.g., Allen et al. 1995; Allen andNewberger 1996), and a surface Ekman spiral hasbeen identified in some data sets (Lentz 1992), ob-served velocities are frequently much more com-


Fig. 4. Satellite-derived sea surface temperature in the PNWon July 18, 1997. The figure is consistent with upwelling alongthe Washington coast and along the coast of Vancouver Island,Canada, west of the Juan de Fuca eddy. The eddy is readilyapparent as a coldwater feature offshore of the strait of Juan deFuca. The white arrow indicates a previously unreported coldupwelling squirt in the lee of the northern headland along theWashington coast. Figure modified from Trainer et al. (2002).

Fig. 5. Annual cycle of the vertically-integrated chlorophyllover the Washington and Oregon shelves. The data illustratethe typically higher chlorophyll on the Washington shelf. Pig-ment profiles were integrated to the 1% light level or the water-sediment interface. Modified from Landry et al. (1989).

plex than those predicted by model studies (Brinket al. 1994). The relatively short alongshore coher-ence scales (10–20 km) belie the large-scale natureof the atmospheric forcing mentioned above. Ingeneral, the cross-shelf flows appear to be highlythree-dimensional, including effects of smaller-scale features in the bottom topography and thecoastline as well as in the wind field.

Meandering jets and an energetic eddy field car-ry much of the variance in the California Currentoff northern and central California (Strub et al.1991). These jets, which extend over at least theupper 200 m of the water column, can carry re-cently upwelled coastal water and associated bio-

logical production seaward of the shelf to distancesof several hundred kilometers. The strongest jetsare generated near coastal promontories whereflow separates from the coast, the resulting jet be-coming unstable (see model studies in, e.g., Bat-teen 1997). The meandering jet that separatesfrom the coast near southern Oregon can betraced southward along the whole California coast(Barth et al. 2000). Unlike the California coast, thecoastline of the PNW is relatively straight, so thatoffshelf jets are rarely observed. Satellite-derivedpatterns of sea surface temperature show only oneregion off the Washington coast where a prom-ontory appears to cause flow separation. Even inthis case the colder water upwelled in this areaflows southward along the shelf rather than acrossthe shelf and coastal margin (Fig. 4).

Mesoscale Features and Along-coast GradientsImportant mesoscale features include river

plumes, submarine canyons, banks, and coastalpromontories. Such features can modulate the lo-cal upwelling response (hence nutrient supply),they can alter flow patterns, and they can changeenvironmental characteristics such as turbidity,mixed layer depth, stratification, and mixing rates.For these reasons, such features are likely to be ofparticular importance to phytoplankton and zoo-plankton production, growth, and retention in agiven region.

ALONG-COAST GRADIENTS IN PRODUCTIVITY ANDWIND FORCING

Seasonal time series of vertically integrated chlo-rophyll a (chl a) for the Washington and Oregonshelves demonstrate that chl a is greater on theWashington shelf (Fig. 5). This result, derived byaveraging data from a number of unrelated surveysbetween 1950 and 1984, is confirmed by satellite-derived images of ocean color (Strub et al. 1990)and also by recent surveys of the Columbia plumeregion (Peterson personal communication). OffOregon, only over Heceta Bank do chlorophyll val-ues approach those seen off the Washington coast.The few available studies of primary productiondemonstrate higher growth rates off the Washing-ton coast (Anderson 1972), suggesting that thealong-coast difference is not simply due to greaterretention of chl a on the Washington shelf, but tohigher growth rates. Greater productivity is also ob-served higher in the food chain, e.g., in euphau-siids and copepods (Landry and Lorenzen 1989).Juvenile salmon are also observed more frequentlyoff the Washington coast (Pearcy 1992).

The apparently greater richness of the Washing-ton coast is particularly surprising because the gra-dients in the primary forcing, alongshore wind


Fig. 6. Topography of the Pacific Northwest illustrating im-portant canyons and banks.

stress, increase in the opposite direction; i.e., theamplitude, duration, and frequency of upwelling-favorable coastal winds decrease northward in thePNW (Hickey 1979). Southward stress frequentlydiffers by almost a factor of two between southernOregon and northern Washington. The durationof coastal upwelling also decreases seasonally to-wards the north. In spite of the alongshore de-crease in wind stress we note that the seasonal var-iation in macronutrient supply to the mid-shelfdoes not differ substantially between the two re-gions (Fig. 2; Landry et al. 1989; Hickey 1989).This result can be attributed to several processes:differences in circulation patterns due to differ-ences in shelf structure, upwelling enhancementby canyons, and influences of the plume from theColumbia River.

The width and shape of the continental shelfvaries substantially in the PNW (Fig. 6). For ex-ample, the width of the shallow nearshore region(arbitrarily defined as the area shallower than 100m) is greater by more than a factor of two (;50km) off Washington than off Oregon, with the ex-ception of Heceta and Stonewall Banks in south-ern Oregon. The shallow nearshore region is fa-vored by the juveniles and returning larvae ofmany species (e.g., Rooper 2002). Model studies inAllen et al. (1995) show that a wider, gently slopingshelf like the Washington shelf results in slower cir-culation (i.e., possibly greater water and particulateresidence times). Also on such a shelf the upwell-ing flow tends to be more concentrated in the bot-tom boundary layer than over a steeper shelf, aresult of the ability of the broader shelf system tomore readily approach a steady state in which sur-face and bottom layer transports are roughly equal.This might explain the apparently similar levels ofmacronutrients on the Washington and Oregonshelves in spite of the substantially weaker windstress to the north; i.e., upwelled water off Wash-ington may come from deeper depths than offnorthern Oregon, compensating for the weakeralongshore wind stress.

BANKS

Because of the large scale nature of the forcing,the oceanography of the PNW is frequently de-scribed in terms of currents and water propertiesthat are similar over large distances. Recent worksuggests that mesoscale features, such as subma-rine banks or islands, may influence nutrient path-ways, residence times, or mixing regimes sufficient-ly to make them important and even critical to theregional marine ecosystem.

A unique example of the general importance ofsuch mesoscale features is shown in a coast-widesurvey of domoic acid that took place in 1998 (Fig.

7; Trainer et al. 2001). Domoic acid frequently re-sults in closures of razor clam beaches along theWashington coast and has been responsible for anumber of mortalities in seabirds and marinemammals in California (Trainer et al. 2000). Highvalues of this toxin were observed in the vicinity ofthe four dominant mesoscale topographic featuresthat might lead to longer residence times and no-where else. These features include the mouth ofthe Santa Barbara Channel, where an eddy usuallyoccurs (Hickey 1992), the Farallon Islands offnorthern California, and, in the PNW, both Hecetabank off Oregon and the banks offshore of theStrait of Juan de Fuca where a seasonal eddy occurs(Freeland and Denman 1982). Maps of ocean pig-ment in the PNW also show a relationship to bank-like features; chlorophyll is greater and extendsfarther offshore in the bank region offshore of theJuan de Fuca Strait as well as over Heceta Bank


Fig. 7. Particulate domoic acid in Pseudo-nitzschia species onthe U.S. Pacific West Coast in 1998. Maximum concentrationsof domoic acid and toxic species are indicated to the right ofeach area of high toxin. Each of these areas is associated withrelatively retentive circulation patterns. From data in figures giv-en in Trainer et al. (2000, 2001, 2002).

Fig. 8. Modeled circulation at depths ranging from 50 to600 m showing cyclonic eddies over and within two submarinecanyons off the Washington coast (Dinneman and Klinck, per-sonal communication). Note that at 50 m the flow is relativelyundisturbed by the canyon topography. The circulation wasforced by an upwelling-favorable wind stress with a magnitudetypical for this region in summer.(Strub et al. 1990). Both chlorophyll and domoic

acid patterns suggest that the physical or biochem-ical environment provided by banks must haveproperties significantly different from nearshoreupwelling regions along straight coastlines.

SUBMARINE CANYONS

The northern half of the PNW coast is indentedby a number of submarine canyons (Fig. 6). Up-welling of nutrient-rich water is enhanced several-fold in the presence of such canyons (see modelstudy in Allen 1996 and observations in Hickey1989). As shown in Fig. 2, seasonal macronutrientsupply appears to be similar off Washington andOregon in spite of the significantly weaker upwell-

ing winds off Washington. We speculate that theadditional upwelling volume from canyons may atleast partially compensate for the weaker upwellingwinds in that region.

Canyons also alter regional circulation patternsin a manner that increases the possibility of localretention (Hickey 1995, 1997). Counterclockwisecirculation patterns are generally observed bothwithin and over submarine canyons, although notnecessarily extending to the sea surface (Fig. 8).Such eddies provide an effective mechanism fortrapping particles such as suspended sediment ororganic detritus (Hickey 1995).


THE COLUMBIA RIVER PLUME

Discharge from the Columbia River varies be-tween 2,500 in late summer to 17,000 m3 s21 inspring over a typical year, and can reach 30,000 m3

s21 in major freshets (Bottom et al. 2001). The Co-lumbia accounts for 77% of the coastal drainageon the U.S. West Coast (Barnes et al. 1972). Theplume from the Columbia River is thought to havemajor ecological effects in the PNW, particularlywith respect to out-migrating juvenile salmon(Pearcy 1992). On a seasonal basis, the plume fromthe Columbia flows northward over the Washing-ton shelf and slope in fall and winter, and south-ward well offshore of the Oregon shelf in springand summer (Fig. 3). In winter, the plume has adramatic effect on the Washington coast, produc-ing time-variable currents in the near surface layers(;20 m) as large as the wind-driven currents(Hickey et al. 1998). In summer, freshwater fromthe Columbia gives rise to the low-salinity signaland associated front used to trace the meanderingcoastal jet that separates from the shelf at CapeBlanco (Huyer 1983). Both observational andmodeling studies show that the plume is a movingtarget, changing direction, thickness, and widthwith every change in local wind strength or direc-tion (Fig. 9; Hickey et al. 1998; Garcia-Berdeal etal. 2002).

In contrast to many U.S. East Coast coastal areas,nitrate input to the ocean from coastal rivers isnegligible even from the Columbia (Conomos etal. 1972). Nitrate provided from the river drainagearea is used within the estuary in summer (Park etal. 1972); mixing of ocean and river water near theriver mouth can entrain nitrate and other nutri-ents from ocean water if coastal upwelling is oc-curring on the shelf at the time the water is ex-ported across the river bar (Conomos et al. 1972).

Although relatively devoid of nitrate, the Colum-bia River plume is rich in iron and silica (Brulandpersonal communication). In summer the plumeoff Oregon is usually located seaward of the shelf,whereas a plume is frequently observed over theWashington shelf even in summer (Horner et al.2000). We expect that water column iron would bemore readily available to phytoplankton north ofthe river mouth. The sediment from the Columbiais deposited near the river mouth but is transport-ed northward by waves in winter storms, producinga ;10 m thick deposit of silt over the mid andouter shelf along the entire Washington shelf (Nit-trouer 1978). Shelf sediment has been suggestedas a primary source of iron during upwelling inthe CCS off California ( Johnson et al. 1999). Tothe extent that the Columbia silt contains iron, theexistence of this source, combined with the fact

that the wider Washington shelf might allow waterto have extensive contact with the bottom, againsuggests that iron is likely more available north ofthe Columbia than south of it. If the system is ironlimited (as yet unknown), these along-coast gradi-ents would be consistent with the observed along-coast gradient in productivity described earlier.

River plumes are generally turbid, providing lesslight for plankton growth, while at the same timeproviding better cover from grazing for higher tro-phic levels. Plumes also provide potentially retentiveareas (i.e., regions with longer residence times);eddy-like features are generated within a plume un-der both steady (Garcia-Berdeal et al. 2002) and un-steady (Yankovsky et al. 2001) outflow conditions.Inshore of the Columbia plume on the Washingtoncoast in winter, a retentive circulation pattern oc-curs during periods of upwelling-favorable winds(Hickey et al. 1998). Deep mixing is inhibited byhigh stratification at the base of the plume, whichtends to keep plankton within the euphotic zone.Plumes alter regional current patterns in the upperlayers, providing along-plume jets for rapid trans-port, and convergences and trapping at frontalboundaries on the edges. The model exampleshown in Fig. 10 illustrates these more retentive cir-culation patterns and also the along-plume jets. In-formation on stratification enhancement is given inGarcia-Berdeal et al. (2002) from a numerical mod-el and in Hickey et al. (1998) from observations ofthe Columbia plume. Recent studies suggest thatplume edges are preferred feeding sites for zoo-plankton. The fact that juvenile salmonids are fre-quently found near the Columbia plume (Pearcy1992) may be due to the local retention patterns orto frontal convergences, either of which might en-hance food availability in this region.

Other than the Columbia, river plumes on thePNW coast are relatively small, and satellite imag-ery suggests that their traceable effects are con-fined to within one or two tidal excursions of themouth of the river or estuary (not shown). Otherriver or estuarine plumes include those from GraysHarbor and Willapa Bay, Washington, and CoosBay, Oregon.

Both the structure and magnitude of the Colum-bia River plume have significant interannual vari-ability. During years of high snowpack in the PNW(such as 1999), very fresh water from the plumecan flood the major coastal estuaries north of theColumbia estuary for prolonged periods, reversingthe normal estuarine density and salinity gradientsover much of the estuaries. Because such plumeintrusions would not occur in estuaries off theOregon coast, the presence or absence of theplume may provide an important environmental


Fig. 9. Modeled response of the Columbia River plume in summer to changes in wind direction. The figure illustrates the evolutionof surface salinity (psu) for southward ambient flow conditions in response to 6 days of downwelling favorable winds (day 13–19), followedby 6 days of upwelling favorable winds (day 20–26) at (a) day 13, (b) day 15, (c) day 16, (d) day 19, (e) day 21, and (f) day 25 with asouthward ambient flow of 10 cm s21. The distance between tick marks is 20 km. Reprinted from Garcia-Berdeal et al. (2002).

distinction between these estuaries as well as be-tween nearshore coastal regions.

THE STRAIT OF JUAN DE FUCA

The counterclockwise, cold eddy offshore of theStrait of Juan de Fuca (also called the Tully eddy;Tully 1942) is situated southwest of Vancouver Is-land and offshore of northern Washington. Theeddy, which has a diameter of about 50 km, formsin spring and declines in fall (Freeland and Den-man 1982). The eddy is a dominant feature of cir-

culation patterns off the northern Washingtoncoast and is visible in summertime satellite imageryas a relative minimum in sea surface temperature(Fig. 4). The seasonal eddy is a result of the inter-action between effluent from the Strait, southwardwind-driven currents along the continental slopeand the underlying topography. A connection be-tween the eddy and the Washington coast was dem-onstrated in July 1991, when oil that spilled in theeddy was found on the Washington coast 6 dayslater (Venkatesh and Crawford 1993). It seems like-


Fig. 10. Modeled velocity structure of the Columbia River plume illustrating potential retentive areas (eddy-like features) andfrontal jets associated with the river plume. Surface salinity (psu) contours and surface velocity vectors (m s21) at day 28 for (a)northward ambient flow of 10 cm s21 and (b) southward ambient flow of 10 cm s21. River discharge for both cases is 7,000 m3 s21.Reprinted from Garcia-Berdeal et al. (2002).

ly that marine organisms residing in the Juan deFuca eddy can, under certain ocean conditions, af-fect the Washington coast.

The photic zone in the Juan de Fuca eddy re-gion is characterized by high ambient macronutri-ents supplied by wind mixing, episodic wind-drivenupwelling, topographically controlled upwelling(Freeland and Denman 1982), and the outflowfrom Juan de Fuca Strait. As with the Columbia,macronutrient output from the strait is thought tobe controlled primarily by coastal upwelling: nu-trients are upwelled into the Juan de Fuca canyon,pass through the canyon, and move into the straitas a compensating flow to the estuarine outflow inthe strait (Mackas et al. 1980). The nutrient-richwater is mixed into the outflowing strait water andreturned to the ocean in near surface waters tofuel coastal productivity. Although the ultimatesource of nutrients for the eddy is the same as thatin a nearshore coastal upwelling region (CaliforniaUndercurrent water), because of the differentpathways to coastal surface waters infusion of up-welled nutrients into the eddy likely occurs on dif-ferent time scales and with different rates than inregions adjacent to the coast.

Repeated surveys on the northern Washington

coast have demonstrated that when domoic acid ispresent in this region it is usually within or nearthe Juan de Fuca eddy (Trainer et al. 2002). Thediatom Pseudo-nitzschia is always present in signifi-cant numbers when the acid is present and thesediatoms are known toxin producers. A relationshipbetween toxification of clams and onshore watermovement in storms was demonstrated in a timeseries (Trainer et al. 2002) for the 1998 toxic eventthat closed the beaches to razor clam harvestingfor an entire year. Growing conditions in this me-soscale feature must differ from the large scaleconditions along the coast where toxin is not usu-ally produced; in this example, the mesoscale dy-namics are as important as the large-scale dynamicsin determining the nature of the biological re-sponse.

Pacific Northwest Estuaries

GEOMORPHOLOGY AND FORCING

The four estuaries studied in PNCERS (GraysHarbor, Willapa Bay, Yaquina Bay, and Coos Bay;Fig. 6) are members of a chain of small estuariesalong the Washington, Oregon and northern Cal-ifornia coasts. Most of these estuaries are drowned


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river valleys, formed from sea-level rise during thelast 10,000 yr. Some have also been shaped byocean-built bars, either partially (e.g., Willapa Bay,Washington) or entirely (e.g., Netarts Bay,Oregon). Emmett et al. (2000) reviews the geog-raphy of this system in detail.

Indices of geomorphology and tide and riverforcing for the PNCERS estuaries are given in Ta-ble 1. For comparison, the same parameters areincluded for the Columbia River estuary, San Fran-cisco Bay and South San Francisco Bay alone, Nar-ragansett Bay, Chesapeake Bay and its tributary theJames River, and Plum Island Sound, a small em-bayment on the Massachusetts coast. Except whereotherwise marked, data are from the National Oce-anic and Atmospheric Administration National Es-tuarine Inventory Data Atlas (NOAA 1985). Vol-ume parameters, which are particularly difficult todefine and measure (e.g., Malamud-Roam 2000),are calculated here by simple, approximate meth-ods for the sake of uniformity, so only gross pat-terns among the area and volume parameters aresignificant. Volume is calculated as the product ofmean depth and surface area at mean sea level(MSL), a method which gives errors up to ;20%in comparison with other published figures(NOAA/EPA 1991). Mean tidal prism volume is re-ported as a percentage of volume at high water,which is calculated as MSL volume plus half thetidal prism itself.

Coos Bay is only a few times larger than tinyPlum Island Sound, but is nevertheless the largestof the Oregon estuaries. Grays Harbor and WillapaBay, the two coastal-plain estuaries north of the Co-lumbia, are an order of magnitude larger, compa-rable in volume and morphology to South SanFrancisco Bay. Both Washington estuaries consistof multiply-connected channels 10–20 m deep sur-rounded by wide mud and sand flats. Half or moreof the surface area of these estuaries lies in theintertidal zone. Significantly, even the smaller, nar-rower estuaries of Oregon have similar percentagesof intertidal area (Table 1; Percy et al. 1974).

Tides on this coast are mixed-semidiurnal, withspring-neap amplitude variation on the order of50% (Emmett et al. 2000). Mean tidal ranges (Ta-ble 1) are generally twice as large as on the outerAtlantic Coast. The combination of large tidalrange with broad, open intertidal surface areayields tidal prisms that are large fractions (30–50%) of total volume. This result holds very gen-erally for PNW outer-coast estuaries, and is amarked difference between these systems and allbut the smallest of their counterparts on otherNorth American coasts. These large tidal prismssuggest that flushing by tidal action is probably im-portant in all these estuaries, even those that re-


ceive significant river flow (Dyer 1973). Tidal ex-cursions, as estimated from current measurementsin Willapa Bay, Grays Harbor, and Coos Bay are 12–15 km, significant fractions (25–50%) of the lengthof the estuaries.

Table 1 includes long-term mean flows for thelowest- and highest-flow months of the year and, asa measure of the strength of river forcing relativeto estuary size, the river-filling time (volume divid-ed by flow rate). The outflow from the ColumbiaRiver is two orders of magnitude larger than riverflow into the other coastal estuaries. With the ex-ception of the Columbia, these estuaries receivefreshwater input from local rainfall only, not fromsnowmelt. Local river flow, like local rainfall, ishigh during winter, when storms are frequent, in-termittent during spring and early summer, andnegligible during late summer (Emmett et al.2000). This seasonality in river flow is generally sev-eral times greater than in East Coast estuaries(NOAA 1985), though flood and drought eventsbeyond the mean seasonal cycle have not beenconsidered here. As a result we might expect thehydrodynamic classification of PNW estuaries tochange dramatically between seasons, or even—where flushing and adjustment times are short—between individual wind events.

This river flow pattern yields a seasonal hydro-graphic cycle that contrasts strongly with tradition-al models of temperate, partially mixed estuaries,with possibly important ecological implications. Ty-ler and Seliger (1980), for example, show that pri-mary production in Chesapeake Bay is controlledby stratification-dependent pathways reminiscentof the seasonal dynamics of the open-ocean mixedlayer. In that estuary, in winter, mixing by wind andtide erases stratification and resuspends nutrients,while in spring and summer increased river flowand solar heating produce strong stratification andreduced vertical exchange. In such a system, strat-ification controls on vertical mixing are crucial todetermining plankton growth rates and the poten-tial for phytoplankton blooms, as in San FranciscoBay (Lucas et al. 1999). In Willapa Bay the patternis quite different with generally weak stratificationduring summer, when river flows are low, andstronger during the winter, when river flow peaks.Vertical, one-dimensional, stratification-centeredmodels of primary productivity would not be ap-propriate in Willapa Bay even at the coarsest level.During the growing season in PNW estuaries, hy-drography, nutrient levels, and biomass all appearto be controlled less by in situ processes than bymesoscale processes in the coastal ocean (Hickeyet al. 2002; Roegner et al. 2002).

STUDY SITES AND OBSERVATIONS

During PNCERS, arrays of moored sensors weremaintained in three coastal estuaries (Grays Har-bor, Willapa Bay, and Coos Bay) as well as at twosites in the nearshore coastal ocean, one off Wash-ington, the other off southern Oregon. Mooredsensors included S4 current meters or acousticDoppler current profilers, and Seabird C-T sensorsor Aanderaa current meters equipped with con-ductivity and temperature sensors. Estuarine in-struments were set in the lower water column ontaut wire moorings. Sampling interval was less than30 min in the estuaries and hourly on the coast.Two arrays were maintained in Willapa Bay andCoos Bay and one in Grays Harbor. The longesttime series (temperature) spans 3 years. In gener-al, salinity time series are much shorter due to foul-ing and clogging problems. Hydrographic sectionswere made with a SeaBird 19 conductivity-temper-ature-depth profiler (CTD) at sporadic intervalswhen mooring instruments were exchanged orcleaned. Data were edited for outliers. Hydro-graphic section data were used to validate datafrom the moored arrays. For subtidal time series,data were filtered with a Butterworth low pass filterand smoothed to hourly intervals.

National Center for Atmospheric Research, Na-tional Centers for Environmental Prediction(NCAR NCEP) six-hourly winds from the Reanal-ysis project (Kalnay et al. 1996) were obtained at2.5 degree intervals and interpolated to Grays Har-bor and Coos Bay mid shelf locations. These dataare provided by the NOAA-Cooperative Institutefor Research in Environmental Sciences ClimateDiagnostics Center, Boulder, Colorado at http://www.cdc.noaa.gov/. These winds are generatedfrom an atmospheric model that includes data as-similated from both coastal buoys and land sta-tions, containing some information on coastal to-pographic effects. The NCEP Reanalysis winds pro-vide a better representation of alongshore gradi-ents in wind than in situ buoys. Coastal buoys arelocated at different distances from the coast, sothat cross-shelf structure in the wind field biasesdifference estimates. Data from National DataBuoy Center Buoy 46029 (the Columbia Riverbuoy) were used for single site analyses.

LINKS TO THE COASTAL OCEAN

Response to Upwelling and Downwelling

The properties of the ocean water presented atthe mouth of the estuary are governed by whetherupwelling or downwelling is occurring along thecoast at that time (Hickey et al. 2002). During up-welling, surface waters move offshore in responseto alongshelf wind stress, and within a few kilo-


Fig. 11. Time series of temperature at selected sites in Wil-lapa Bay illustrating up-estuary propagation of a coastal upwell-ing-downwelling signal. Time series of salinity in the estuary andalongshore wind on the nearby coast illustrate the response ofthe estuary salinity to coastal upwelling events, high salinity dur-ing periods of upwelling-favorable winds and low salinity duringperiods of downwelling-favorable winds, with a ;1.5 day lag be-tween wind and estuary salinity.

Fig. 12. Schematic illustrating baroclinic coupling betweenthe coastal ocean and a coastal plain estuary in the PacificNorthwest during upwelling and downwelling events for a lowriver flow, summer period. Reprinted from Hickey et al. (2002).

meters of the coast cold, saltier, nutrient-rich watermoves onshore and upward. Depending on strati-fication, bottom slope, and wind stress magnitudeand duration, upwelled water may be confined tothe bottom few meters or it may move onshore inthe central water column (Huyer 1983; Allen et al.1995). In general, the source of upwelled water liesat or just below the shelf break, in the waters ofthe California Undercurrent. Phytoplankton seedstock are also upwelled into the euphotic zoneand, fueled by the high nutrient level, begin togrow (Roegner et al. 2002). The growing phyto-plankton move offshore as new seed stock is up-welled so that the highest biomass may be situatedsome distance from the coastal wall and themouths of the estuaries. During downwelling,warmer, fresher, nutrient-depleted surface watersmove inshore and downward, and offshore phyto-plankton blooms likewise are advected back to thecoast.

Oceanic phytoplankton can enter a coastal es-tuary by two routes. During upwelling events, seedstock can be pulled into the estuary, where a localbloom is fueled by the high nutrients brought inwith the upwelled water (de Angelis and Gordon1985). During downwelling events, phytoplanktonfrom a prior offshore bloom can be pulled directlyinto the estuary as observed in Willapa Bay duringone upwelling-downwelling sequence (Roegner etal. 2002). This biomass, although nutrient-poorand declining rather than growing, may provide a

direct food source to secondary production, par-ticularly near the mouth of the estuary. In thePNW, transitions between upwelling and down-welling occur at 2–10 d intervals (Hickey 1989),and so the ocean end member of estuarine waterproperties can change significantly over just a fewtidal cycles.

In Willapa Bay both upwelling and downwellingwater presented at the mouth of the estuary havebeen observed to travel up-estuary in the lower wa-ter column at a rate on the order of 10 km d21,modifying the gravitational circulation of the es-tuary as they pass (Hickey et al. 2002). These mod-ulations of circulation and water properties lag lo-cal wind stress fluctuations (hence, upwelling ordownwelling) by more than a day (Fig. 11). Thismode of up-estuary propagation is consistent withthe suggestion by Duxbury (1979) that modulationof the gravitational circulation by upwelling anddownwelling is responsible for increased meanflushing rates in summer months in Grays Harbor.Such a baroclinic coupling between ocean and es-tuary is schematized in Fig. 12, with values takenfrom typical early-summer conditions in WillapaBay. During upwelling events, high ocean salinitiesincrease the along-channel salinity gradient whichincreases the magnitude of baroclinic exchange(Hansen and Rattray 1965; Monteiro and Largier1999). During downwelling events, the salinity con-trast between ocean and estuary is reduced, whichthen reduces the strength of the exchange flow.

It is likely that the propagation of oceanic signalsinto these estuaries is accomplished not by baro-clinic, density-driven mechanisms alone, but also inpart by diffusive, density-independent mechanisms,like lateral stirring by tidal and wind-driven cur-rents (e.g., Wang 1979, Geyer 1997). In general,tide- and wind-driven mechanisms are expected todominate in shallow, well-mixed estuaries, and


Fig. 13. (a) Time series of temperature in three estuaries in the Pacific Northwest, illustrating simultaneous response to largescale upwelling-downwelling events along the open coast during spring-fall 1999. All data are from the lower water column and fromstations near the mouth of each estuary (right panels). The solid line along the x-axis indicates the interval expanded in (b, c, d).(b) Time series of temperature in Grays Harbor and Coos Bay. Arrows indicate downwelling events (warmer water) prominent onlyin the northern estuaries. (c) Time series of alongshore wind interpolated to latitudes close to Grays Harbor and Coos Bay. Arrowsillustrate wind events that cause downwelling seen in panel b. (d) Time series of salinity for the same period, illustrating effects ofmore persistent upwelling-favorable winds at more southern locations.

density-driven exchange is expected to dominatein deeper, partially stratified systems (e.g., Hansenand Rattray 1966). The PNW outer-coast estuariesspan both of these broad categories, and we canexpect the relative role of river-driven baroclinicexchange, baroclinic coupling between ocean andestuary, and tide- and wind-driven exchange tovary significantly between systems and over time ina single system. In particular, a number of generalcharacteristics of PNW estuaries—large tidalprisms, complex bathymetry, and highly variableriverflow and stratification that are close to zeroduring much of the summer—make it likely that

tidal stirring is important to the overall flushingrate of these systems during the growing season.

Along-coast Correlation between Estuaries

If these estuarine-ocean coupling processes aresimilar among the estuaries, we might expect waterproperties in many PNW estuaries to vary coher-ently because wind-driven coastal processes in sum-mer have scales of several hundred kilometers. ThePNCERS data confirm that this is generally thecase; time series of temperature data collected si-multaneously in Grays Harbor, Willapa Bay, and


Fig. 14. (a) Time series of the north-south component ofnearshore wind from late winter to early summer, 2000. Thedates of the two CTD transects of Willapa Bay shown in Fig. 15are indicated along the top. (b) Salinity and (c) the local along-channel salinity gradient near the mouths of Willapa Bay andGrays Harbor during a three-week period April–May 2000, show-ing a brief upwelling event, an intrusion of the Columbia Riverplume, and a recovery from that intrusion. In (b), both 30-minand subtidal (48-h-Butterworth-filtered) data are shown. Dotsmark times of high slack water in Willapa Bay. In (c), the dif-ference between high-slack and low-slack salinity divided by thetidal excursion for each semidiurnal tidal cycle has been filteredas above to provide a subtidal, single-station time series of thealong-channel salinity gradient.

Coos Bay demonstrate that in the spring-fall grow-ing season all three estuaries, which span 400 kmof the PNW, are highly correlated (Fig. 13a). Cor-relation is highest in late summer-early fall (day201–271, r 5 0.95 between Grays Harbor and Wil-lapa Bay, 0.91 between Coos Bay and Grays Har-bor).

The spring-summer period is further analyzed inthe next three panels of Fig. 13, showing temper-ature, alongshore wind, and salinity. Comparisonof temperature and wind illustrates the general re-sponse to upwelling and downwelling favorablewinds, with the wind-property lag (1.5 d at bothGrays Harbor and Coos Bay) discussed above.Some slight differences between the northern andsouthern estuaries are observed, namely, a fewtemperature peaks (indicated with arrows) occuronly in the northern estuaries or have much stron-

ger amplitudes there, reducing correlation in thespring-summer period to 0.74. For example, fromday 165–175 temperature decreases in Coos Baywhile it increases in Grays Harbor (Fig. 13b). Com-parison with alongshore wind (Fig. 13c) demon-strates that this difference is caused by the fact thatdownwelling-favorable winds are stronger duringthis period near Grays Harbor than near Coos Bay.Several other similar examples of alongshore dif-ferences in estuary water temperature caused bysmall but significant alongshore differences inwind amplitude or direction can be seen in therecords.

Another example is illustrated in salinity recordsfrom northern and southern estuaries from thesame period (Fig. 13d). In the southern estuary(Coos Bay) the upwelling signal (higher salinity inthe first half of the record) is of significantly longerduration than in Grays Harbor. The latter experi-enced actual downwelling winds rather than justweakened upwelling winds during this period.Note that the greater salinity range and overall low-er salinity at the northern estuary is consistent withthe generally lower regional salinity due to theproximity of the Columbia plume (see next sec-tion).

Columbia River Plume IntrusionsAlthough the estuaries of Washington and

Oregon generally respond to ocean forcing coher-ently as discussed above, the Columbia Riverplume can cause major asymmetries between theestuaries. Since the plume moves offshore when itflows southward past Oregon during periods of up-welling-favorable winds, it does not impinge uponmost Oregon estuaries directly under those con-ditions. When the plume flows north under down-welling winds, it fills the nearshore water columnnorth of the river mouth past the depth of theestuary mouths (Garcia-Berdeal et al. 2002; Roeg-ner et al. 2002). The plume may also impact es-tuaries on the northern Oregon coast duringdownwelling, when the southwest-tending plumeformed under the preceding upwelling conditionsand seasonal southward ambient flow moves shore-ward. Mixing during the downwelling event wouldresult in much less density contrast than off theWashington coast, where the plume is relativelynew and fresher. The effect of the plume on theestuaries is most dramatic and sustained in latespring and early summer, when local river flow hasslackened but the Columbia is still running highwith snowmelt.

Lower water column salinities from moorings in-side the mouths of Willapa Bay and Grays Harborduring April and May 2000 are shown in Fig. 14.For each station, the along-channel salinity gradi-


Fig. 15. Salinity from CTD transects along the main channel of Willapa Bay on (a) May 3, 2000 near the end of a Columbia Riverplume intrusion and (b) May 30, 2000 during a strong upwelling event that replaced plume water (;21.5 psu) with much saltierwater ($29 psu). A reversal of the along-channel salinity gradient is marked in (a). Triangles at the top of the salinity sections givethe location of CTD casts. Tidal stage and transect route are also given for each section. The wind preceding and during each CTDsection is shown in Fig. 14a.

ent has also been calculated as a subtidal time se-ries, by dividing the difference between high- andlow-water salinities by the tidal excursion for eachsemidiurnal tidal cycle, and then filtering the re-sulting discrete series. This method takes advan-tage of the fact that each station effectively samples;15 km of the channel through tidal advection.This allows us to calculate along-channel gradientswithout requiring pairs of stations to obtain differ-ences. An upwelling event, which brings ;32 psuwater into the estuaries and produces strong along-channel gradients (on April 19, ;5 psu over onetidal excursion), is followed by a plume intrusion,indicated by a dramatic decrease in salinity andweak along-channel salinity gradients. When down-welling-favorable winds slacken after ;April 27, sa-linity and the along-channel salinity gradient in-crease again. The 5-mo wind time series shown inFig. 14a suggests that this intermittent alternationof upwelling and plume intrusion continues fromlate winter through early summer.

During the onset of plume intrusions the along-channel salinity gradient in the estuary can reversefor sustained periods. In Fig. 14b, for example, asthe plume intrusion intensifies during the periodApril 20–28, salinity at the Willapa Bay mooring athigh slack water (indicated by dots) is generallylower than the subtidal average, indicating thateach flood tide is bringing somewhat fresher waterinto the estuary. This reversal of the expected gra-

dient between mid-estuary and ocean water is il-lustrated in a CTD transect along the main chan-nel of Willapa Bay on May 3, 2000, during the re-covery from the plume intrusion (Fig. 15a). Salin-ity increases downstream from the head to .21.8psu, drops to ,21.4 psu, and then increases againwithin one tidal excursion of the mouth.

Vertical gradients weaken during plume intru-sions along with the longitudinal gradients. Thevertical salinity difference in the interior of the es-tuary in the May 3 transect is on the order of 0.1psu. In comparison, a transect on May 30 duringthe onset of an upwelling event after a period ofintermittent winds (Fig. 15b) shows vertical salinitydifferences of about 2–4 psu within a tidal excur-sion of the mouth. During a plume intrusion, thereduced salinity contrast between the river andocean end members of the estuary presumablyweakens baroclinic pressure gradients and thusstratification to the point where vertical shear dis-persion can completely homogenize the water col-umn. In contrast to input of freshwater from thelocal rivers, which tends to increase stratificationand gravitational exchange, input from the Colum-bia River via the coastal ocean tends to producenear-complete mixing in Washington estuaries.

Downwelling conditions tend to reduce estua-rine salinity gradients even in the absence ofplume intrusions (Hickey et al. 2002), though to amuch lesser extent. The effect of the Columbia


River plume, then, is to greatly intensify the con-trast between spring and summer upwelling anddownwelling conditions in the Washington estuar-ies in comparison with Oregon estuaries. Thisasymmetry between the two coasts would likely beobserved not just on the event scale, but on inter-annual scales as well. Following wet (La-Nina-like)winters like 1998–1999, but not following dry (El-Nino-like) winters like 1997–1998, sustained plumeintrusions would be expected in the Washingtonestuaries during May and June.

SPATIAL VARIABILITY IN THE INTERTIDAL ZONE

Because much of the physical forcing importantto estuarine productivity is coherent over tens orhundreds of kilometers on this coast, the responseof the coastal estuaries to this forcing may be co-herent and generalizable on this scale as well. Atthe same time, pervasive, significant variation incurrents and hydrography is possible on muchsmaller scales, as short as 100 m, in estuaries withcomplex bathymetry, particularly in very shallowregions, which often are most important biologi-cally. These small-scale variations, which can bethought of as creating estuarine microenviron-ments, easily confound attempts to generalize frommeasurements that do not integrate over largerscales.

New data allow us to describe the two mecha-nisms of lateral variability best resolved by tidalscale observations in the Washington estuaries: di-rect solar heating of bank water and the creationof persistent lateral gradients by tidal advection. Afull account of the transverse structure of these es-tuaries—which must consider competition and in-teraction between tidal currents, density-drivenflows, rotational effects, and wind-driven circula-tions, all of which are shaped by bathymetry (e.g.,Friedrichs et al. 1992; Valle-Levinson andO’Donnell 1996)—is beyond the scope of availabledata.

Solar HeatingCoordinated longitudinal (along-channel) and

transverse (bank-to-channel-to-bank) CTD tran-sects were obtained in Willapa Bay and Grays Har-bor during the summers of 1999 and 2000. Theseobservations frequently suggest solar heating of wa-ter on shallow intertidal flats, either by direct heat-ing of the water at high tide or by transfer to thewater of heat stored in the mud flats themselvesfrom insolation at low tide. Consider a late-after-noon, early-flood transect along the main channelof Grays Harbor during a period of fair weather inJune 1999 (Fig. 16). The warmest water in thechannel is associated with neither the ocean northe river end member, but rather appears near the

surface over a broad middle reach of the channel.CTD casts along this transect were separated byonly ;4 km, and the spatial structure of this warmwater may be patchier than contouring betweencasts allows. We interpret this signal as evidence ofwater warmed during the midday high tide that hascirculated back into the main channel on the fol-lowing ebb. A temperature-salinity (T-S) diagramof this transect (Fig. 16c) shows clearly that thissignal represents warming of water at intermediatesalinity, and effectively constitutes a third mixingend member, toward which the T-S profile of thechannel is inflected. Transverse, channel-to-shoalsurveys on the day of the along-channel transectand over the next four days locate a similar warmwater mass in depths ,5 m at higher stages of thetide (dots in Fig. 16c).

Surveys in Willapa Bay from June and July 2000(Fig. 17) show similar results: warmest tempera-tures on banks in the interior of the estuary, in-flection of the main-channel T-S profile that liftsintermediate water above the mixing line betweenthe ocean and river end members. The warmestpoints in the June 2000 survey, more than 48Cwarmer than main-channel water of the same salin-ity, represent the shallowest water sampled, water,0.5 m deep sampled by foot with a hand-heldmeter.

Note that since the fair-weather events that bringincreased insolation also bring cold, upwelled wa-ter, an estuary’s response to direct heating may bemasked on the event scale and better resolved byan integration over many events. In Willapa Bay,where time series of along-channel transects exist,the inflection of the T-S profile tends to increaseas the summer proceeds, though not monotoni-cally (not shown).

Differential Tidal AdvectionNot all bank-to-channel hydrographic variations

result from solar heating or other transformationof water properties. Consider the along- and cross-channel flood-tide transects from July 1999 in Wil-lapa Bay shown in Fig. 18. The along-channel sa-linity gradient is ;5 psu over one tidal excursion(15 km); across a shallow, narrow bank adjacent tothe main channel during late flood, the salinitygradient is ;4 psu over only 1.3 km. Huzzey (1988)likewise found that in the York River, which likeWillapa Bay consists of a deep central channelflanked by shoals, the freshest water in a cross sec-tion at high slack water was located on the banks.A T-S diagram of the July 1999 transects (Fig. 18c)shows that the bank and channel water masses, un-like those shown in Figs. 15 and 16, are indistin-guishable. The lateral variation in salinity and tem-perature must have arisen from advective rear-


Fig. 16. (a) Temperature and (b) salinity from a CTD transect along the main channel of Grays Harbor on June 11, 1999 duringa time of strong solar heating. Triangles at the top of the sections give the location of CTD casts. (c) Temperature-salinity profile ofthe along-channel transect (lines) and CTD casts on shoals adjacent to the channel June 11–15 (dots). Location and tidal stage ofbank and channel surveys are also shown. Dots on inset maps indicate location of bank stations.

Fig. 17. Temperature-salinity diagrams for surveys of Willapa Bay during (a) June and (b) July 2000, showing the hydrographicsignature of direct solar heating. Line segments represent CTD casts within the main channels of the estuary; dots represent wateron banks with depths ,5 m.

rangement, not transformation, of main-channelwater in the intertidal zone.

Since these strong gradients appear on intertidalbanks that are submerged for only a few hourseach tidal cycle, they must be the result of tidal-time scale processes and not tidal-residual ones. In-deed, large lateral gradients can arise solely fromdifferential advection by tidal currents (Huzzeyand Brubaker 1988; O’Donnell 1993); i.e., the fact

that on a shallow bank tidal motion is slowed byfriction so that a given flood or ebb moves waterparcels farther longitudinally in a channel than onan adjacent shoal. This shearing of the flow effec-tively transfers the along-channel gradient overone tidal excursion, or some fraction thereof, intoa cross-channel gradient. In support of this expla-nation for the lateral variation seen in Willapa Bayin July 1999, repeated channel-to-bank surveys in


Fig. 18. Salinity from CTD transects on July 14, 1999 (a) along the main channel of Willapa Bay and (b) from the channel toshore across a shallow, narrow bank. The vertical bar in (a) near 18 km marks the location of the cross bank transect in (b). Locationand tidal stage are shown. Triangles at the top of the sections give the location of CTD casts. The nearly identical temperature-salinityprofiles of the along channel and cross channel transects are confirmed with a T-S diagram in (c).

the same location have shown that the transversegradient there at high water follows the along-channel variation. On November 1–2, 1999, for ex-ample, the along-channel salinity gradient in thecentral reach of the estuary was much weaker thanthat shown in Fig. 18, only 0.5 psu over one tidalexcursion and the salinity variation over the bankwas likewise 0.5 psu (Banas unpublished data, notshown).

The differential-advective effect would be ex-pected to be strongest on the shallowest banks(like that shown in Fig. 18b) where the effect offriction is presumably greatest, and less importanton deeper, subtidal shoals. Such lateral structurein tidal advection may have important local andbiological consequences. Sessile organisms in ashallow region with strong lateral gradients may ex-perience mean temperatures or rates of nutrientor food supply appreciably different, more likeconditions a large fraction of a tidal excursion up-estuary, than organisms in deeper water a shortdistance away. At the same time, differential tidaladvection may contribute to overall estuarine flush-ing if these lateral shears are a lateral-dispersionmechanism similar to the models of tidal trappingreviewed by Fischer (1976).

Summary and ConclusionsRecent work in the PNW coastal zone has in-

cluded papers on water property variability in Wil-lapa Bay (Hickey et al. 2002), chlorophyll intru-sions into Willapa Bay (Roegner et al. 2002), anda model of Columbia plume variability in responseto variable winds (Garcia-Berdeal et al. 2002). In-

formation in these and other papers as well asfrom historical research in this region (e.g., Pruterand Alverson 1972; Landry and Hickey 1989) pro-vides a framework for better understanding someaspects of ecosystem variability. With respect to thecoastal ocean, the large scale nature of upwellingand nutrient supply as well as mesoscale featuressuch as variations in shelf width or slope, the Co-lumbia River plume, the semi-permanent eddy off-shore of the Strait of Juan de Fuca, and submarinecanyons are likely important to nutrient supply,turbidity, residence time, and transport pathways.In the outer coast estuaries, strong coupling be-tween the estuaries and the inner shelf waters aswell as other mechanisms such as solar heating anddifferential tidal advection on banks control estu-arine water properties and residence time duringthe summer growing season.

This synthesis of historical and recent observa-tions has revealed an important and persistent pat-tern that has been overlooked in the literature:productivity is higher off the coast of Washington(with the exception, at times, off Heceta Bank insouthern Oregon) in spite of the fact that upwell-ing-favorable wind stress is weaker by as much as afactor of two off the coast of Washington than offthe coast of Oregon. Three possible reasons arepresented: the difference in shelf structure (wider,more gently sloping off Washington, possibly re-sulting in upwelling of water from deeper layersthat are richer in nutrients than off narrowershelves), the presence of submarine canyons (withtheir enhanced upwelling volume) only on theWashington coast, and the role of micronutrients


in plankton growth, in particular, the potential roleof iron from both the water or sediments of theColumbia plume.

Results from the new data for three PNW estu-aries indicate that the coupling of estuarine andcoastal water properties was similar in all three es-tuaries sampled. Changes in water properties fol-lowed changes in the direction of the local coastalwind field, hence, upwelling or downwelling, by afew hours to days. Water property variability in es-tuaries separated by distances of over 400 km alongthe coast are highly correlated in the summergrowing season when weather systems, and coastalwinds, are predominantly large scale. In spring,when coastal weather systems may not include theentire PNW, some subtle property differences areobserved. Fine-scale processes within each estuarysuch as solar heating of tidal flats and differentialtidal can create bank to channel gradients as largeas along-channel gradients on scales as small as;100 m. Intrusions of water from the Columbiaplume can remove stratification and even reversealong-estuary density gradients in estuaries northof the river mouth. Such plume intrusions can cre-ate significant water-property contrasts betweenWashington and Oregon estuaries during winter,spring, and early summer.

Coupling between ocean and estuary is centralto the dynamics of any small embayment, but itsimportance is perhaps amplified in PNW estuariesfor two reasons: the ocean rather than local riversis the dominant source of nutrients and biomassalong this coast, and oceanic water properties areextremely variable on the scale of the residencetime or adjustment time of the estuaries them-selves (i.e., on the event scale). This coincidenceof time scales makes the estuaries highly variableand unsteady themselves, far more so, during sum-mer, than fluctuations in local river input wouldforce on their own.

The PNW estuaries constitute a fruitful set fordynamical or ecological comparison; they haveenough commonalities (similar tidal forcing, sim-ilar river flow patterns, event-scale synchrony) tobe usefully compared, and at the same timeenough diversity (in overall size and cross-sectionalshape, relative river flow magnitude, and relationto the Columbia plume) to form an interestingnatural experiment. The data collected and anal-yses to date, sufficient to determine dominant timescales and modes of variability as well as some spa-tial complexities in both the estuaries and the ad-jacent coastal ocean, constitute the first importantstep toward a complete understanding of the linkbetween variability in physical properties and themarine ecosystem in the PNW.

ACKNOWLEDGMENTS

Data collection was supported by PNCERS (grant #NA76RG0485 and NA96OP0238 from the Coastal Ocean Pro-gram of the NOAA). Analysis was supported by PNCERS, Wash-ington Sea Grant (grant #NA16RG1044-R/ES-42 and#NA16RG1044-R/F-137), by a grant (#OCE-0001034) to B.Hickey from the National Science Foundation as part of GlobalOcean Ecosystem Dynamics, and by the NOAA Coastal OceanProgram under award #NA07OA0310 to the University of Wash-ington as part of the Olympic Region Harmful Algal Bloomprogram. This is contribution number 401 of the U.S. GLOBECprogram, jointly funded by the National Science Foundationand NOAA. Susan Geier was responsible for overseeing equip-ment deployment, calibration, and data analysis. Jim Johnsonand Bill Fredericks were responsible for the moored array de-ployments and recovery. Nancy Kachel and Nicolaus Adams as-sisted in some figure displays. We would also like to thank Dr.Jan Newton and Mr. Eric Seigel of the Washington State De-partment of Ecology for graciously providing additional CTDdata for Willapa Bay and Dr. John Klinck and M. Dinneman forproviding modeled circulation to illustrate canyon effects.

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SOURCES OF UNPUBLISHED MATERIALS

BRULAND, K. personal communication. University of California,Santa Cruz, Ocean Sciences Department, A446 Earth and Ma-rine Sciences Building, Santa Cruz, California 95064.

DINNEMAN, M. personal communication. Old Dominion Univer-sity, Center for Coastal Physical Oceanography, Norfolk, Vir-ginia 23529.

KLINCK, J. personal communication. Old Dominion University,Center for Coastal Physical Oceanography, Norfolk, Virginia23529.

PETERSON, W. personal communication. Oregon State Univer-sity, College of Oceanic and Atmospheric Sciences, HatfieldMarine Science Center, 2030 SE Marine Science Drive, New-port, Oregon 97365.

Received for consideration, July 19, 2002Revised, May 23, 2003

Accepted for publication, August 8, 2003Revised, August 26, 2003

Documents

Oceanography of the U.S. Paciﬁc Northwest Coastal Ocean ...coast.ocean.washington.edu/coastfiles/HickeyBanas-PNWcoast.pdf · Oceanography of the U.S. Paciﬁc Northwest Coastal