CONFLICTING MANAGEMENT GOALS: MANATEES AND …

571

Ecological Applications, 14(2), 2004, pp. 571–586q 2004 by the Ecological Society of America

CONFLICTING MANAGEMENT GOALS: MANATEES AND INVASIVECOMPETITORS INHIBIT RESTORATION OF A NATIVE MACROPHYTE

JENNIFER HAUXWELL,1,3 CRAIG W. OSENBERG,1 AND THOMAS K. FRAZER2

1University of Florida, Department of Zoology, P.O. Box 118525, Gainesville, Florida 32611-8525 USA2University of Florida, Department of Fisheries and Aquatic Sciences, 7922 NW 71st Street,

Gainesville, Florida 32653-3071 USA

Abstract. Vallisneria americana is a native macrophyte in freshwater and oligohalineecosystems, often forming meadows that significantly affect ecosystem carbon and nutrientcycling and provide structural habitat. Vallisneria has declined in abundance in severalFlorida lakes, rivers, and estuaries. We conducted an 11-month field experiment in KingsBay, Florida, USA, to determine factors that might affect successful transplantation ofVallisneria and to guide potential future restoration efforts by water resource managementagencies. To determine the effects of herbivores and other primary producers on Vallisneriaproduction, we conducted a 2 3 2 factorial experiment at three sites in which we allowedor denied: (1) relatively large herbivores (.3 cm) and (2) other primary producers (e.g.,Myriophyllum spicatum, Hydrilla verticillata, Lyngbya sp.) access to 1.5 3 1.5-m trans-planted plots of Vallisneria. Within one month, Vallisneria disappeared from 80% of her-bivore-access plots due primarily to consumption by manatees. Vallisneria density wasreduced a variable amount (0–50%) in response to competitors, due to site-specific variationin natural abundance of other primary producers (at two sites, we observed extensivecolonization by other primary producers and a strong treatment effect). Most of this com-petitive effect was attributable to Eurasian watermilfoil (Myriophyllum spicatum); afterpooling Vallisneria density data across sites, we observed a negative exponential relation-ship between shoot density of Vallisneria and stem density of Myriophyllum for everysampling date. We observed variable recruitment of Vallisneria transplants into larger sizeclasses among sites, but not between treatments, and large, established plants grew at similarrates whether in monospecific or mixed stands, with evidence for reduced mass-specificgrowth in dense stands. Hence, a probable mechanism by which Myriophyllum reducesVallisneria occurs via space limitation, and reduction in Vallisneria densities and/or re-cruitment and growth of new shoots. We conclude that two different management goals(restoration of Vallisneria and protection of manatees) are in conflict and must be simul-taneously considered to devise effective and sustainable ecosystem management scenarios.Due to the highly managed nature of freshwater systems worldwide, the limitations ofextrapolating experimental results and the importance of site-specific pilot experimentsshould also be recognized.

Key words: aquatic restoration; herbivory; Hydrilla verticillata; Lyngbya; manatee; milfoil;Myriophyllum spicatum; nutrient; seagrass; transplant; Vallisneria americana; wild celery.

INTRODUCTION

Over the past several decades, loss of native sub-mersed aquatic vegetation (SAV) has been a reoccur-ring phenomenon in shallow freshwater, estuarine, andmarine systems worldwide (Bayley et al. 1978, Rybickiand Carter 1986, Schloesser and Manny 1990, Kimberet al. 1995, Short and Wyllie-Echeverria 1996, Rogerset al. 1997, Valiela et al. 1997, Duffy and Baltz 1998,Hauxwell et al. 2001), and can often be attributed to arelatively limited number of factors. Repeatedly, hu-man-induced alterations to chemical, physical, and/or

Manuscript received 31 July 2002; revised 12 May 2003; ac-cepted 27 June 2003. Corresponding Editor: L. Deegan.

3 Present address: Wisconsin Department of Natural Re-sources, Department of Natural Resources Research Center,1350 Femrite Drive, Monona, Wisconsin 53761 USA.E-mail: [email protected]

biological features of landscapes or adjoining watershave caused declines in SAV. Anthropogenic nutrientloading from watersheds to aquatic systems is one ofthe most pervasive human effects on aquatic systems(GESAMP [Joint Group of Experts on the ScientificAspects of Marine Pollution] 1990, National ResearchCouncil 1994, U.S. Geological Survey 1999, Chamberset al. 2001) and has led to extensive loss of light-limitedSAV via competitive exclusion by fast-growing, nu-trient-limited algae (Kemp et al. 1983, Short et al. 1993,Carter et al. 1996, Short and Wyllie-Echeverria 1996,Valiela et al. 1997, Hauxwell et al. 2001). Sedimenttransport as a result of development and agriculturalactivities in adjoining land parcels may also result indeclines in SAV (Kemp et al. 1983). Particularly ininland systems, the introduction of an exotic plant(Lind and Cottam 1969, Van et al. 1976, 1999, Titusand Adams 1979, McFarland and Rogers 1998) or her-

572 JENNIFER HAUXWELL ET AL. Ecological ApplicationsVol. 14, No. 2

bivore (Hestand and Carter 1978, Roberts et al. 1995,Wilson 2002) also may significantly reduce areal cov-erage by native macrophytes.

Because identification of the various factors that maycause declines in SAV has occurred relatively recently(in the past 10–30 years), the daunting task of resto-ration is a relatively new challenge for ecologists andwater resource managers. To achieve long-term suc-cess, restoration of SAV habitat must involve at leastthree phases: (1) understanding the mechanisms bywhich initial loss of SAV habitat occurred, (2) restoringthe original ecological setting, so that physical, chem-ical, and biological conditions are again hospitable tothe target species, and (3) identifying a successfulplanting protocol that results in the persistence anddispersal of the population. Considerable effort hasbeen expended in estuarine and marine environments,where, for example, (1) identification of the relation-ship between nutrient loading from watersheds and de-cline in areal coverage of seagrasses has been estab-lished (Valiela and Cole 2002, Hauxwell et al. 2003a),(2) reductions in nutrient loads have been achieved sothat seagrasses have returned to systems in which theyhad been lost (e.g., Tampa Bay [Johansson and Green-ing 2000]), (3) planting protocols for several specieshave been established (Zostera marina [Davis andShort 1997, Orth et al. 1999], Thalassia testudinum[Tomasko et al. 1991], and Halodule wrightii [Sheridanet al. 1998]), and (4) experimental work on transplantsuccess has been conducted around the world (Ches-apeake Bay region, USA [Orth et al. 1999], Florida,USA [Tomasko et al. 1991], North Carolina, USA[Kenworthy and Fonseca 1992], California, USA [Zim-merman et al. 1995], New Hampshire, USA [Davis etal. 1998], Texas, USA [Sheridan et al. 1998], BritishColumbia, Canada [Harrison 1990], and Australia[West et al. 1990]).

In contrast to the availability of numerous publishedreports focused on seagrass restoration, there is a pau-city of published information on restoration of nativemacrophytes in primarily freshwater environments(Smart et al. 1998). In this study, we were interestedin identifying the factors that were likely to affect res-toration of Vallisneria americana Michx. in a spring-fed system in Florida. Vallisneria americana is a nativemacrophyte in freshwater and oligohaline ecosystems,ranging from Central America to Canada (Korschgenand Green 1988), and like many seagrass species, oftenforms meadows that significantly affect ecosystem car-bon and nutrient cycling, bind sediments, fuel signif-icant secondary productivity, and provide structuralhabitat to a myriad of aquatic organisms. Vallisneriaalso has declined in abundance in many freshwater andestuarine systems (Bayley et al. 1978, Rybicki andCarter 1986, Schloesser and Manny 1990, Kimber etal. 1995, Rogers et al. 1997, Duffy and Baltz 1998).Restoration efforts aimed at revegetating shallow fresh-water systems with Vallisneria have been initiated in

several systems in Florida, but at considerable cost andwith variable success (B. Hujik [Florida Fish and Wild-life Conservation Commission], personal communi-cation; see also Jaggers 1994).

Despite the relative similarity of causes for declinein SAV, the factors that affect its re-establishment maybe much more diverse and may include ‘‘natural’’ aswell as anthropogenic causes. As with other native spe-cies of submersed aquatic vegetation, several factorsmay affect successful restoration of Vallisneria. In-creases in anthropogenic nutrient supply or sedimentloads to inland waters, accompanied by degraded waterclarity, has resulted in loss of light-limited rooted mac-rophytes in freshwater environments (Chambers andKalff 1985, Barko et al. 1986) and may limit potentialfor restoration. Infestations of nonnative macrophytes,including Eurasian watermilfoil (Myriophyllum spi-catum L.) and hydrilla (Hydrilla verticillata (L.f.) Roy-le) may also competitively exclude native freshwatermacrophytes, including Vallisneria (Lind and Cottam1969, Van et al. 1976, 1999, Titus and Adams 1979,McFarland and Rogers 1998), and inhibit restorationefforts. Nonnative herbivores (e.g., grass carp (Cten-opharyngodon idella Val.) or the rusty crayfish (Or-conectes rusticus) may severely affect native SAV(Hestand and Carter 1978, Roberts et al. 1995, Wilson2002), and along with native herbivores (e.g., turtles,manatees, muskrats, waterfowl; Carter and Rybicki1985), may also inhibit restoration efforts. Given therange of potential anthropogenic and natural processesthat may affect restoration, small-scale pilot experi-ments are a potentially powerful way to understand theimportance of different factors in controlling macro-phyte dynamics and, thus, the feasibility of alternativemanagement strategies, before large-scale transplan-tation is undertaken.

Study site

We conducted an experiment in Kings Bay, Florida,USA, designed to assess the feasibility of large-scalerestoration of Vallisneria americana and to identifyfactors that might affect successful restoration. KingsBay (Citrus County, Florida; Fig. 1) is a spring-fed,tidally influenced oligohaline/freshwater system (seeFrazer et al. 2001b, Hoyer et al. 2001) which connectsto the Gulf of Mexico via the 11-km long Crystal River(Frazer et al. 2001a). Water-based recreational use ofKings Bay is extensive (Southwest Florida Water Man-agement District [SWFWMD] 2001), with ;70 000 an-nual visitors, and includes boating, fishing, scuba div-ing, and snorkeling. Many of these visitors travel toKings Bay, specifically, to observe the hundreds ofmanatees (Trichechus manatus Linnaeus) that seek therelatively warm, 258C, spring waters as refuge duringwinter.

Long-time visitors and residents of Kings Bay havenoted both degrading water clarity and alterations inpopulations of submersed aquatic vegetation over the

April 2004 573FACTORS AFFECTING VALLISNERIA RESTORATION

FIG. 1. Map of the Kings Bay/Crystal River estuarine/spring system. Site locations of transplanted plots of Vallisneriaamericana are indicated with X’s. The inset shows the study location in Citrus County, Florida, USA.

past several decades, including an overall decline inareal coverage of Vallisneria and a concomitant in-crease in coverage by both exotic macrophytes andnutrient-limited algal taxa (Romie 1990). Althoughthere are anecdotal accounts that Vallisneria once dom-inated the benthic vegetation of Kings Bay, this nativemacrophyte is now restricted to limited patch meadows(Frazer and Hale 2001). Exotic macrophytes, includingHydrilla verticillata (introduced to Kings Bay in the1960s; Langeland 1990) and Eurasian watermilfoil(Myriophyllum spicatum, also introduced in the 1960s;Blackburn and Weldon 1967) are now prominent com-ponents of the community of SAV in Kings Bay (Frazerand Hale 2001, Hoyer et al. 2001), and have beenshown to displace Vallisneria in other systems (Lindand Cottam 1969, Van et al. 1976, 1999, Titus andAdams 1979, McFarland and Rogers 1998). Lyngbyasp., a native, filamentous cyanobacteria, has alsobloomed in areas of Kings Bay (mid-1980s; Romie1990), and altered habitat, impaired recreational activ-ities, and caused odor problems. Lyngbya forms densemats on the bottom of lakes and rivers, and as trappedgases within canopies of Lyngbya increase throughoutthe day, these mats often float to the surface. These

floating mats clog waterways, shade other vegetation,and transport Lyngbya, and other attached incidentalmacrophytes, to other areas of the bay. Dense canopiesof filamentous macroalgae, similar in form and functionto Lyngbya, have also been shown to displace aquaticplants (Hauxwell et al. 2001).

In 1992, in response to complaints of degraded waterclarity and shifts in macrophyte community structure,the City of Crystal River (Florida) diverted treatedwastewater effluent that had been directly dischargedinto Kings Bay by employing an upland spray disposalmethod. Nutrient concentrations were subsequently re-duced in a large portion of the Bay (Terrell and Canfield1996). Presumably, it is possible that the growth ofnutrient-limited competitors of Vallisneria, includingLyngbya and phytoplankton, may have also been re-duced (see, however, Terrell and Canfield 1996), mak-ing Kings Bay a potential site for future large-scalerestoration. This is possible, of course, only if: (1) ini-tial decline of Vallisneria had been a result of degradedwater clarity and/or increased competition from nutri-ent-limited taxa, (2) improvement in water quality wassufficient to ameliorate these historical effects, and (3)


TABLE 1. Summary of environmental data in Kings Bay, Florida.

Site

Total

Phosphorus(mg/L)

Nitrogen(mg/L)

Chlorophyll(mg/L)

Temperature(8C)

Salinity(‰)

Sediment characteristics

Organicmatter (%)

Silt/clay(%)

Cedar CoveParker IslandBuzzard IslandBaywide mean

28 6 428 6 528 6 328 6 4

244 6 17236 6 44254 6 51239 6 16

7 6 39 6 69 6 59 6 6

23 6 225 6 425 6 324 6 3

0.3 6 0.11.4 6 0.31.6 6 0.31.5 6 0.2

3.70.81.6

2.95.02.9

Notes: Values given are annual means 6 1 SD and are for water quality data for sites sampled monthly in 2000 in thevicinity of Cedar Cove, Parker Island, and Buzzard Island (Florida LAKEWATCH, University of Florida, Gainesville, Florida,USA; methods described in Hoyer et al. [1997]). Sediment characteristics are taken from Belanger et al. (1993).

other factors do not prevent Vallisneria from respond-ing to improved water quality.

This study was designed to facilitate the successfulmanagement and restoration of Kings Bay and othersimilarly affected spring-fed systems. Our generalgoals were threefold: (1) to ascertain whether KingsBay may be a suitable site for future successful res-toration of Vallisneria, (2) to determine whether trans-plant success might vary among sites within Kings Bay,and (3) to determine what factors might affect suc-cessful restoration. In this paper, we specifically de-scribe results of a field experiment designed to assessthe effects of: (1) large herbivores (especially mana-tees), and (2) other native or exotic primary producers,on a suite of demographic features of transplanted Val-lisneria.

MATERIALS AND METHODS

Experimental design and transplanting protocol

To determine the effect of both herbivores and otherprimary producers (as well as interactive effects) onsuccess of transplants of Vallisneria, we conducted a2 3 2 factorial field experiment in which we allowedor denied: (1) relatively large herbivores (.3 cm; e.g.,manatees, turtles, waterfowl, certain fishes) and (2) oth-er primary producers (i.e., Myriophyllum, Hydrilla,Lyngbya) access to transplanted plots of Vallisneria.Comparisons between plots with allowed or denied ac-cess to herbivores and/or fast-growing opportunisticproducers allowed us to assess their effects on Vallis-neria density and other demographic parameters, andwhether active exclusion of herbivores and other pri-mary producers might be necessary for the successfullong-term restoration of Vallisneria.

In January 2001, a team of scuba divers transplantedindividual Vallisneria rosettes (purchased from Aquat-ic Plants of Florida, Sarasota, Florida, USA) within 1.53 1.5-m plots (naturally void of dense vegetation) atthree sites. Within a given site, plots were situated ;2m apart in depths of ;0.8–1.2 m at mean low water.Shoot planting density was ;200 m22, and 15–25-cmtall plants were used. Within a given site, there werethree replicates of each of the following treatments. Intreatment 1, herbivores were allowed and other primaryproducers were allowed. In treatment 2, herbivores

were allowed and other primary producers were ex-cluded. In treatment 3, herbivores were excluded andother primary producers were allowed. In treatment 4,herbivores were excluded and other primary producerswere excluded

To exclude herbivores (treatments 3 and 4), weplaced plastic fencing (2.5-cm mesh) around the sidesof plots (leaving the top open). This fencing was tallenough (;2 m) to extend above the water surface athigh tide (the typical tidal range in Kings Bay is ;1m). The mesh fence excluded herbivores .3 cm, whileminimally affecting water circulation and light (seeHauxwell et al. 2001). Plots in treatments 1 and 2 wereleft open to herbivores. To exclude other primary pro-ducers (treatments 2 and 4), plots were routinely mon-itored, and producers other than Vallisneria were hand-removed by snorkelers every two weeks. We were ableto effectively remove macrophytes and loose clumpsof filamentous algae, but did not attempt to removeattached epiphytes. In treatments 1 and 3, we allowed(but did not facilitate) colonization by other primaryproducers over the 11-month study period.

To incorporate spatial heterogeneity in response pa-rameters, we conducted this experiment at three sites:(1) within the northwest inlet of Cedar Cove, (2) alongthe northern shore of Parker Island, and (3) along thesouthern shore of Buzzard Island (Fig. 1). The exper-imental design resulted in 36 total plots, requiring;17 000 transplanted shoots. In the days immediatelyfollowing the initiation of the experiment, sites werevisited to qualitatively assess transplant success andidentify potential herbivores.

Environmental data for the study sites are providedin Table 1, and indicate little differences among sitesfor total phosphorus, total nitrogen, or chlorophyll con-centrations. Physical variables like temperature and sa-linity were also similar among sites. Sediments at allsites were dominated by fine and very fine sands, withsome variation in the percentage of organic matteramong sites (Table 1).

Measurements of response variables

Response variables monitored within plots over time(January to November 2001) included: Vallisneriashoot density (monthly), aboveground growth rates (bi-


monthly), shoot size distributions (quarterly), and den-sity of other primary producers (stem density and per-cent cover, both measured monthly). Measurementswere made within the centermost 1 m2 of plots to allowsome buffering (;25 cm) of potential edge effects. Tocompare transplant performance to that of naturallyoccurring Vallisneria, we simultaneously collecteddensity and growth measurements (using the samemethods as those described for experimental plots)from a meadow located adjacent to the plots at Buz-zards Island.

To quantify densities of Vallisneria, Myriophyllum,and Hydrilla within each plot, scuba divers countedtotal numbers of shoots or stems enclosed within threehaphazardly placed 30 3 30-cm quadrats. Within thesame quadrats, divers also quantified the percent cov-erage of Lyngbya sp. and less common plants (includ-ing Stuckenia pectinata, Najas guadalupensis, and Cer-atophyllum demersum).

Measurements of in situ aboveground growth by rel-atively large, established plants ($4 leaves per shootand $30 cm tall) were made on a bimonthly basisthroughout the course of the experiment, after an initialacclimatization period following planting. We modifiedthe hole-punching technique commonly used for sea-grasses (Zieman and Wetzel 1980), and appropriate formany wide-bladed aquatic plants exhibiting a basalmeristem (Hauxwell et al. 2003b). Scuba divers taggedseven relatively large, established plants in each plot,and punched two needle holes (18 gauge) at the baseof each leaf per shoot. Tagged plants were retrievedone month after punching. Shoots were transported tothe laboratory and frozen until total length, width, andleaf elongation for each leaf of each shoot could bemeasured. Because growth may also vary with shootsize, we simultaneously collected shoot mass data (totalleaf surface area) on plants selected for growth mea-surements.

To convert aboveground shoot characteristics andleaf growth from units of surface area to shoot mass,we calculated a mean leaf specific density from ;120shoots set aside prior to transplantation. We determinedleaf specific density, by first removing epiphytic ma-terial from each leaf of each shoot (using a glass slide),measuring leaf surface area, and then drying leaves at708C to a constant weight, and weighing. A conversionfactor (0.0034 g/cm2, r2 5 0.92, P , 0.0001) was cal-culated and applied to length and width data of trans-planted shoots to estimate final aboveground mass (mg/shoot) and the mass of new tissue produced for theinterval between punching and retrieval. Daily growth(mg·shoot21·d21) was estimated by dividing the pro-duction of new tissue by the time elapsed since punch-ing, and mass-specific leaf growth rates (%/d) weredetermined by dividing daily growth rates by finalaboveground shoot mass 3 100 (Dennison 1987).

Changes in size distributions of plants within treat-ment plots can yield information regarding the effect

of other primary producers on: (1) recruitment of newshoots and (2) recruitment of transplanted shoots intolarger size classes. To obtain nondestructive estimatesof Vallisneria individual shoot mass and size distri-butions within plots, we developed a relationship be-tween mass and morphological parameters of Vallis-neria that could be easily and repeatedly obtained byscuba divers without harvesting plants. Using 300plants that we had set aside prior to the transplantationexperiments, shoot mass was determined and detailedmorphological characteristics of shoots were measured.Leaf area proved a reliable predictor of plant mass:Aboveground shoot mass (in g) 5 0.0030((leaves pershoot 3 length of longest leaf (in cm) 3 width oflongest leaf (in cm))0.915). This equation correlated wellwith our data (P , 0.0001, r2 5 0.93). To assess sizedistributions in situ, scuba divers counted the numberof leaves and measured the length and width of thelongest leaf from 10 plants within a defined area ofeach plot three and five months after the initial planting.Together with the previously fitted regression equation,this yielded estimates of Vallisneria shoot mass andsize distributions within plots over time.

RESULTS

Effect of herbivores and other primary producerson Vallisneria shoot densities

The density of Vallisneria in plots exposed to her-bivores declined rapidly, and within one month, plantswere absent from 80% of the open plots (Fig. 2). Val-lisneria, however, was still present and relatively abun-dant in all of the fenced plots (Fig. 2). At Cedar Coveand Parker Island, the effect of herbivory was mostpronounced; within one week of initiating the experi-ment, we observed a near total loss of transplants inall open plots. The effects of herbivory at BuzzardIsland were dramatic, but less pronounced (;50% ofopen plots had some plants remaining), possibly be-cause plots at this site were in slightly shallower depths,which may have reduced access by large herbivoresduring low tides. These results greatly contrast withthose observed for naturally occurring Vallisneria (Fig.2, bottom, dashed line), suggesting either that trans-planted plants were selectively targeted by herbivoresor that in the absence of herbivores natural meadowswould exhibit dramatic increases in density.

Manatees were the most likely herbivore causingthese large effects, and we observed manatees grazingin our plots soon after initiating the experiment. A dailymean of .200 manatees were counted within the 1.8-km2 Kings Bay during the initial stages of the exper-iment (aerial surveys by Joyce Kleen, U.S. Fish andWildlife Service), and manatees consumed plants di-rectly from the hands of divers as they attempted toplant Vallisneria in two independent, yet similar indesign, experiments (D. Tomasko, D. Bristol, and G.Sheehy, personal communication; Hauxwell et al., in


FIG. 2. Density of transplanted shoots of Vallisneriaamericana in plots in which herbivores or other primary pro-ducers were either allowed or excluded during 2001 at theCedar Cove (top), Parker Island (middle), and Buzzard Island(bottom) sites in Kings Bay, Florida (means 6 1 SE). Thedashed line in the bottom panel indicates density of naturallyoccurring Vallisneria in a patchy meadow adjacent to thetransplanted plots at Buzzard Island.

press). Additionally, we observed very limited effectsof other grazers (e.g., assessed by observation of ac-tivity in the area and by grazing scars imprinted onleaves), and based upon the rapid removal of plantsduring this experiment, conclude that manatees are themost significant grazer in the system with regard toVallisneria restoration.

After the initial mortality, plants did not return toany of the plots that were initially affected by manatees.Since manatees consumed almost all of the plants inexposed plots, and these plots remained void of Val-lisneria, we ceased monitoring the open plots in July2001. Given the obvious detrimental effect of herbi-vores on transplants, the remaining results (and accom-panying statistical analyses) focus only on the effectof other primary producers (i.e., using herbivore ex-clusion plots).

Even in plots that excluded large herbivores, densitydeclined by ;50% in the first month, probably due to

mortality induced by handling and transplantation (Fig.2). After the first month, shoots in these plots appearedhealthy and were actively growing; after three monthsdensity increased. Overall, long term fluctuations in thecaged plots were similar to the pattern observed in anadjacent natural meadow, with early summer peaks inshoot density followed by decreases in density at theend of the growing season.

Response of Vallisneria densities to the presence orabsence of other primary producers varied among sites(Fig. 2, Table 2), due to site-specific differences in thedensity or coverage of other primary producers thatcolonized plots (Fig. 3). At Cedar Cove, where colo-nization of other primary producers was minimal in allplots (Fig. 3, top panels), we observed no effect ofother primary producers on Vallisneria densities (Table2), and Vallisneria coverage extended beyond the pe-rimeter of the original plots (J. Hauxwell, personalobservation). There were, however, significant effectsof other primary producers on Vallisneria densities atthe other sites (Fig. 2, Table 2), where colonization byother primary producers was extensive relative to plotsin which other primary producers were actively ex-cluded (Fig. 3, middle and bottom panels). At both theBuzzard Island (strong significant effect, Table 2) andParker Island (marginally significant effect, Table 2)sites, Vallisneria density in plots where colonizationby other primary producers was allowed was ;50%the density in plots in which other primary producerswere actively excluded.

Primary producers that colonized plots included Eur-asian watermilfoil (Myriophyllum spicatum), hydrilla(Hydrilla verticillata), sago pondweed (Stuckenia pec-tinata), southern naiad (Najas guadalupensis), coontail(Ceratophyllum demersum), and filamentous cyano-bacteria (Lyngbya sp.) (Fig. 3). Routine hand removalof weeds by snorkelers was effective in maintainingvery low biomass of other primary producers in ex-clusion plots (Fig. 3, compare left and right panels).Where allowed, colonization by other primary produc-ers at the Cedar Cove site was minimal (Fig. 3). How-ever, at Parker Island and Buzzard Island, Myriophyl-lum became a dominant feature of the vegetation inplots in which we allowed colonization, extending intothe water column over one meter and reaching densitiesgreater than twice that of Vallisneria in those plots. Atboth of these sites, Myriophyllum densities peaked inlate June. Based on data simultaneously obtained frombenthic cores, these high densities correspond to driedbiomass values greater than 400 g/m2. Hydrilla andother species were generally rare and exhibited, atmost, only minor peaks at both sites.

Because Myriophyllum spicatum was the most abun-dant competitor, we assessed the effect of Myriophyl-lum on Vallisneria density with regression analysesusing monthly stem and shoot density data for bothtaxa in all of the herbivore-exclusion plots at all sites.Though our experimental design did not include ma-


TABLE 2. Results of three-factor (treatment, site, and time) and two-factor (treatment andtime, within each site) repeated-measures ANOVAs used to test for differences in densitiesof transplanted Vallisneria americana over time in plots that either excluded or allowedcolonization by other primary producers at the three study sites in Kings Bay, Florida (de-picted in Fig. 2).

Site and factor df MS F P

AllTreatmentSiteTreatment 3 sitePlots within treatmentsTime

122

127

33 64231 602

935825538301

13.1812.38

3.67

15.01

0.00350.00120.0572

,0.0001Time 3 treatmentTime 3 siteTime 3 treatment 3 siteTime 3 plots within treatments

7141484

38723081079

553

0.704.171.95

0.6720,0.0001

0.0320

Cedar CoveTreatmentPlots within treatmentsTimeTime 3 treatmentTime 3 plots within treatments

1477

28

613047735

208472

0.01

16.400.44

0.9491

,0.00010.8677

Parker IslandTreatmentPlots within treatmentsTimeTime 3 treatmentTime 3 plots within treatments

1477

28

18 644334721281138

803

5.57

2.651.42

0.0777

0.03110.0238

Buzzard IslandTreatmentPlots within treatmentsTimeTime 3 treatmentTime 3 plots within treatments

1477

28

33 708301030551199

384

11.20

7.953.12

0.0287

,0.00010.0145

nipulating stem densities of Myriophyllum, differentialcolonization of the various treatment plots providedsufficient variation in Myriophyllum densities to be use-ful in a regression approach. We found a significantnegative exponential relationship between shoot den-sity of Vallisneria and stem density Myriophyllum foreach sampling date (Fig. 4).

Effect of other primary producerson Vallisneria shoot growth rates

Although we targeted relatively large, establishedplants for measuring growth, and specifically avoidedsmall newly recruiting plants, shoots that were sampledspanned a range of size classes. We were, however,successful in targeting the larger plants; in 80% of thecases, plants were .1 g, falling within the upper 75%of the size distribution.

Because growth increased with plant size (Fig. 5;regression analyses for pooled treatments, P , 0.05 inall cases), we included size as a covariate in a mixedeffects model to test for differences in growth rates ofVallisneria over time in plots that either excluded orallowed colonization by other primary producers at thethree study sites (Table 3; Littell et al. 1996, SAS PROCMIXED with fixed factors of treatment, site, and timeand plot as a random factor). While we observed sig-

nificant differences in shoot growth rates among sitesand dates, absolute growth rates of plants respondedmarginally, if at all, to treatment effects of other pri-mary producers (including inconsistent responses, giv-en the significant treatment 3 site 3 time interaction;Table 3). This effect of competitors, if real, was gen-erally in the opposite direction from what we antici-pated; plants grew similarly or worse when competitorswere removed (Figs. 5 and 6).

To better visualize the growth responses, we esti-mated absolute and mass-specific growth rates. In situgrowth of transplanted shoots varied among sites andover time (Table 4), ranging from 7 to 32 mg·shoot21·d21

(Fig. 6, top panels), representing turnover rates rangingfrom 0.7 to 1.6% per d (Fig. 6, bottom panels). Absoluteand mass-specific growth rates generally decreasedover the sampling period (Fig. 6). Absolute growth ofplants at Cedar Cove and Parker Island was generallygreater than that at Buzzards Island, largely becauseplants at Buzzard Island were smaller (Fig. 5 and seealso Fig. 8). As expected, absolute growth of trans-plants was somewhat lower than that of naturally oc-curring Vallisneria for much of the study period (Fig.6, upper right panel, dashed line). The results for spe-cific growth (Fig. 6, bottom panels) are approximatelycomparable to our statistical analyses described above


FIG. 3. Stem density of Eurasian watermilfoil (Myrio-phyllum spicatum) and hydrilla (Hydrilla verticillata) and per-cent cover of additional primary producers over time at theCedar Cove (top), Parker Island (middle), and Buzzard Islandsites (bottom) in Kings Bay, Florida, in transplanted plots ofVallisneria americana in which colonization of other primaryproducers was excluded (left) or allowed (right; means 6 1SE). Additional primary producers that colonized experimen-tal plots included a filamentous cyanobacterium (Lyngbyasp.), sago pondweed (Stuckenia pectinata), southern naiad(Najas guadalupensis), and coontail (Ceratophyllum demer-sum).

FIG. 4. Stem density of Eurasian watermilfoil (Myrio-phyllum spicatum) vs. shoot density of Vallisneria americana(raw data of means in Figs. 2 and 3) in plots in which weexcluded or allowed colonization by other primary producersfor each date over the sampling period in 2001 at the threeexperimental sites (symbols represent means within plots,treatments not distinguished; open circles 5 Cedar Cove, graycircles 5 Parker Island, black circles 5 Buzzard Island) inKings Bay, Florida. Each regression was significant at P ,0.001: March, y 5 107(e20.013x), r2 5 0.27; April, y 568(e20.018x), r2 5 0.23; May, y 5 93(e20.027x), r2 5 0.68; June,y 5 131(e20.010x), r2 5 0.49; July, y 5 136(e20.009x), r2 5 0.82;August, y 5 72(e20.014x), r2 5 0.81; September, y 5 66(e20.016x),r2 5 0.50; October, y 5 44(e20.042x), r2 5 0.57.

(Table 3; Fig. 5): i.e., the slope of the relationship be-tween log(growth) and log(mass) was 1.18 (fairly closeto, albeit slightly greater than a slope of 1, which wouldbe expected if specific growth did not vary with plantsize). Indeed, Fig. 6 (bottom panels) shows (1) thedetected interaction between site, time, and treatment,and (2) the slightly reduced growth of Vallisneria inweeded plots, as well as (3) lower specific growth ratesof plants at Cedar Cove, where Vallisneria densities inall plots were relatively high (Fig. 2). After factoringout the effect of plant size, both the magnitude andseasonal pattern of mass-specific growth of transplantswas similar to that of naturally occurring Vallisneria(Fig. 6, lower right panel, dashed line).

Because weeding reduced the density of competitors,but increased the density of Vallisneria, it is possiblethat the reduced (or lack of increased) growth in weed-ed plots was related to increased Vallisneria density.In fact, when pooled across all sites and treatments,

there was a negative linear relationship between meanVallisneria mass-specific growth rate and Vallisneriashoot density on each date (Fig. 7; although this wasstatistically significant on only one of the four dates).Repeatedly, points from the Cedar Cove site clusteredin a manner indicating low specific growth rates at highshoot densities.

Effect of other primary producers on Vallisneriashoot size distributions

Using the relationship between morphometric vari-ables and shoot size (described in the Materials andMethods: Measurement of response variables) and insitu field measurements of the necessary variables with-in transplanted plots over time, we were able to assesswhether there were shifts in size distributions of plants(1) within sites over time, (2) between treatments, and(3) among sites. Results, including initial shoot mass


FIG. 5. Growth rates of individual Vallisneria americana shoots vs. aboveground shoot mass in plots in which we excluded(open circles) or allowed (black circles) colonization by other primary producers for each date over the sampling period in2001 at the three experimental sites in Kings Bay, Florida (all as dry mass). Because there were no differences in slopes ofregressions between treatments in any individual case (P . 0.05; comparison of multiple slopes, Sokal and Rohlf 1995),regression lines and statistics correspond to regression data pooled between treatments (Cedar Cove: May, y 5 11.4(x1.17),r2 5 0.69; July, y 5 9.7(x1.07), r2 5 0.58; September, y 5 8.2(x1.06), r2 5 0.66; November, y 5 5.5(x1.13), r2 5 0.33. ParkerIsland: May, y 5 15.9(x0.95), r2 5 0.79; July, y 5 13.1(x0.92), r2 5 0.64; September, y 5 7.2(x1.34), r2 5 0.48; November, y513.0(x1.31), r2 5 0.88. Buzzard Island: May, y 5 13.6(x0.96), r2 5 0.77; July, y 5 11.1(x1.21), r2 5 0.87; September, y 5 9.1(x1.30),r2 5 0.77; November, y 5 8.0(x1.45), r2 5 0.74).

from laboratory measurements and subsequent (afterfive months) field measurements of shoot mass, areshown in Fig. 8, and provide before and after snapshotsof Vallisneria in plots at the three sites.

At planting, the frequency distribution of shoot masswas skewed toward smaller plants (,0.5 g, 15–25 cmtall). After three months (data not shown), size fre-quency distributions shifted toward larger plants, in-dicating active growth of transplants. After fivemonths, size frequency distributions of plants were sig-nificantly different than the initial distribution (Fig. 8).

At five months, there were also significant differencesin plant sizes among sites with smaller plants at theBuzzard site and larger plants at the Cedar Cove site(Table 5). Plants at Cedar Cove, in fact, extended tothe surface at low tide (up to 1.5 m tall). The presenceof very small plants at all sites indicated recruitmentof small, new plants. Though there was a significanteffect of other primary producers on Vallisneria den-sities, removal of competitors had no demonstrable ef-fect on the mean size of Vallisneria at any of the sites(Table 5).


TABLE 3. Results of a mixed effects model (Littell et al.1996, SAS PROC MIXED) used to test for differences ingrowth rates of Vallisneria americana over time in plotsthat either excluded or allowed colonization by other pri-mary producers at the three study sites in Kings Bay, Flor-ida, and that incorporated shoot mass as a covariate withshoot growth (fixed factors 5 treatment, site, and time;random factor 5 plot; data were log transformed and aredepicted in Fig. 5; see also Fig. 6, bottom panels).

Factor df F P

TreatmentSiteTimeShoot massTreatment 3 siteTreatment 3 timeSite 3 timeTreatment 3 site 3 time

1, 122, 123, 321, 3862, 123, 326, 326, 32

3.598.11

16.83711.13

2.180.931.042.80

0.0820.006

,0.0001,0.0001

0.1560.4380.4160.027

FIG. 6. Absolute (top, in mg dry mass) and mass-specific (bottom) growth rates of Vallisneria americana shoots overtime (midpoint of punching and retrieval dates) in plots in which we excluded or allowed colonization by other primaryproducers during 2001 at the three study sites in Kings Bay, Florida (means 6 1 SE). The dashed lines in the right panelsindicate growth rates of naturally occurring Vallisneria in a patchy meadow adjacent to the transplanted plots at BuzzardIsland.

DISCUSSION

Productivity of naturally occurringand transplanted Vallisneria

Despite the many ecological values inherent in pre-serving Vallisneria habitat and the widespread effortsto restore Vallisneria in areas in which it has declined,there remains a paucity of information on even the mostbasic demographic parameters in Vallisneria popula-tions, including annual dynamics of density andgrowth, let alone ecosystem effects on carbon fixation

or nutrient cycling, i.e., what are we restoring? Baselineecological data are crucial for assessing successful res-toration of Vallisneria, as well as the ecological im-plications of restoration (including the likely effects ofrestoration on system productivity). To our knowledge,this study provides the only information on transplantand natural stand productivity to date. Notably, mini-mum rates of Vallisneria production (;4.5 g·m22·d21,Figs. 2 and 6; Hauxwell et al. 2003b) were estimatedto be approximately twice that of the maximum growthrate of a ‘‘highly productive’’ seagrass (Thalassia tes-tudinum) meadow off of Florida’s central Gulf coast(Zieman et al. 1989). These data suggest that, first, onan ecosystem scale, the magnitude of carbon fixed byVallisneria and available as standing biomass for het-erotrophic consumption may be relatively large, em-phasizing the importance of Vallisneria to whole eco-system functioning in springs, lakes, and oligohalinereaches of many estuaries.

In comparison with productivity of natural stands,transplants exhibited similar relative growth rates (0.8–1.8% per day). At no point during our yearlong ex-periment, however, did overall productivity reach val-ues exhibited by natural stands, due to lower shootdensities in transplanted plots. It is not yet clear howlong (if ever) it might take a transplanted plot toachieve productivity of a natural stand. However, pro-ductivity may not be the appropriate benchmark from


TABLE 4. Results of three-factor (treatment, site, and time) repeated-measures ANOVAs usedto test for differences in absolute and mass-specific growth rates of Vallisneria americanaover time in plots that either excluded or allowed colonization by other primary producersat the three study sites in Kings Bay, Florida (depicted in Fig. 6).

Factor df MS F P

Absolute growth rateTreatmentSiteTreatment 3 sitePlots within treatmentsTimeTime 3 treatmentTime 3 siteTime 3 treatment 3 siteTime 3 plots within treatments

122

103366

30

0.0000020.0001030.0000240.0000360.0010000.0000710.0001670.0000630.000025

0.072.890.67

31.172.786.582.49

0.79840.10200.5332

,0.00010.05810.00020.0448

Mass-specific growth rateTreatmentSiteTreatment 3 sitePlots within treatmentsTimeTime 3 treatmentTime 3 siteTime 3 treatment 3 siteTime 3 plots within treatments

122

103366

30

0.2510.3120.0360.0610.8620.0500.0200.0710.037

4.15.10.6

23.31.40.51.9

0.06990.02950.5699

,0.00010.27450.77200.1101

which to judge ‘‘success.’’ Indeed, there are many as-pects of habitat restoration (physical, chemical, bio-logical, as well as aesthetic and educational) that var-ious audiences representing different interests considerfavorable targets.

Effect of herbivores on Vallisneria restoration

Experimental results demonstrate strong effects ofmanatees on Vallisneria restoration efforts in KingsBay, and potentially in other spring-fed systems alongthe Gulf coast of Florida. Although natural Vallisneriabeds adjacent to transplanted plots remained intact overthe course of our experiment, grazing by manatees onnewly transplanted Vallisneria in Kings Bay was tar-geted and intense, regardless of site, and would be avital element to consider when formulating a restora-tion strategy. From grazing scars on leaves, we roughlyestimate small invertebrate grazing to be minimal(;,0.1% standing biomass; J. Hauxwell, unpublisheddata). Field observations of turtles, a class of herbivoreshown to significantly reduce success of Vallisneriatransplants in other systems in Florida (Lake Monroe;Jaggers 1994), Alabama (Guntersville Reservoir;Doyle and Smart 1993), and Maryland (Potomac River;Carter and Rybicki 1985), were also minimal; whilewe encountered manatees daily, in the collective.4000 hours spent diving for this and other experi-ments, we observed only one turtle. Also, herbivorouswaterfowl were never observed in the vicinity of plots.

Despite the marked effect of manatees on Vallisneriain our experimental plots, we do not, however, suggestthat the presence of manatees (1) was the cause forlong-term decline of Vallisneria in Kings Bay, or (2)necessarily precludes restoration of Vallisneria in

Kings Bay. While feeding within natural meadows ofVallisneria, manatees primarily crop plants (T. K. Fra-zer, personal observation), leaving the belowgroundportion, along with several centimeters of leaf material,intact. Vallisneria and other true seagrasses (e.g., Thal-assia and Halodule) have evolved to cope with her-bivory by large herbivores (including manatees, du-gongs, turtles, waterfowl), and can repeatedly regrowthe cropped leafy portion (Cebrian et al. 1998), pre-sumably using within-shoot belowground energy re-serves or translocated energy reserves from a neigh-boring clone. In several studies, seagrasses have beendocumented to persist, and in some cases, even increaseproductivity (Valentine et al. 1997), despite extensive,sometimes repeated, grazing by sea turtles, manatees,dugongs, or sea urchins (Greenway 1976, Ogden et al.1983, Zieman et al. 1984, Williams 1988, Masini et al.2001). We, in fact, have conducted simulated grazingexperiments within natural stands of Vallisneria inKings Bay and have found that, one week after clip-ping, we could not recognize clipped plants from con-trols (J. Hauxwell, personal observation). In the ex-periment described here, however, newly plantedshoots were completely uprooted by manatees. It ispossible that transplanted Vallisneria must be protectedduring some initial acclimatization period (e.g., enoughtime to allow plants to become well-rooted and to de-velop a more mature belowground rhizome system),and if so, it would be a relatively simple task to excludeherbivores during this period, using spatial (fencing,necessary year-round; Smart et al. 1998) combinedwith temporal strategies (planting in summer whenmanatee abundances are lowest, although still present).If this is indeed the case, however, the time required


FIG. 7. Mass-specific growth rates of Vallisneria americana shoots vs. Vallisneria shoot densities in plots in which weexcluded or allowed colonization by other primary producers for four dates in 2001 at the three study sites in Kings Bay,Florida (symbols represent means within plots; treatments not distinguished).

for plants to establish is beyond the five-month periodtested in a follow up experiment (Hauxwell et al., inpress).

Results from this experiment illustrate a potentiallydifficult management scenario and a topic of ongoingdebate: ‘‘save the ecosystem, [vs.] save the species’’(Senior 2003). The manatee, a federally endangeredspecies, negatively affected restoration of a nativeaquatic plant. Though simultaneous protection of man-atees and potential restoration of Vallisneria would re-quire creative solutions that are not necessarily mu-tually exclusive, results of this experiment illustratethe potential conflicts that can arise when species-spe-cific approaches to management are employed withinecosystems.

Effect of other primary producerson Vallisneria restoration

Recently planted shoots of Vallisneria in Kings Baymay be competitively reduced by other plants, espe-

cially the nonnative macrophyte Myriophyllum spica-tum. In plots where these competitors were routinelyremoved, or where natural densities of Myriophyllumremained low (Cedar Cove), there remained relativelydense and productive stands of Vallisneria. Therefore,the recent status of water clarity in Kings Bay does notappear to preclude Vallisneria restoration. Lyngbya,though abundant in the vicinity of enclosures at CedarCove, was not common in our experimental plots. Hy-drilla verticillata was present but did not achieve nui-sance levels. Compared to herbivory, competition withMyriophyllum likely represents a much more chronicand difficult problem to confront in restoration, andmay have been a factor related to the initial decline ofVallisneria in Kings Bay. Myriophyllum has beenshown to competitively exclude native vegetation, in-cluding Vallisneria, in other systems in North America(Lind and Cottam 1969, McFarland and Rogers 1998).

Potential mechanisms by which Myriophyllum re-duces Vallisneria in shallow-water systems might in-


FIG. 8. Histograms showing the size distribution of Val-lisneria americana shoots (aboveground modeled values, allin g dry mass) that were initially planted in all plots (toppanel) and five months after planting in plots in which weexcluded or allowed colonization by other primary producersduring 2001 at the Cedar Cove (second panel), Parker Island(third panel), and Buzzard Island sites (bottom panel) inKings Bay, Florida. Extended dashed lines indicate means(pooled for treatments within a site).

TABLE 5. Results of a two-factor ANOVA comparing meansizes of shoots of Vallisneria americana (modeled above-ground shoot mass, depicted in Fig. 8) in transplanted plotsin which we either excluded or allowed colonization byother primary producers at the three experimental sites (Ce-dar Cove, Parker Island, and Buzzard Island) in Kings Bay,Florida.

Factor df MS F P

SiteTreatmentSite 3 treatmentResidual

212

12

0.3230.0010.0050.019

16.830.070.25

0.00030.80300.7803

Note: Data were log transformed.

clude (1) competition for space and (2) light limitationof newly recruiting shoots. Established tall plants ofVallisneria grew similarly whether in monospecificstands or in mixed stands with Myriophyllum. Densitiesof Vallisneria, however, decreased rapidly as Myrio-phyllum stem density increased. It is possible that My-riophyllum affects growth of short, newly recruitingVallisneria shoots as a result of inherent differencesbetween their growth forms. Titus and Adams (1979)demonstrated that in one meter depths (similar to KingsBay), Myriophyllum formed a leafy canopy concen-trated at the surface (68% of its aboveground biomasswas within 30 cm of the surface), whereas young shootsof Vallisneria concentrated the majority of leaf biomassnear the bottom (62% of its biomass was within only30 cm of the bottom; see also Duarte and Roff 1991).Growth of Vallisneria in systems with degraded waterclarity can be stimulated by increased light, demon-strating general light limitation of growth (Carter et al.1996). Therefore, it is possible that in Kings Bay,growth of young, newly recruiting plants may be re-

duced by competition with Myriophyllum, althoughgrowth of taller Vallisneria (which extend to the water’ssurface) is not (Figs. 5 and 6). This mechanism wouldalso affect the number of plants recruiting to larger sizeclasses. In our experiment, where Myriophyllum col-onization was negligible (Cedar Cove), size distribu-tions of Vallisneria spanned the greatest range, andafter five months, a large percentage of plants extendedto the surface, even at high tide (.1.5 m tall). It ispossible, that plants at Cedar Cove reached highenough densities (Fig. 2) and attained large enoughsizes (Fig. 8) for growth to be limited due to self-shading (Fig. 7). Alternatively, increased allocation ofcarbon reserves to belowground growth in the absenceof interspecific competition might account for the ab-sence of an effect of competitors on abovegroundgrowth.

Implications for Vallisneria restorationand management of aquatic habitats

Based on results of this study, we propose the fol-lowing recommendations regarding potential futurerestoration of Vallisneria in Kings Bay. First, trans-plants must be protected from manatees. Second, ef-fects of competition must be relatively small. As aresult of spatial variation in competition, success ofrestoration is likely to be highly site specific. For ex-ample, we observed large spatial heterogeneity in over-all success of transplants in the experiment, due to site-specific differences in natural abundances of other pri-mary producers. At two of our three sites, restorationdoes not appear to be feasible without considerable sitemaintenance (i.e., routine mechanical or targeted chem-ical removal of potential competitors); colonization byMyriophyllum was rapid, and it was able to competi-tively reduce densities of Vallisneria transplants. Atthe Cedar Cove site, however, natural densities of com-petitors were low (Figs. 3 and 4) and we observed: (1)generally high densities of plants (Fig. 2), (2) generallyhigh absolute growth rates (Fig. 6), (3) greatest re-cruitment of small shoots into larger size classes (Fig.8), and (4) general extension of Vallisneria coveragebeyond original plot perimeters. Many factors may con-tribute to the site-to-site differences in colonization by


other primary producers and the general success ofVallisneria in transplanted plots, including physical,geological, and geochemical differences among sites(Titus and Adams 1979, Titus and Stone 1982, Barkoand Smart 1986, Twilley and Barko 1990, Sullivan andTitus 1996, Kraemer et al. 1999, Van et al. 1999, Koch2001).

It is important to note that these results are from ahighly ‘‘managed’’ system (as are most freshwater sys-tems), and may be affected if any one of the manymanagement practices were altered or if new strategieswere implemented. During our experiment, the herbi-cide endothall (Aquathol, Cerexagri, King of Prussia,Pennsylvania, USA) was applied in back canals to limitgrowth of hydrilla (Mark Edwards, Citrus CountyAquatic Services, Citrus County, Florida, USA, per-sonal communication). In addition, mechanical remov-al of vegetation assemblages in the littoral zone (pri-marily hydrilla, Lyngbya, and Eurasian watermilfoil)occurred sometimes daily. These management strate-gies are not specific to Kings Bay, and may, in manycases, be the norm in many freshwater environmentsaround the world. Indeed, there exists a very full andimaginative toolbox accessible to those interested inameliorating recreational or aesthetic problems stem-ming from cultural eutrophication in freshwaters, in-cluding: application of herbicides, application of alumor lime to bind nutrients, withdrawal of nutrient-richhypolimnetic waters, water level manipulation, impos-ing light limitation using dyes or opaque screens, liningthe bottom with plastic to inhibit plant growth, whole-lake aeration to reduce internal phosphorus loading,dredging, introduction of herbivores (e.g., grass carp,milfoil weevil), mechanical harvesting, and repeateddisturbance of sediments (‘‘weed rollers’’) (Holdren etal. 2001). In best cases, these methods may be used inconjunction with improved watershed managementpractices; in many cases they are used in lieu of longer-term solutions. As is the case for many other waterbodies, numerous management agencies, representingcity, county, regional, state, and federal interests arepresently active within the Kings Bay system. Hence,experimental results for Kings Bay and other highlymanaged systems must be viewed within this contextand the limitations of extrapolating these results to oth-er systems—and the importance in site-specific pilotexperiments—should also be recognized, particularlyfor freshwater environments.

In conclusion, we argue that only by understandingthe ecological factors that affect the successful re-es-tablishment of Vallisneria, can we develop the mostsuccessful, cost-effective techniques for restoration.Restoration might not be feasible in many targeted sys-tems, and it is important to understand not only thecauses of initial degradation and factors that might af-fect future restoration, but also, to characterize the ap-propriate measures of ‘‘success.’’ Detailed demograph-ic studies of natural populations of Vallisneria must at

least accompany, if not precede, investment in large-scale restoration efforts, and the possibility that ‘‘re-stored’’ systems may differ significantly from naturalsystems in terms of productivity, habitat complexity,and genetic diversity (Williams 2001), should also berecognized.

ACKNOWLEDGMENTS

This research was supported by the Southwest Florida Wa-ter Management District (SWFWMD) through the SurfaceWater Improvement and Management (SWIM) program andthe Coastal Rivers Basin Board (SWFWMD Contract Number01C0N000007), and by the Wisconsin Department of NaturalResources (WDNR) through support to J. Hauxwell duringfinal stages of manuscript development. We greatly appreciatethe efforts of Denise Bristol (SWFWMD), who provided as-sistance and guidance throughout various phases of this re-search. Mark Hammond (SWFWMD), David Tomasko(SWFWMD), Linda Deegan, and two anonymous reviewersprovided helpful comments on earlier drafts of the manu-script. We also thank Paul Rasmussen (WDNR) for providingassistance with statistical analyses. A number of people pro-vided invaluable field and laboratory support including Grif-fin Sheehy, Sharon Stevens, Mellissa Warren, Kori Blitch,David Giehtbrock, and Sky Notestein. Transplant crew mem-bers included Jason Hale, Stephanie Keller, Jaime Greena-walt, Rikki Grober-Dunsmore, James Vonesh, Laurent Vig-liola, Mark Butler, Doug Marcinek, and Dave Birch, in ad-dition to SWFWMD personnel and those named above. Wealso thank Seth Blitch and staff at the St. Martins AquaticPreserve (Florida Department of Environmental Protection)for the use of their facilities in staging our field work. Thisarticle is Journal Series No. R-09667 of the Florida Agri-cultural Experiment Station.

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