MARINE ECOLOGY PROGRESS SERIESMar Ecol Prog Ser

Vol. 494: 191203, 2013doi: 10.3354/meps10572

Published December 4

INTRODUCTION

A central goal of ecology is to understand patternsof abundance and distribution of organisms. Deter-mining the key factors that drive population dynam-ics requires knowledge of demographic inputs (birthand immigration) and losses (death and emigration).In marine systems, the process of larval dispersal

adds additional complexity to quantifying populationinputs as many marine organisms release dispersivelarvae that act as the primary agents coupling birth atone site to immigration at another. Therefore, larvalrecruitment to coastal populations relies not only onreproductive output of parent populations and post-settlement processes, but also on oceanographic fac-tors that affect the dispersal and delivery of settlers

Inter-Research 2013 www.int-res.com*Email: knickols@stanford.edu

Spatial differences in larval abundance withinthe coastal boundary layer impact supply to

shoreline habitats

Kerry J. Nickols1,4,*, Seth H. Miller1, Brian Gaylord1,2, Steven G. Morgan1,3, John L. Largier1,3

1Bodega Marine Laboratory, University of California at Davis, Bodega Bay, California 94923, USA2Department of Evolution and Ecology, University of California at Davis, Davis, California 95616, USA

3Department of Environmental Science and Policy, University of California at Davis, Davis, California 95616, USA

4Present address: Hopkins Marine Station, Stanford University, Pacific Grove, California 93950, USA

ABSTRACT: Explorations of the dynamics of nearshore regions of the coastal zone are missingfrom many efforts to understand larval transport and delivery to suitable habitats. Larval distribu-tions in the coastal ocean are variable and depend on physical processes and larval behaviors,leading to biophysical interactions that may increase larval retention nearshore and bolster theirreturn to natal sites. While recent evidence suggests that many larvae are retained within a fewkilometers from shore, few studies incorporate measurements sufficiently close to shore to plausi-bly assess supply to the shoreline benthos. We measured cross-shore distributions of larvae ofbenthic crustaceans between 250 and 1100 m from shore (i.e. just beyond the surf zone) within thecoastal boundary layer (CBL) a region of reduced alongshore flow and simultaneously quan-tified a suite of physical factors that may influence larval distributions. We found high larval abun-dance within the CBL, with a peak at 850 m from shore, and a decrease in abundance along theshoreward edge of the sampled transect. We also found distinctly different larval assemblages atouter stations within the CBL, as compared to inner stations that are more influenced by shorelinedynamics. These patterns persisted across sample dates, suggesting that the spatial structure ofnearshore larval assemblages is at least somewhat robust to temporal changes in physical condi-tions. Thus, while larval abundance appears to be high within the CBL, larvae appear to be sparsewithin the narrow band of water adjacent to the surf zone. Low larval supply adjacent to suitablehabitats has important implications for the coupling of supply and recruitment, and resultingdynamics of shoreline populations.

KEY WORDS: Dispersal Invertebrate larvae Retention Nearshore Transport

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(Cowen 1985, Gaines et al. 1985, Roughgarden et al.1988, Gaylord & Gaines 2000, Morgan 2001, Under-wood & Keough 2001, Largier 2003, Prairie et al.2012). While larval recruitment is a critical determi-nant of population structure (Gaines & Roughgarden1985, Underwood & Fairweather 1989, Menge et al.2004) and larval delivery has long been recognizedfor its role in driving population dynamics (Thorson1946), larval transport pathways and connectivity aredifficult to quantify due to the small size of larvae andthe difficulty of tracking them (Levin 2006). Thesedifficulties are exacerbated by a lack of understand-ing of how larval supply varies over time and space.

Relatively few studies have measured larval supplyand settlement concurrently in such a way as to ex -plicitly link them. Those that have done so havereached mixed conclusions: some found supply andsettlement to be coupled (e.g. Gaines et al. 1985,Bertness et al. 1992, Gaines & Bertness 1992, Dudaset al. 2009) while others did not (e.g. Yoshioka 1982,McCulloch & Shanks 2003, Rilov et al. 2008). Suchdifferences may be due to the magnitude of recruit-ment, with larval supply acting as a strong predictorof adult dynamics in regions where recruitment islimited (Connell 1985), but less so in areas where re -cruitment is high. Connections between supply andsettlement or recruitment may be easier to assess inother systems; for instance those involving dispersalof macroalgal spores (see, e.g. Reed et al. 1988, Gay-lord et al. 2002, 2004, 2006, 2012, Reed et al. 2006).

Much of the west coast of North America is recruit-ment-limited to one degree or another, such that lar-val supply is a critical determinant of populationdynamics in many species. This feature derives fromthe fact that the west coast sits within a major up -welling region. During times of strong equatorwardwinds, the predominant currents flow equatorwardand surface waters move offshore. The potential forupwelling waters to move larvae offshore has beenwidely recognized (Yoshioka 1982, Roughgarden etal. 1988, Botsford et al. 1994), and is consistent withobservations that larval settlement and supply in persistent upwelling regions is higher during relax-ation events when wind speeds decrease or reversedirections (Farrell et al. 1991, Botsford 2001). In -creases in settlement and supply during relaxationevents can also result from poleward advection of lar-vae (Wing et al. 1995a,b). More recent work hasshown the persistence of sequential larval stages innearshore plankton during upwelling conditions,indicating that many taxa are not swept offshore inupwelling regions (Morgan et al. 2009b, Shanks &Shearman 2009). Larvae appear to be able to at least

partially avoid offshore transport associated withupwelling through physical and behavioral mecha-nisms. Nearshore retention zones arising from topo-graphic effects on coastal circulation have been ob -served (Graham & Largier 1997, Wing et al. 1998,Roughan et al. 2005), and are associated with higherlarval abundances and settlement (Mace & Morgan2006, Morgan et al. 2009a, 2011, 2012). Avoidance ofsurface waters by larvae can favor retention and de -crease offshore transport (Morgan et al. 2009b,c,Shanks & Shearman 2009, Morgan & Fisher 2010,Morgan et al. 2012) and there is mounting evidencethat larval concentrations are high close to shore,even in areas of strong upwelling that are tradition-ally viewed as being recruitment-limited. Here weinvestigate this phenomenon closer to shore to see ifhigh abundances extend over the inner shelf andinward to the shoreline.

Nearshore processes play an important role in lar-val ecology, and may be relevant for a significantportion of pelagic larval durations. A number of re -cent studies measured larval abundance in cross-shore transects and found increases in abundancecloser to shore in a range of invertebrates and fishes(Borges et al. 2007, Tapia & Pineda 2007, Morgan etal. 2009b,c, Shanks & Shearman 2009). For example,Morgan et al. (2009b) measured larval abundance ofbenthic crustaceans from 1 km from shore out to theshelf break (30 km offshore) along the open coast ofnorthern California and found that the highest larvalabundances were within 3 km from shore. The com-bination of larval behaviors (e.g. swimming and ver-tical migration) and nearshore processes may in -crease retention of larvae close to shore and to theirnatal site. Such retention is consistent with evidencefrom a range of species and systems that shows self-recruitment is higher and dispersal distances smallerthan previously thought (Swearer et al. 2002, Levin2006, Shanks 2009), and further emphasizes theimportance of understanding the role of nearshoreprocesses in larval supply and population dynamics.

A number of nearshore processes may reducescales of dispersal. Adjacent to the shore within thesurf zone, rip tides can create recirculation zones(MacMahan et al. 2010) and onshore wave transportcan lead to accumulation of water-borne material(Monismith 2004, McPhee-Shaw et al. 2011). There isalso a region termed the coastal boundary layer(CBL) that extends beyond the surf zone and is char-acterized by reduced speeds (Nickols et al. 2012). Inparticular, average alongshore velocities in the CBLare an order of magnitude larger than cross-shorevelocities (Lentz et al. 1999, Gaylord et al. 2007), and

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alongshore velocities increase strongly with distancefrom shore until reaching a free-stream value off-shore (Nickols et al. 2012). Such decreased velocitiesprovide another potential mechanism for reducingscales of dispersal in coastal populations (Nickols etal. 2012, Nickols et al. unpubl. data), but require thatlarvae spend sufficient time within the CBL forreduced flow to influence net transport. Because pre-vious studies of coastal larval distributions did notextend into the CBL or only just entered the CBL(McQuaid & Phillips 2000, Morgan et al. 2009b,c,Shanks & Shearman 2009), or sampled over an insuf-ficient temporal scale to fully characterize patterns(Tapia & Pineda 2007), the general role of the CBL ininfluencing patterns of larval transport and supplyremains unknown.

The goals of this study were therefore to addressthe following questions: (1) what is the spatial patternof larval abundance within the CBL, (2) are there differences in larval assemblages close to shore (i.e.are different larvae found inshore versus offshorewithin the CBL), and (3) do time-dependent or space- de pendent physical processes tend to dictate vari-ability in larval abundance? We recognize in target-ing these goals that it is not possible to definitivelyascribe particular mechanisms to observed patterns;rather, our aim is to present the first description of thespatio-temporal distribution of larval assemblageswithin the CBL, which is critical for understandingthe potential for nearshore transport processes toimpact larval supply to shoreline habitats. We addi-tionally hope that this study will also inform themethodological question of where supply should bemeasured to best address relationships between set-tlement and supply when exploring questions aboutmarine population dynamics, particularly for recruit-ment- limited regions.

MATERIALS AND METHODS

Study system and species

This study was conducted along the open coast innorthern California, USA, near Bodega Head, Cali-fornia (Fig. 1), a region characterized by strong sea-sonal upwelling during spring and summer thatdrives prevailing currents equatorward and pushessurface waters offshore (Winant et al. 1987, Largier etal. 1993). When the winds weaken or reverse direc-tion (inducing a relaxation event), currents movepoleward, often responding within a day or less invery nearshore regions (Send et al. 1987). Inner shelf

currents, observed previously on the scale of kilome-ters, are slower than currents farther offshore, have ahigher tendency to move poleward (Kaplan et al.2005), and are associated with increases in inverte-brate settlement (Wing et al. 1995a,b). Benthic crus -ta cean larvae can be present during both relaxationand upwelling conditions within areas of topographicretention and along the open coast, and many can beretained in areas within 1 to 3 km of the shore via acombination of physical and behavioral mechanisms(Morgan et al. 2009b, Morgan & Fisher 2010, Morganet al. 2012). The present study focused on waterswithin 1 km of the shore.

Our efforts focused on larvae of benthic crusta -ceans (primarily barnacles and crabs), which are thebest-studied meroplankton in this region. From priorwork, it is known that larval abundance of benthiccrustaceans peaks during the spring and summer,coinciding with the upwelling season. Barnacles moltthrough 6 larval stages (nauplii) and a postlarvalstage (cyprid) and spend about 2 to 4 wk in the water

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Fig. 1. Map of the study region showing the cross-shelf tran-sect located off Bodega Head in northern California, USA.Circles indicate locations of moorings where plankton towsand CTD casts began. The moorings on the 10, 15, and 22 misobaths included bottom mounted ADCPs and thermistors.The mooring on the 15 m isobath contained a thermistorstring measuring temperature at depths of 4, 7, 10, and 14 m.Wind velocity was measured via an anemometer located on-shore at the Bodega Marine Laboratory, indicated by the

square

Mar Ecol Prog Ser 494: 191203, 2013

column (Strathmann 1987). Barnacle larval releasegenerally begins in spring (Gaines et al. 1985, Strath-mann 1987) and is continuous through the summermonths (Shanks & Eckert 2005). Barnacle speciesfrom both subtidal (Balanus crenatus, B. nubilus) andintertidal (B. glandula, Chthamalus spp., and Polli -cipes polymerus) habitats are common in this region(Morgan & Fisher 2010). Crab larvae spend weeks tomonths in the plankton, and peak recruitment formost species in this region is during the spring andsummer (Shanks & Eckert 2005, Mace & Morgan2006). The most common taxa include members ofthe families Pinnotheridae, Porcellanidae, Cancri -dae, and Majidae.

Larval samples

Cross-shore distributions of nearshore larvae weresampled during 6 daytime cruises using a 0.5 m dia -meter, 200 m mesh net equipped with a mech ani -cal flow meter (Model 2030, General Oceanics). Thenet was modified with a sled to accommodate tow-ing along the bottom. Cruise dates spanned 3 moduring the upwelling season, from May throughJuly 2010, and occurred approximately every 10 d(Fig. 2) under a variety of oceanographic conditions.We sampled 4 stations in a cross-shore transectalong the 10, 15, 22, and 30 m isobaths, correspon-ding to ap proximately 250, 425, 850, and 1100 mfrom shore (Fig. 1). We refer to the 10 and 15 m iso-bath stations as inner CBL stations, and the 22 and30 m isobath stations as outer CBL stations. Weconducted a single 10 min oblique tow at each station, which sampled from the bottom to the sur-face of the water column. Larvae were sorted andidentified to species, or the lowest taxonomic grouppossible, and developmental stage. Larval abun-dances were calculated per m3 to standardize acrossstations.

Physical data

To provide physical context during our study, wemeasured currents, temperature, salinity, and winds.Current speed and direction were measured through-out the water column using moored acoustic Dopplercurrent profilers (Workhorse Sentinel ADCP, 1200kHz; Teledyne RD Instruments). Instruments werelocated on the 10, 15, and 22 m isobaths near thestarting position for plankton tows. The ADCPs col-lected 1 min bursts of 0.75 Hz velocity data every 2

min in 1 m vertical bins that typically extended from~1.5 m above the bottom to ~1.5 m below the surface.The velocity record at the 10 m station ended earlyon 10 June 2012 when its anchor was dislodged. Toquantify general velocity patterns, the raw velocitytime series were depth-averaged, ro tated onto theirprincipal axes, and low-pass filtered with a 33 h cut-off to remove dominant tidal motions (Rosenfeld1983). The major principal axes aligned parallel toshore and along-isobath, corresponding to an angleof 300.

Bottom temperature was recorded at the ADCPmooring sites every minute over the duration of thestudy at the 10, 15, and 22 m stations, and tempera-tures at depths of 4, 7, 10, and 14 m were recorded atthe 15 m station (SBE 37 and SBE 39, Sea-Bird Elec-tronics). During cruises, temperature, salinity, anddensity were profiled at each station throughout thewater column using a conductivity, temperature, anddepth profiler (SBE 19-Plus, Sea-Bird Electronics),with the exception of the cruise on 25 June.

Wind data during this study were available from ananemometer located on the shore at the Bodega Mar-ine Laboratory, within 1 km of the study locations, ata height of 20 m (38 19 3.35 N, 123 4 17.20 W;RM Young 05103 Wind Monitor; data available on -line http://bml.ucdavis.edu/boon/).

Data analysis

We performed multivariate analyses to determineif larval abundance and larval assemblages variedwith distance from shore and with time. We exam-ined patterns of cross-shore abundance for all taxa,for crab and barnacle larvae separately, and accord-ing to larval stage. All statistical analyses were con-ducted using the multivariate statistical softwarepackage PRIMER v. 6.1.10 (Clarke & Gorley 2006).We determined whether larval assemblages changedwith distance from shore and with sampling dateusing nonparametric analysis of similarity (ANOSIM)and hierarchical cluster analysis and ordination. Datawere fourth-root transformed to reduce the hetero-geneity of variance among samples and assembledinto a Bray- Curtis dissimilarity matrix with a dummyvariable of 1. The resultant dendrogram was testedfor group differences using a similarity profile test(SIMPROF), and the percentage contribution (SIM-PER) of each species and stage to the significant clus-ters was as sessed to classify species-stage combina-tions by their cross-shore distributions and samplingdate. We used non-metric multidimensional scaling

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(NMDS) to exa mine separation of assemblagesaccording to sample date and distance from shore. Toassess if community composition was structured inspace or time, we repeated each of these analyses onuntransformed data that were standardized by totalsample abundance. For all analyses, when there wassignificant structure among samples we then deter-mined which species and stages contributed to thepatterns.

RESULTS

Physical conditions

During the study period, we cap-tured a variety of oceanographic con-ditions, including upwelling, relax-ation, and post-relaxation (Fig. 2).However, our larval sampling datesgenerally occurred during low-windconditions due to logistical constraintson field operations. Daily- averagedalongshore wind speeds during eachsampling date ranged from 8 m s1,indicative of northwesterly winds, to8 m s1, indicative of southeasterlywinds, but winds were predominantlyupwelling favor able over the studyperiod (Fig. 2A). While depth-aver-aged alongshore currents are knownto alternate be tween equatorwardand poleward in this region (Largieret al. 1993, Roughan et al. 2005, Ka-plan & Largier 2006, Morgan et al.2012), on all sampling days of ourstudy the alongshore current waspoleward (Fig. 2B). In general, depth-averaged alongshore velocities meas-ured at inner CBL locations (10 and15 m isobaths) were slower than ve-locities measured at the outer CBL in-strument on the 22 m isobath(Fig. 2B), characteristic of a coastalboundary layer. Exceptions occurredduring onset of strong up wellingwinds on 20 May and 6 June and dur-ing flow reversals.

Water column properties duringlarval sampling dates ranged fromwell-mixed to stratified (Figs. 2 & 3).The 17 May, 9 June, and 25 Junesampling events represented up well - ing conditions, with a cold homo -geneous water column across sta-

tions: temperatures below 10C and salinitiessimilarly uniform, with the exception of some low-salinity water on the surface near the outer station on17 May (Figs. 2, 3A & 3C). Alongshore wind speedson the day preceding these sampling dates werefrom the northwest and reached up to 10 m s1, characteristic of strong upwelling conditions andaccounting for the presence of cold isothermal condi-tions over the inner shelf. On 27 May and 4 July, the

Fig. 2. (A) Alongshore wind velocity measured at Bodega Marine Laboratory.(B) Alongshore depth-averaged and 33 h low-pass filtered current velocitymeasured by bottom-moored ADCPs at the 10, 15, and 22 m isobaths. Positivealong shore velocity is poleward and negative alongshore velocity is equator -ward. (C) Bottom temperature measured on the ADCP moorings at the 10, 15,and 22 m isobaths. (D) Temperature at depths of 4, 7, 10, and 14 m measured at

the 15 m isobath. Vertical gray bars indicate when sampling occurred

Mar Ecol Prog Ser 494: 191203, 2013

water column was stratified at the 2 inner CBL sta-tions (Figs. 2, 3B & 3D), following a day of southeast-erly or weak winds. After 4 July temperatures in -creased substantially, and the water column wasstratified at all stations on the 14 July sampling day(Fig. 2C). Winds during this period were substantiallyweaker than previous sampling events (Figs. 2C, 2D& 3E).

Larval abundance

We identified larvae of 22 crustacean species dur-ing our study (Table 1). The outer CBL station, alongthe 30 m isobath, had the highest number of species,with 10 species on average as compared to 6 to 7 spe-

cies at the other stations. Throughout the study, lar-vae were most abundant along the 22 m isobath,850 m from shore (Fig. 4A). This pattern was drivenby high abundances of barnacle larvae (Fig. 4B).Early, middle and late stage barnacle larvae werepresent at all stations with the highest abundancealong the 22 m isobath (Fig. 5A). Barnacle postlarvae(cyprids) were found in similar abundance across sta-tions. Crab larvae were most abundant near the 30 misobath, 1100 m from shore (Fig. 4C). All larval crabstages were most abundant at the 2 outer CBL sta-tions, although early stage crab larvae dominated thesamples and abundance decreased with increasingstage (Fig. 5B). Very few crab larvae were found atthe inner CBL stations, and the majority of thosewere early stage larvae.

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Fig. 3. Contours of temperature (left) and salinity (right) at the beginning of larval tows along the 10, 15, 22, and 30 m isobathson (A) 17 May 2010, (B) 27 May 2010, (C) 9 June 2010, (D) 4 July 2010, and (E) 14 July 2010. Sample locations are indicated by

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Nickols et al.: Larvae in the coastal boundary layer

Larval assemblages

Considering all taxa, larval assemblages within theCBL differed among stations (2-way ANOSIM av =0.322, p = 0.017) and were similar among dates. Be -cause overall larval assemblages were not structuredby date we performed a 1-way ANOSIM to detectspatial differences among assemblages at the differ-ent stations. The innermost station drove differencesbetween assemblages, and it differed from the 2outer stations (1-way ANOSIM pairwise test: for 10 mvs. 30 m stations: R = 0.463, p < 0.01; for 10 m vs. 22 mstations: R = 0.609, p < 0.01). These differences were

echoed in the dendrogram from cluster analysis, andthe NMDS ordination, which both revealed spatialstructure with 2 main clusters: one defined by lownumbers of larvae, with samples primarily from innerCBL stations, and the second defined by high num-bers of larvae, with samples primarily from the outerCBL stations (Fig. 6).

Partitioning the analysis by taxa, we found thatpatterns of barnacles and crabs differed. Crab larvalassemblages by themselves did not differ by stationor date, but barnacle larval assemblages did differ bystation and sampling date (2-way ANOSIM: stationav = 0.322, p = 0.02; date av = 0.35, p < 0.01). Thedendrogram from cluster analysis and the NMDSordination revealed 2 main clusters of barnacle sam-ples. One cluster occurred early in the season andmostly offshore, and it was composed of samplesfrom all stations during 17 May and samples from theouter CBL stations during June and July. The othercluster occurred late in the season (June throughJuly) and consisted primarily of samples from theinner CBL stations (Fig. 7). In the early-season, outerCBL assemblage, barnacle larval abundances wereas high as 7200 larvae m3, whereas in the late-season, inner CBL assemblage, barnacle concentra-tions were less than 500 larvae m3.

We also analyzed changes in the composition of thelarval assemblage. We divided our larval counts (bothspecies and stage) by the total number of larvae ineach sample, providing a fraction of the total samplefor each species and stage, to standardize the data fordifferences in larval abundance across time. Larvalcommunity composition within the CBL differedmostly among stations, but also by date (2-wayANOSIM station av = 0.394, p < 0.01; date av = 0.282,p = 0.026). As noted previously, date was not signifi-cant when abundance was considered in the analysisof larval assemblages. In this analysis of composition,

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Fig. 4. Average larval abundance and standard error across date by station for (A) all benthic crustaceans, (B) barnacles, and (C) crabs. Note that the y-axis of (C) is an order of magnitude smaller than (A) and (B)

Family Taxon

Cirripedia Balanus crenatus Balanus glandula Balanus nubilus Chthamalus dalli Lepas spp. Pollicipes polymerus

Cancridae Cancer antennarius Cancer magister Cancer productus Carcinus maenas

Grapsidae Hemigrapsus oregonensis

Hippidae Emerita analoga

Majidae Mimulus foliatus Pugettia producta Pugettia richii Scyra acutifrons

Paguroidae Pagurid spp.

Pinnotheridae Pinnotheridae

Porcellanidae Pachycheles spp. Petrolisthes cinctipes

Thalassinidae Neotrypaea californiensis

Xanthidae Lophopanopeus bellus

Table 1. Crustacean larvae identified in the study

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Fig. 5. Average larval abundance and standard error across date by station for (A) barnacles according to early, mid, late, and postlarval (PL) stage and (B) crabs according to early, late, and PL stage

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Fig. 6. (A) Hierarchical clustering dendrogram (using group-average linking) of larval assemblages of benthic crus-taceans from 24 samples taken over 6 d (indicated by sym-bols and legend entry) at 4 sampling stations across thecoastal boundary layer (along the 10, 15, 22, and 30 m iso-baths, indicated by numbers adjacent to symbols), usingtransformed data. Solid black lines indicate significantgroup structure at the 5% level. Dashed lines represent non-significant group structure. Sample station isobaths are re-ported beneath each symbol. (B) Non-metric multidimen-sional scaling plot (2D stress, 0.15; 3D stress, 0.09) from the24 samples with superimposed significant clusters at simi -larity levels of 40% (solid lines) and 55% (dashed lines).

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Fig. 7. (A) Hierarchical clustering dendrogram (using group-average linking) of barnacle larval assemblages from 24samples taken over 6 d (indicated by symbols and legendentry) at 4 sampling stations across the coastal boundarylayer (along the 10, 15, 22, and 30 m isobaths, indicated bynumbers adjacent to symbols). Solid black lines indicate sig-nificant group structure at the 5% level. Dashed lines repre-sent non-significant group structure. Sample station iso-baths are reported beneath each symbol. (B) Non-metricmultidimensional scaling plot (2D stress, 0.12; 3D stress,0.07) from the 24 samples with superimposed significantclusters at simi larity levels of 55% (solid lines) and 75%

(dashed lines). Symbols and numbers as in (A)

Nickols et al.: Larvae in the coastal boundary layer

the effect of date was driven by one particular sam-pling event on 9 June, when there was a distinct innerCBL assemblage (10 and 15 m stations; 92% similar-ity). The dendrogram from cluster analysis and theNMDS ordination showed 2 additional as semblages(Fig. 8): an inner CBL assemblage (68% similarity)and a main CBL assemblage containing nearly all ofthe other samples from the CBL, the 2 outer CBL sta-tions as well as the 15 m station (67% similarity).

The barnacles Balanus crenatus and B. nubilus lar-vae dominated the composition of the assemblageand drove differences between clusters. The assem-blage at the inner CBL stations on 9 June (Fig. 8) wascomposed of >75% B. crenatus cyprids. All other

samples were composed of 30% or less B. crenatuscyprids. In addition, the 9 June inner CBL assem-blage contained

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da (2007) measured larval concentrations of Balanusglandula and Chtha ma lus spp. at 3 cross-shore sta-tions within 1100 m from shore over a period of 7 d.While concentrations of most larval stages of B. glan-dula were similar among stations, concentrations ofthird through sixth stage Chthamalus nauplii werelower at the innermost station, 300 m from shore(Tapia & Pineda 2007). Even over a short temporalperiod, barnacle concentrations exhibited spatialstructure and nauplii may potentially have avoidedvery nearshore waters.

Despite low concentrations of larvae in the inner-most waters of the CBL, the high concentrations of alllarval stages of barnacles in the CBL along the 15, 22,and 30 m isobaths suggest that many barnacle larvaemay be retained within the CBL and develop in waters within 1100 m from shore. All larval stagesof crabs occurred at the 2 outer CBL stations, sug-gesting that they may also complete developmentwithin the CBL. These findings are consistent withother studies in both weak and strong upwelling re-gions where high abundances of all larval stages oc-curred within a few kilometers from shore (Tapia &Pineda 2007, Morgan et al. 2009b,c, Shanks & Shear-man 2009, Morgan & Fisher 2010, Morgan et al. 2011,2012). Benthic crustacean larvae exhibit depth pref-erences that can aid nearshore retention for mostspecies in upwelling regions (Miller & Morgan 2013).By remaining near the bottom, larvae can take ad-vantage of slower velocities in the bottom boundarylayer (in both the along- and cross-shore directions),as well as avoid offshore transport in the surface Ek-man layer (Morgan et al. 2009b, Shanks & Shearman2009, Morgan & Fisher 2010, Morgan et al. 2012).

Inshore and offshore assemblages within the CBL

A distinct larval assemblage occurred closest toshore within the CBL. Although some features of lar-val assemblages changed with time, these assem-blages were predominantly defined by space: notonly was there spatial structure in larval abundance,there was spatial structure in the composition of theas semblage. This spatial structure occurred eventhough physical conditions were variable amongsampling dates (flow velocity, water temperature,stratification) and on many days there was no cleardifference in physical parameters between the inner-most station and those within the rest of the CBL(Figs. 2 & 3).

The spatial boundary between assemblages ofinner and outer stations within the CBL is dynamic.

Larval assemblages on half of the sampling dates atthe 15 m station were most similar to the 10 m station,and on the other half were more similar to the 22 mstation. There is no clear physical difference betweenthese groupings of days apparent from our data, asthey spanned oceanographic conditions. For exam-ple, on 9 June, 4 July, and 14 July the 15 m stationmatched most closely with the 10 m station, yet thewater column profiles from each of these days arequite distinct (Fig. 3CE). One possible physical fac-tor we did not explore that could influence thedemarcation of the inshore community is the width ofthe surf zone and associated rip current zone, whichis itself a dynamic boundary, dependent on the sig-nificant wave height and tidal elevation (Lentz et al.1999, Brown et al. 2009). Surf zone characteristicsappear to impact shoreline settlement of inverte-brates, with low settlement observed at reflectivebeaches which are characterized by high beachslopes and standing waves and are thought to havereduced cross-shore exchange (Shanks et al. 2010).Rocky shores are hypothesized to be similar to reflec-tive beaches, and if so, the associated reduction incross-shore ex change might explain low settlementat some locations and low abundances of larvae insurf zone waters (Shanks et al. 2010). Our studyfound low larval concentrations in waters just beyondthe surf zone, but at distances that could be influ-enced by surf zone processes through the action ofrip currents. Specifically, off Horseshoe Cove (Fig. 1,just downcoast of station locations), wave-driven cir-culation has been observed to extend as a macro-ripup to distances ~250 m offshore (J. L. Largier,unpubl. drifter data), comparable with the distance toour inner CBL station along the 10 m isobath. Thissuggests that the influence of wave-driven processeson larval transport may extend offshore (contrary tothe idea of reduced exchange off rocky shores, assuggested by Shanks et al. 2010).

In addition to potential physical differences be -tween the habitat of the inshore and offshore as -semblages, predation may be higher within the nar-row band of inner CBL water than farther offshore.Habitat along the 10 m isobath at our study site fea-tures rocky substrate with some areas supportingstands of the bull kelp, Nereocystis luet keana. Incentral California, larval abundances were found tobe negatively correlated with kelp density, and lowerlarval abundances on the inshore edges of kelpforests were attributed to predation (Gaines &Rough garden 1987). Although the kelp in our regionis much more sparse than the giant kelp Macrocystispyrifera beds in central California, predation is still a

Nickols et al.: Larvae in the coastal boundary layer

possible explanation for decreased abundance at themost inshore station of our study.

Implications of cross-shelf larval structurewithin the CBL

Larvae are clearly spending time within the coastalboundary layer; some may even complete their entiredevelopment within the CBL, which could impactestimates of population connectivity. During theirtime in the CBL, larvae are exposed to slower movingalongshore flows than farther offshore, which willhave an impact on overall dispersal distance (Nickolset al. 2012, Nickols et al. unpubl. data). Although wedid not have current velocity measurements through-out the water column beyond the 22 m isobath, con-current measurements of surface currents by high- frequency radar showed that current velocities werefaster farther offshore (data not shown). The radardomain begins 2 km offshore, and generally has highagreement with measurements from ADCPs (Kaplanet al. 2005). This gradient is also observed in a cross-shore array of moorings deployed during WEST(Wind Events and Shelf Transport; Largier et al.2006) and in other unpublished data from BML. Esti-mates of dispersal distance in this region shouldtherefore consider current velocities within 1 km orless from shore, as this is where the majority of larvaeappear to be concentrated. Such consideration mayimprove estimates of dispersal distance derived frompelagic larval durations, which are often larger thandispersal distances estimated from genetics, tagging,and natural tracers (Palumbi 2004, Jones et al. 2009,Shanks 2009, Lpez-Duarte et al. 2012). Refining ourunderstanding of dispersal distances will improveour ability to accurately model population dynamicsand assess population persistence (Botsford et al.2009, White et al. 2010, Burgess et al. in press).

The coast of northern California generally haslower recruitment than other regions along the westcoast of North America (Connolly et al. 2001), and alongstanding question has been whether or not thispattern is linked to larval supply. Although it wasproposed that larval supply is diminished when lar-vae are forced offshore by strong upwelling (e.g.Roughgarden et al. 1988), numerous studies nowsuggest strongly that many larvae of multiple speciesare retained nearshore during both upwelling andrelaxation conditions (Tapia & Pineda 2007, Morganet al. 2009b,c, Shanks & Shearman 2009, Morgan &Fisher 2010, Morgan et al. 2011). Our study alsofound high abundance of larvae close to the shore,

and extended closer to shore than previous work inthis region of strong upwelling.

An important finding of our study is the observa-tion of low larval concentrations and a different lar-val assemblage in the innermost waters of the CBL,indicating a potential disconnect between high larvalabundance in the CBL and larval supply to shorelinerecruitment habitat. Further, this disconnect appearsto occur in waters beyond the surf zone, in contrast torecent work by Shanks et al. (2010) that suggests thatsurf zone processes may disrupt the supply of near -shore planktonic larvae to shoreline habitats. Whilethese results are from a single location, they repre-sent a diversity of oceanographic conditions and theobserved mismatch raises important questions abouthow general this result may be. Our study also fo -cuses attention on the need to understand the mech-anisms that control transport of larvae to shorelinehabitats, while highlighting methodological concernsof studies that explore links between supply and set-tlement. As we endeavor to better understand thelinks between larval dispersal and population dy -namics, it is essential that the nearshore zone bestudied in greater detail and that we work to addressthe spatial pattern of recruitment limitation in coastalsystems.

Acknowledgements. We thank J. Demmer, R. Fontana, andN. Weidberg for assistance in the field as well as D. Dann,M. Robart, and J. Herum for help with deployments ofoceanographic instrumentation. J. Fisher provided guidanceand feedback. This work was funded by California SeaGrant (NA08AR4170669) and the National Science Founda-tion (OCE-0927196 and OCE-0927255). K.J.N. was also sup-ported by a Bodega Marine Laboratory Graduate StudentFellowship.

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Editorial responsibility: Lisandro Benedetti-Cecchi, Pisa, Italy

Submitted: March 28, 2013; Accepted: September 20, 2013Proofs received from author(s): November 14, 2013

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