A comparison of eelgrass, sea lettuce macroalgae, and marsh creeks as habitats for epibenthic fishes and decapods

Estuarine,

Coastal

and Shelf

Science

(1991) 33,501~519

A Comparison of Eelgrass, Sea Lettuce Macroalgae, and Marsh Creeks as Habitats for Epibenthic Fishes and Decapods

Susan

M. Sogard”

and Kenneth

W. Able

Marine Field Station, Institute of Marine and Coastal Sciences, Rutgers University, Great Bay Boulevard, Tuckerton, NJ 08087, U.S.A. Received

18 October

Keywords: Habitat ment, New Jersey

1990 and in revisedform

quality,

23 May

estuaries, Zostera

1991

marina,

Ulva

lactuca,

recruit-

Densities of epibenthic fishes and decapod crustaceans (excluding xanthids and pagurids) were quantified with daytime throw trap sampling in shallow water habitats of New Jersey estuaries. We compared eelgrass (Zostera marina), sea lettuce macroalgae (Ulva lactuca), unvegetated sand/mud substrates adjacent to these vegetation types, and saltmarsh creeks. The highest total density of fishes occurred in marsh creeks, due primarily to high abundances of Menidia menidiu. The highest total decapod density was also in a marsh creek, but only slightly surpassed the density in Zostera. Results of apriori comparisons tests for individual species demonstrated that vegetation (either Zostera or Ulva) was superior in quality (based on fish and decapod densities) to adjacent unvegetated substrates. Sites with Zostera as the dominant vegetation had higher densities of most fish species than sites with Ulva as the dominant vegetation, but only one decapod, Hippolyte pleuracanthus, was more abundant at eelgrass sites. Ulva lactuca, therefore, was an important habitat in areas lacking Zostera marina; for the decapods the two vegetation types were comparable in habitat quality, but for fishes Ulva did not provide an equivalent substitute for Zostera. Marsh creeks supported very high densities, but only for a few species that were also common in other habitats. Comparison of recruitment patterns suggested many species do not begin exploiting these estuarine habitats until relatively late in the summer, perhaps as a result of peak spawning in mid-summer.

Introduction

The importance of eelgrass(Zostera

marina) and other seagrasses ashabitat for epibenthic

fishes and decapod crustaceans is well documented. cantly higher fauna1 densities in seagrass relative

Numerous studies have found signifito unvegetated sand or mud substrates

(seereview in Orth et al., 1984), and experimental studies have demonstrated the value of seagrassas a refuge from predation (Heck & Thoman, 1981; Coen et al., 1981; Stoner, 1982; Leber, 1985; Heck &Wilson, 1987; Main, 1987; Wilson et al., 1987). The role of “Present address: Hatfield Marine Science Center, Oregon State University, Newport, OR 97365, U.S.A. 0272-7714/91/l

10501+

19 $03.00/O

@ 1991 Academic Press Limited

502

S. M. Sogard & K. W. Able

Figure 1. Location of sampling sites in the Little Egg Harbor-Great Bay estuarine system of southern New Jersey. Sites Gl (grassl) and G2 (grass2) were in dense Zostera marina beds with intermittent patches of unvegetated sand/mud. Sites Ul (Ulval) and U2 (Ulvn2) were sand/mud substrates with patchy accumulations of Ulna lactuca. Sites Cl (creekl) and C2 (creek2) were in shallow marsh creeks.

Zostera beds as a nursery habitat for newly settled juvenile fishes and decapods has also received much attention (Adams, 1976; Heck & Thoman, 1984; Orth & van Montfrans, 1987; Heck et al., 1989). Rich, well developed Zostera beds are a dominant habitat in northern New Jersey estuaries. From the middle of Little Egg Harbor south through Delaware Bay, however, Zostera marina is virtually absent (Figure 1). Cottam and Munro (1954) noted that formerly extensive grassbeds in this area never recovered following the wasting disease of the 193Os, and were replaced by beds of macroalgae (Ulva and Enteromorpha spp.). This pattern appears to have stabilized, with little Zostera recovery in the 37 years since their report. Macroalgal mats can have negative effects on resident organisms. Exudates of Ulva lactuca are toxic to larval winter flounder (Johnson, 1980) and larvae of five different crab species (Johnson & Welsh, 1985). Similarly, adult barnacles (Balanus balanoides) experience high mortality rates when placed in contact with Ulva in simulated tide pools (Magre, 1974). An additional detrimental effect results from the low dissolved oxygen levels occurring within and under macroalgae mats at night or when flushing is poor

Habitat preference infish and decapods

503

(Edwards &Welsh, 1982; Reise, 1983;Johnson&Welsh, 1985;Hull, 1987). Ulva proliferation has been associatedwith eutrophication (Guist & Humm, 1976; Rosenberg, 1985; Soulsby et al., 1985) and is often considered an indicator of pollution. Chlorophyte mats can also have beneficial attributes. The macroalgae provide an important food source for someorganisms,through either direct assimilation or indirectly through detritus-based food chains(Lubchenco, 1978;Price & Hylleberg, 1982;Lenanton et al,, 1982; Soulsby et al., 1982; Warwick et al., 1982; Robertson & Lenanton, 1984; Hull, 1987). The structure of the algae interferes with predator efficiency, providing protective refuge from aquatic (Wilson et al., 1990a) and avian predators (Lenanton et al., 1982; Robertson & Lenanton, 1984). These refuge and food benefits result in the utilization of macroalgal mats asnursery habitat for somefish species(Lenanaton et al., 1982; Lenanton & Caputi, 1989; Wright, 1989). An additional shallow water habitat available to epibenthic fishes and decapods in estuaries is the saltmarsh creek. Extensive networks of creeks ranging from small intertidal rivulets to major subtidal tributaries occur throughout the salt marshes in New Jersey. Although fish and decapod abundances in creek habitats have been examined in several studies (Richards & Castagna, 1970; Cain & Dean, 1976; Shenker & Dean, 1979; Weinstein, 1979; Mense & Wenner, 1989), few direct comparisonswith Zostera have been made. Weinstein and Brooks (1983) found a greater diversity of fishes and decapods in Zostera beds, but creeks supported higher total densities, due to the overwhelming abundance of spot (Leiostomus xanthurus). Several less abundant species were more common in Zostera than in creeks. In New Jersey estuaries, initial comparisons of fish and decapod communities in shallow water habitats were conducted by Wilson et al. (in review), who quantitatively sampled vegetated and unvegetated substrates at a Zostera-dominated site and an Ulvadominated site, along with a marsh creek site. For both fishes and decapods, Ulva supported slightly lower densities relative to Zostera, but both vegetation types had higher densities than adjacent patches of bare sand. The highest total fish densities were in the creek habitat, whereas total decapod densitieswere highest in Zostera. In the present study we expanded on the efforts of Wilson et al. (in review), quantifying fish and decapod densities in vegetated and unvegetated substratesat two sites in Zostera marina beds (grass sites) and two sites with patches of Ulva lactuca (Ulva sites), and comparing these values with densities in two marsh creeks (creek sites). We considered natural densities to be a quantitative measure of habitat quality, with higher densities reflecting either behavioural selection of preferred habitats or higher survival rates relative to other habitats. Our primary objectives were to compare the degree of habitat enhancement provided by Zostera marina and Ulva lactuca over adjacent unvegetated substrates, and to determine the value of marsh creeks and macroalgal mats relative to Zostera beds. In addition, we examined recruitment patterns for the various speciesutilizing shallow estuarine habitats in New Jersey, and compared the Zostera fauna1community with other Zostera ecosystemsalong the U.S. east coast. Materials

and methods

Sampling was conducted at six sites (Figure 1) in the Little Egg Harbor-Great Bay system. Grass1 (near Marshelder Island) and grass2(near Ham Island) were extensive Zostera marina bedswith isolated patches of unvegetated sand/mud. Ulval (in Little Bay.) and UZva2 (near Little Beach Island) were shallow sand/mud substrates with varying

504

S. M. Sogard & K. W. Able

densitiesof unattached Ulva lactuca mats. Both creek siteswere on a saltmarsh peninsula dividing Little Egg Harbor and Great Bay; creek1 was a narrow side channel in Hatfield Creek, and creek2 was at the mouth of Foxboro Creek. At the grass and Ulva sites, sampling was conducted in both vegetated and unvegetated areas. The marsh creek sitesusually lacked substantial patches of vegetation. All six sites were similar in depth, averaging 20 to 40 cm at mean low tide. Epibenthic fishes and decapod crustaceanswere collected with a throw trap, an open aluminum box 1 m2in areaand 45 cm in height. The height of the trap wasextended to 1 m with a band of 3-mm meshnetting attached to float cord, preventing escapeover the top of the trap. To sample, the trap was thrown on to the desired substrate and immediately pushed into the sediment. Animals were removed with a 1-m wide framed net with 3-mm meshuntil three consecutive net passesproduced no fish or decapods. This gear provided quantitative 1-m’ samples,allowing direct comparison of different habitats. Capture efficiencies of enclosure traps for epibenthic speciesare very high on both vegetated and unvegetated substrates(70”, to nearly 100%; Kushlan, 1981; Pihl & Rosenberg, 1982), but are probably lower for active water column species.Xanthid and pagurid crabs are also inefficiently sampled becausetheir burrowing habits make them difficult to capture; they were excluded from analysis in this study. All other decapodsand fishes were identified and measured (standard length for fishes, carapace length for shrimps and carapacewidth for crabs). Throw trap sampleswere collected on a biweekly basisfrom May through September in 1988, with two samplesfrom vegetated and two samplesfrom unvegetated substratesat each grass and UZva site. In 1989, samphng effort was increased to three throw traps per habitat per site, and the two marsh creeks were added to the sampling schedule. Collections were again taken on a biweekly schedule from May through September. All sampleswere collected closeto low tide during daylight hours. Salinities were measured with a refractometer in conjunction with each sampling collection. In 1989, water temperatures were continuously monitored at the grass and Ulva siteswith Ryan Tempmentor submersiblethermographs placed in PVC housings in the immediate sampling area. The sensorwas positioned at the sediment-water column interface and temperature recorded every 20 min. Continuous records were not available for 1988 sampling or for creek sitesin 1989. The accumulation of Ulva lactuca at the Ulva sites varied markedly over the summer seasonin both years, ranging from absenceto near total coverage. We attempted to throw the trap into relatively densealgal mats. When accumulation was particularly sparsethis required searching for algae. When algae were dense, a similar search was sometimes required to find bare sand. In 1988, no mats of Ulva could be found in one August collection at the Ulval site and two August collections at the U/vu2 site. In 1989 the macroalgae were present throughout the summer, although often in very small patches. Ulva was sporadically present in the marsh creeks. It was present in throw trap samples only at the creek2 site, where it was usually very sparseexcept for one sample in June and three samplesin July. At the grass sites vegetated and unvegetated sections were persistent. Vegetation densitieswere measuredin conjunction with each throw trap. At grasssites, two 10.2-cm diameter cores were collected with each Zostera throw trap. Shoot density and standing crop (dry weight in g of photosynethetic portion of blades) per m2 were calculated for each core. At Ulva sites, all Ulva within the l-m2 sampling area of each throw trap was removed and dried to a constant weight at 60 “C. The densities reported

Habitat preference in fish and decapods

505

here reflect the amount of algae present in patches selected for throw trap sampling (usually the densestareasavailable), not the averageacrossthe site. For statistical analysis, densitiesand standing crop of Zostera were compared between the two grasssitesand the 2 years of sampling with two-way ANOVAs. Ulva lactuca densities in the selected patches were likewise compared between the two Ulva sitesand 2 years. All vegetation values were log-transformed prior to analysis, successfully reducing heterogeneousvariances to nonsignificant levels. To compare densities of the dominant speciesacrossthe various sitesand habitats, we conducted one-way ANOVAs followed by a priori multiple comparisons tests, using the method of testing means outlined by Sokal and Rohlf (1981). All densities were logtransformed prior to analysis. Heterogeneity of the variances was substantially reduced for all the tested species, but was non-significant only for Crangon septemspinosa and Callinectes sapidus. However, due to the robustnessof ANOVA and the highly significant differences among groups (PC 0.001 for the overall ANOVA for all tested species),we felt that results of the ANOVAs and planned comparisons could be interpreted reliably. Mean densities (including all sampling dates and both years of the study) were compared for 10 habitat groups: group 1= vegetated substrate at the grass1 site; 2 = unvegetated at grass1; 3 = vegetated at grass2; 4 = unvegetated at grass2; 5 = vegetated at Ulval; 6 =unvegetated at i-J&al; 7 =vegetated at Ulva2; 8=unvegetated at Ulva2; 9 =creekl; lO=creek2. With 10 groups, nine multiple comparisons are possible. We chosethe following seven due to their ecological importance:

Contrast no. 1 2 3 4 5 6

7

Comparison

Groups used

Grass sitesvs. Ulva sites Zostera vs. adjacent unvegetated Ulva vs. adjacent unvegetated Creek sitesvs. other sites Grass1 vs. grass2

1,2,3,4 vs. 5,6,7,8 1 and3vs.2and4 5and7vs.6and8 9 and 10 vs. 1 through 8 land2vs.3and4 5and6vs.7and8 9vs. 10

Ulval

vs. Ulva2

Creek1 vs. creek2

These comparisonswere designedto determine the relative value (in terms of density) of the different habitats for the common epibenthic fishesand decapods.Becausethe same substrate types did not occur at all sites, it was not possible directly to compare habitats without someinfluence of site location in the estuary. Thus, these multiple comparisons can provide only a general view of habitat relationships. Results

and discussion

Site characteristics

Salinities ranged from 21 to 34 ppt over the 2 years of sampling, but differed little among sites. On average, the maximum difference in salinity acrossall sites within a sampling period was only 2 ppt. Overall, salinities were lower in 1989, but clear seasonalpatterns were not observed. Thus, we felt that contrasts in salinity were not strong enough to contribute substantially to community differences among the various sites.

506

S. M. Sogard & K. W. Able

May

18

Jun. 0

Jun. 29

Jul. 20

Aug. IO

Aug. 31

Sep. 2 I

Date

Figure 2. Mean daily water temperature from May to September, the 72 measurements recorded each day, at the two grass sites (sites (. ,) used for throw trap sampling.

1989, averaged ) and two

over

Ulva

Basedon detailed records from the continuous thermographs in 1989, water temperatures were very similar between the two grasssites,and tended to be warmer than the two Ulva sites, which were very similar to each other (Figure 2). The contrast in temperature between grassand Ulva sitesdecreasedby the end of the summer, when all four siteswere very similar. Temperatures climbed steadily during the spring, and were consistently above 20 “C after 1 June until the end of September. Unfortunately, wo do not have corresponding temperature data from creek sites. Vegetation densitiesvaried acrossthe summer at all four sites(Figure 3). Basedon twoway ANOVAs using site and year as factors, Zostera shoot densities and standing crop declined significantly from 1988 to 1989. Both shoot density and standing crop declined around mid-summer each year, possibly asa result of the increased turbidity associated with brown tide (as described by Cosper et al., 1987), which occurred both years. Mean Zostera shoot densitieswere not significantly different between the two sites, but because Zostera bladestended to be longer and more robust at the grass1site, standing crop values were significantly higher at grassl. Ulva densities in selected patches fluctuated widely acrossthe summer of 1988, and were substantially higher at the Ulval site. In 1989, Ulva densities were very low at both sites, resulting in a significant factor of year but a nonsignificant factor of site in the two-way ANOVA (Table 1). Fauna1 composition

Overall, 25 fish speciesand 11 decapod species(excluding xanthids and pagurids) were collected from the six sites(Table 2) over the 2 years of the study. For the fishes, the most abundant species were: Zostera residents, including fourspine stickleback (Apeltes quadracus), naked goby (Gobiosoma bosci), northern pipefish (Syngnathus fuscus), and rainwater killifish (Lucania parva); and small schooling species,including Atlantic silverside (Menidia menidia), mummichog (Fundulus heteroclitus) and bay anchovy (Anchoa

Habitat preference in fish and decapods

507

160

May

Jun.

Jul. 1988

Aug.

Sep.

May

JUIl

Jul. 1989

Aug.

Sep.

Figure 3. Mean values of vegetation parameters sampled biweekly in summers of 1988 and 1989. (a) Zostera marina density (shoots m ‘) at the two grass sites (, grassl; ----, grass2). (b) Zostern marina standing crop (dry weight in g m “) at the two grass sites (--, grassl; - -- -, grass2). (c) Ulva luctucu standing crop (dry weight in g mm*) at the two Ulvu sites (p, Ulvu 1; - - - -, Ulvu2). Ulva values reflect densities in patches selected for throw trap sampling and are not an estimate of the overall standing crop at each site.

mitchilli). Together these speciesrepresented 97.391, of the total fishes collected. Only four decapod specieswere abundant, the grassshrimps Palaemonetes vulgaris and Hippolyte pleuracanthus, the sand shrimp Crangon septemspinosa, and the blue crab, Callinectes sapidus. These speciescomprised 99.276 of the total decapods. Few other studies of fish and decapod communities in shallow estuarine habitats along the U.S. east coast have used quantitative sampling, limiting our ability to make geographic comparisons. Adams (1976) used drop nets in North Carolina Zostera beds and Weinstein and Brooks (1983) used Wegener rings in Virginia Zostera and tidal creek habitats. To broaden our base of comparison for the fishes we included studies using seining and trawling methods in North Carolina tidal creeks (Weinstein, 1979), Virginia

508

S. M. Sogard & K. W. Able

1. Results of two-way ANOVAs comparing Zostera marina shoot densities and standing crop at the two grass sites and Ulva standing crop at the two Ulva sites in the 2 years of sampling (1988 and 1989). Ulva measurements were made in patches selected for fauna1 sampling and were not randomly collected. All vegetation parameters were log-transformed prior to analysis TABLE

Factor Grass sites-shoot Site Year Site x year Grass sites-standing Site Year Site x year Ulva sites-Ulva Site Year Site x year

d.f.

MS

F

r

1 1 1

0.02 2.28 0.06

0.24 25.76 0.72

0.625 0.000 0.398

1 1 1

0.90 1.86 0.04

9.68 20.11 0.43

0.003 0.000 0.513

0.53 2.18 0.98

1.99 8.24 3.68

0,161 0,005 0.058

density

crop

standing

crop 1 1 1

tidal creeks (Richards & Castagna, 1970), Virginia Zostera beds (Orth & Heck, 1980), New York Zostera beds and unvegetated areas(Briggs & O’Connor, 1971), Connecticut unvegetated shallows (Warfel & Merriman, 1944) and Massachusetts Zostera beds and unvegetated areas(Heck et al., 1989). As expected, the New Jersey fish fauna was similar to New York and intermediate in composition compared to more northern and more southern areas.In New Jersey, the fish community in shallow waters was not dominated by spot (Leiostomus xanthurus), and other sciaenidswere rare compared to Virginia and North Carolina estuaries. Pinfish (Lagodon rhomboides), the most abundant speciesin North Carolina grassbeds,and severalother southern speciesadding to the fauna1richness of Virginia and North Carolina, were not collected in our throw trap samples.We did not collect any gadoids, but they occurred from New York to Massachusetts. Speciescommon in New Jersey but rare in Massachusetts included Gobiosoma bosci, toadfish (Opsanus tau) and Lucania parva. The dominant New Jersey speciesof S. fuscus, M. menidia and F. heteroclitus were common in all of these areas, and A. quadracus was common from Virginia northward. Information on decapod communities wasmore limited, including only North Carolina (Weinstein, 1979), Virginia (Heck & Orth, 1980), and Massachusetts (Heck et al., 1989). New Jersey estuariessupported very high densities of Palaemonetes vulgaris, which was rare in Massachusettsbut very common in Virginia. The high density of Hippolytepleuracanthus in New Jersey compared to other areasmay have been due to the greater efficiency of throw trap sampling for this small species. Marsh (1973) reported relatively high densities of H. pleuracanthus collected directly from Zostera blades in Virginia. Three speciesof penaeid shrimp were abundant in North Carolina, and Penaeus aztecus was common in Virginia, but no penaeids occurred in New Jersey or Massachusetts. We collected high numbers of Callinectes supidus, similar to Virginia and North Carolina, but this specieswas not present in Massachusetts. In a previous quantitative study of New Jersey habitats (Wilson et al., 1990b), however, C. sapidus was far less abundant, suggesting high inter-annual variation. Green crabs (Carcinus maenas) and rock crabs (Cancer irroratus) were common in Massachusetts but relatively rare in New Jersey. Crangon septemspinosa was a dominant fauna1component from Virginia northward.

Habitat preference in fish and decapods

509

In general, both the fish and decapod communities in shallow New Jersey habitats are dominated by specieswith relatively wide geographic ranges. The fauna appears to be transitional, with declining numbers of many southern speciesand initial appearance of several northern species. Habitat

comparisons

Fish and decapod densities differed markedly across the different sites and between vegetated and unvegetated substratesat the grassand macroalgaesites.Total fish densities were generally higher in Zostera or Ulva than in adjacent unvegetated substrates (Figure 4). Densities in Zostera were comparable to or higher than previously reported seagrass densities using similar sampling methods in Texas (Huh, 1984) and Florida (Sogard et al., 1987), but were overwhelmed by densities in the marsh creeks, which ranged up to 578 fish me2 (575 of the fish in this sample were M. menidia). Decapod densities were also higher in vegetation than adjacent sand/mud at the grass and Ulva sites. The highest density occurred at the creek1 site, but wasnot markedly greater than that in Zostera at the grass1site. Individual fish speciesdiffered in their habitat utilization (Figure 5). A. quadracus wasa Zostera specialist; it was present in high densities in Zostera at the grass sites, but was absent at other sites. S. fuscus inhabited both grassand Ulva sites, where it was consistently more abundant in vegetation than sand/mud. Pipefish were rarely caught in creeks. G. bosci was more abundant at grasssitesand was denser in vegetation at grassand Ulva sites, but was also abundant in the marsh creeks, which generally lacked the physical structure provided by vegetation. M. menidia was present throughout the estuary, but attained remarkable densities in the creeks. Silversides were largely responsible (80°, of the total fish) for the overall high fish abundance in creeks. F. heteroclitus and A. mitchilli also contributed to the high creek densities, but most other fish speciesdid not make extensive use of creek habitats (Table 2). Several fishes were clearly more abundant in vegetation; winter flounder (Pseudopleuronectes americanus), however, wasmost common on unvegetated substrates. For the decapods, P. vulgaris was much more abundant in vegetation (either Zostera or Ulva) than in adjacent unvegetated areas(Figure 6), but the highest densities were at the creek1 site, which ranged up to 558 shrimp m-‘, and accounted for the high total decapod densitiesat this site. H. pleuracanthus wasclearly dependent on Zostera habitat, but we did not consider it a true habitat specialist, since numerous individuals occurred in macroalgaeat the Ulva sites. C. septemspinosa wasa habitat generalist, abundant at all sitesand in all habitats, with slightly lower densities in creeks compared to the other sites. C. sapidus wasalsoa generalist, occurring in all habitats. The high blue crab densities in marsh creeks indicated that this habitat is an important nursery area in New Jersey estuaries. While blue crab densities at the Ulva sites were higher in the algae, there was no difference between vegetated and unvegetated substrates at the grass sites. Wilson et al. (19906) also found that blue crabs in New Jersey are habitat generalists. This pattern contrasts from that described by Orth and van Montfrans (1987) in ChesapeakeBay, where juvenile C. sapidus are highly dependent on Zostera as nursery habitat; mean densities in the Chesapeakewere 35.4 crabs m -2 in grassbedsand 1.3 crabs me2 in an adjacent tidal creek. Results of a priori multiple comparisons tests revealed several significant habitat contrasts (Table 3). Of the 11 speciestested, six were more abundant at grasssites than Ulva sites (contrastl). Fish densities were more closely linked to grass sites than were decapod densities, however, with only H. pleuracanthus more abundant at grasssitesthan

Veg

48

29 660 53 96 76 25 51 21 0 18 0 2 0 0

Species

Effort

Fishes: Menidia menidia Apeltes quadracus Fundulus heteroclitus Gobiosoma bosci Syngnathus fuscus Anchoa mitchilli Lucania parva Opsanus tau Pseudopleuronectes americanus Anguilla rostrata Fundulus majalis Tautoga onitis Hippocampus erectus Leiostomus xanthurus

Grass1

300 1 0 3 26 0 0 3 8 0 0 0 0 1

48

Unveg

53 238 2 77 18 1 71 14 1 3 0 0 0 0

48

Veg

Grass2

334 40 0 32 8 7 2 4 4 0 0 0 0 0

47

Unveg

193 0 0 16 17 11 0 2 3 0 0 16 0 1

47

Veg

mm1

206 0 0 6 10 26 0 2 7 0 0 0 0 2

51

Unveg

Site and habitat

70 0 1 11 35 0 0 0 4 0 0 3 6 0

43

Veg

Vlva2

35 0 6 6 11 1 0 0 6 0 0 0 0 2

50

Unveg

988 0 315 33 1 0 0 0 0 0 0 0 0 0

27

Unveg

Creek

1

1236 0 82 24 2 81 1 0 0 1 22 0 0 0

27

Unveg

Creek2

TABLE 2. Number of individuals of fishes and decapods collected in throw trap sampling in vegetation (either Zostera marina or Ulva lactuca) and unvegetated mud/sand at two grass sites, two Ulva sites, and two creek sites in southern New Jersey estuaries. All of the creek samples were considered to be unvegetated substrates. Effort = number of throw trap samples in each habitat type at each site

Habitat

preference

oo+oooooooo

-000000000~

o--o~oooooo

ooooooo--00

NOOOOOOOOOO

-0000000000

ON000~00000

00000000000

in fish and decapods

511

512

S. M. Sogard & K. W. Able

Ulva sites.At the grasssites, eight specieswere more abundant in Zostera than on adjacent bare patches (contrast 2); none of the speciestested was more abundant on unvegetated substrate. Similarly, at the Ulva sites (contrast 3), those specieswith a significant differencebetween Ulva and adjacent unvegetated sand/mud were always more abundant in the algae. Several of the specieslacking significant differences for this particular contrast were simply rare at the UZva sites(Table 2). When the two creek siteswere compared against all other sites (both vegetated and unvegetated habitats included), five specieswere more abundant at other sites (contrast 4). For the very common M. menidia, F. heteroclitus, P. vulgaris and C. sapidus, however, marsh creekssupported greater densities. Significant differences between the two sites of a particular habitat type were less frequent, but suggested that the grassl, Ulval and creek1 sites were superior in quality for several species,supporting higher densities than the grass2, Ulva2 and creek2 sites respectively (contrasts 5,6 and 7). Although the total catch of M. menidia washighest at the creek2 site (Table 2), it was dominated by two extreme samples.Following log-transformation, the meanMenidia density was higher at the creek1 site (Table 3). Our results comparing the distribution of individual speciesamong different habitats were similar to those of Wilson et al. (in review). Gobiosoma bosci, however, was higher in Ulva patches than in Zostera in Wilson et al. (in review). The mean standing crop of Ulva in selected patches was lower in the present study, particularly near the end of the summer, when gobies were settling in the estuaries. In extremely dense Ulva patches (mean = 407 g dry weight me2), we have found G. bosci densities of over 70 fish mm2 (unpubl. data). This speciesmay select Ulva patches as habitat, but only when the algae forms thick mats that are more likely to remain in place than the small, temporary patches present at the Ulval and Ulva2 sites. Another contrast of our results with Wilson et al. (in review) was the lower total fish densities in the latter study, perhaps a result of inter-annual variation. Fish densities increasedmore than two-fold from 1986 to 1987(Wilson et al., in review) but still did not approach the densities we observed in 1988 and 1989. Differing capture efficiencies between suction samplers and throw traps may also explain this contrast. The former method involved moving a cumbersome l-m diameter cylinder into position on the desired substrate, perhaps allowing more fish to escape.Differences in decapod densities between

the two studies

were primarily

due to our exclusion

of xanthids

and pagurids

from the analysis; Dyspanopeus sayi, for example, was very abundant and showed a clear preference for Zostera habitat in Wilson et al. (in review). Suction sampling techniques may be preferable for decapods, with more accurate sampling of burrowing species,but throw traps may be more effective in capturing fishes. Results of this study were generally similar to those of Sogard (1989) for Zostera and adjacent sand/mud substrates (macroalgae and creek habitats were not examined in the latter study). Exceptions were the higher densities of C. septemspinosa and G. bosci in unvegetated areasthan in Zostera beds in Sogard (1989). For the ubiquitous C. septemspinosa, this difference was probably due to chance and not habitat preference. For G. bosci, the length-frequency distribution of fishes collected from sand/mud in Sogard (1989) was skewed toward smaller sizes, suggesting that gobies initially settled on unvegetated substrateswith subsequentmigration into Zostera beds. At the Zostera sites used in the previous study, 409;, of the gobies collected were < 15 mm standard length (61o,, of the gobies collected on sand were < 15 mm). In the present study, fewer newly settled gobies were collected, with only 197, of the total < 15mm. These smaller individuals were not proportionately more abundant on unvegetated substrates,where 207; of the gobieswere

Habitat

preference

in$sh

and decapods

513

120

(0)

(bl

100

60 40 20 0

GI

G2

Ul

u2

Cl

c2

me

Figure 4. Mean densitiesm-* of (a) fish (all species combined) and (b) decapods (all species combined except xanthids and pagurids) collected at six sites in New Jersey estuaries. Error bars are standard errors. Samples were collected from May through September in 1988 and 1989 at grass (Gl and G2) and Ulva sites (Ul and U2), with both vegetated ( q ) and unvegetated ( W) substrates sampled. Creeks (Cl and C2) were sampled only in 1989. All creek samples were considered to be unvegetated; thus, there are no vegetated creek habitats.

“E h

05 ”

2 E 2

GI

G2

UI

U2

Cl

0.0

C2

GI

G2

u2

UI

Cl

c2

1

P 2-5 I 2.0 I.5

0.5 0.0

i GI

G2

u2 UI Site

Cl

c2

Cl

(

Site

Figure 5. Mean densities m 2 of common fish species collected at six sites in New Jersey estuaries. (a) Apeltes quadracus, (b) Syngnathus fuscus, (c) Gobiosoma bosci, (d) Menidia menidia. Error bars are standard errors. Samples were collected from May through September in 1988 and 1989 at grass (Gl and G2) and Ulva sites (Ul and U2), with both vegetated (ffl) and unvegetated (B) substrates sampled. Creeks (Cl and C2) were sampled only in 1989. AH creek samples were considered to be unvegetated; thus, there are no vegetated creek habitats.

S. M. Sogard & K. W. Able

514

“E h

25 0

2c

0

GI

G2

UI

GI

G2

UI

U2

Cl

C2

GI

G2

UI

u2

Cl

c

GI

G2

UI

Site

U2

Cl

C2

U2

Cl

C2

We

Figure 6. Mean densities m * of common decapod species collected at six sites in New Jersey estuaries. (a) Palaemonetes vulgaris, (b) Crangon septemspinosa, (c) Callinectes sapidus, (d) Hippolyte pleuracanthus. Error bars are standard errors. Samples were collected from May through September in 1988 and 1989 at grass (Gl and G2) and Ulva sites (Ul and U2), with both vegetated (PI) and unvegetated ( W) substrates sampled. Creeks (Cl and C2) were sampled only in 1989. All creek samples were considered to be unvegetated; thus, there are no vegetated creek habitats.

< 15mm. Thus, the hypothesized settlement pattern was not supported in the present study, but we did not encounter enough early recruits to adequately test it. Relative

importance

of different

habitats

Vegetation is a vital element of habitat quality in the shallow bays of New Jersey estuaries.Both Zostera and Ulva supported significantly higher densitiesthan on adjacent unvegetated substrates.Between the two vegetation types, Zostera appearedto be superior habitat to Ulva for epibenthic fishes. Ulva provides a significant refuge from predation (at least for juvenile blue crabs; Wilson et al., 1990a) and supports faster growth than Zostera habitats for somejuvenile fishes (Sogard, 1990), but fish densities were usually higher in Zostera. BecauseUlva lactuca is an ephemeral, unpredictable habitat, with standing crops varying greatly in time and space,it may be a lesspreferred habitat compared to Zostera beds, which are more predictable in location and seasonaldevelopment. Thus, Ulva is an important habitat in areas lacking seagrass,but cannot be considered an equivalent substitute for Zostera. The only fish speciesthat appeared to be dependent on Ulva asa nursery habitat wasthe tautog (Tautoga onitis). Although not abundant enough to be included in apriori comparisons,tautogs occurred only in vegetation and were more abundant in Ulva than Zostera in both this study and Wilson et al. (in review). The brilliant green colouration of juvenile T. onitis closely matchesthat of Ulva lactuca, providing a cryptic background. In addition, growth rates of juvenile T. onitis are higher in Ulva than in Zostera habitats (Sogard,

Habitat

preference

in fish and decapods

515

TABLE 3. Results of a priori multiple comparisons, testing for significant differences between pre-determined sets of habitat groups. Design of contrasts is explained in the Methods section. For significant differences, the habitat group of higher density is listed, with the level of significance. Blanks indicate no significant difference between the two habitat groups of that contrast Contrast

Species Fishes: Apeltes quadracus Gobiosoma bosci Syngnathus fiscus Lucania parva Menidia menidia 0psanus tau Fund&s heterocfitus Decapods: Palaemonetes vulgaris Crangon septemspinosa Hippolyte pleuracanthus Callinectes sapidus

1 Grass sites vs. Ulva sites

2 Veg vs. unveg (grass sites)

Grass‘ Grass’ Gras9 Grass‘

Grass’ Grass’ Grass’ Grass’

Grass‘

Grass’ Grass” Grass<

Ulva’ Grass’ Ulva’

Grass’

3 Veg vs. unveg (Ulva sites)

Ulvab

4

Creeks other

Grass1 vs. grass2

Other

Grassl’

Othef

Grasslc

VS.

Creeks OtheP Creeks Ulva’ Ulva’ Ulva”

5

Creeks’ Other‘ Other Creeks’

6

Ulval

7

Creek1

VS.

vs.

Ulva2

creek2

Creekl” Grassl“

Grass2

Creekl.

Ulvalb

Creekl’ Creekla

“P < 0.05. bP
1990). For this species, Ulva may be preferred over Zostera despite the ephemeral nature of the macroalgaemats. Of the four common decapod species,in contrast to the fishes, only H. pleuracanthus had higher densities in Zostera than in Ulva. The rarity of this species in other habitats demonstrated a dependence on Zostera marina. For the other three species, Ulva was comparable or superior in quality to Zostera. The largely unvegetated areas that we sampled in the marsh creeks supported high fauna1densities, in marked contrast to unvegetated substrates outside the marshes.High densities in the creeks occurred for only a few species,however, all of which were also common in other habitats. Wilson et al. (1990~) measured predation rates on juvenile Callinectes sapidus in the samehabitats used in this study. Vegetation (either Zostera or L&a) provided a significant refuge from predation, but marsh creeks had predation rates as high as those on unvegetated substrates outside the creeks. Thus, the creek habitat probably does not provide a protective refuge from predation, and some other factor promotes the high densities of fishesand decapods. Of the abundant speciescollected in this study, only two were clear habitat specialists, with Zostera marina a critical habitat for Apeltes quadracus and Lucania parva. Hippolyte pleuracanthus was highly dependent on Zostera but was also present at the Ulva sites. Several other specieshad higher densities in Zostera, but were also common in other habitats.

quadracus

sapidus

pleuracanthus

Callinectes

Hippolyte

ocellatus

septemspinosa

Ovalipes

americanus

vulgaris

Crangon

Decapods: Palaemonetes

mitchilli

Anchoa

heteroclitus

Fundulus

parva

tau

Opsanus

Lucania

onitis

Tautoga

bosci

menidia

l’seudopleuronectes

Gobiosoma

Menidia

Syngnathusfuscus

Apeltes

Fishes:

Species

(645) 0.06 (35.4)

O-06 (23.0)

6.28 (105) 18.50 (3.7) 0.28 (35.0) 3.64

0.02 (54.0) 0.00 0.00

7.32 (11.7) 25.14 (3.9) 0.48 (38.0) 3.32 (7.2) 0.04 (28.3)

0.00

0.07 (94.0) 1.23 (29.0) 0.00

0.23 (46.2) 0.00

0.18 (40.0) 0.00 0.00

Late

4.20 (21.0) 0.48 (46.9) 6.02 (18.1) 0.00

June

3.90 (16.0) 0.18 (45.0) 3.20 (15.0) 0.00

Early

(6.3)

0.23 (25.0) 3.04

(8.2)

8.33 (11.4) 12.27

0.06 (42.0) 1.00 (45.0) 0.00 0.00

1.63 (13.0) 0.13 (1115) 0.11 (10.0) 0.04 (24.0) 0.65 (30.0) 0.00

May

0.02 (3.9)

(4.8)

7.72 (10.8) 50.78 (3.9) 0.39 (39.6) 3.56

2.67 (22.0) 1.02 (70.9) 7.41 (38.0) 0.00 0.06 (56.0) 0.09 (12.0) 0.09 (21.0) 1.93 (23.0) 0.02 (19.0) 0.00 -

Early

July

6.69 (5.4) 0.06 (10.7)

(84)

9.26 (9.4) 42.59 (4.0) 2.30

2.24 (23.1) 0.87 (74.1) 3.35 (24.3) 0.26 (12.0) 0.02 (68.0) 0.06 (10.8) 0.06 (19.7) 1.89 (26.0) 0.11 (12.1) 0.00

Late

0.13 (8.8)

(5.6)

9.72 (7.5) 31.17 (42) 3.17 (9.0) 3.72

1.98 (24.0) 0.52 (91.1) 5.74 (40.3) 0.78 (16.0) 0.04 (45.0) 0.22 (21.6) 0.30 (29.7) 1.72 (31.0) 0.54 (15.1) 0.00 -

Early

Late

17.99 (8.9) 15.57 (4.1) 3.54 (13.0) 5.04 (4.0) 0.00 -

1.00 (25.1) 0.44 (102.2) 2.24 (46.0) 1.39 (22.0) 0.04 (81.5) 0.03 (40.0) 0.11 (41.0) 0.25 (30.0) 0.42 (21.4) 1.53 (22.0)

August

25.13 (9.8) 15.72 (4.1) 4.54 (12.9) 8.17 (4.1) 0.02 (9.0)

1.00 (28.0) 0.28 (109.0) 2.50 (26.0) 1.50 (25.6) 0.00 0.04 (44.2) 0.13 (37.0) 0.22 (32.0) 1.06 (19.4) 0.59 (21.0)

Early

Late

43.98 (10.6) 16.63 (4.6) 4.80 (11.0) 11.37 (4.4) 0.02 (16.8)

0.85 (28.0) 0.30 (110.0) 4.17 (41.0) 1.85 (26.0) 0.02 (79.9) 0.00 0.11 (58.0) 2.02 (35.4) 0.33 (22.7) 0.33 (14.0)

September

TABLE 4. Mean density m 1 and median size in mm (in parentheses) of individual species collected during each sampling period. Sizes are standard length for fishes, carapace length for shrimps and carapace width for crabs. Data from all sites and habitats and both years combined

Habitat

preference

Recruitment

infish

and decapods

517

patterns

With our biweekly sampling schedule we were able to track the recruitment of juveniles to estuarine nursery grounds (Table 4). Median sizesof individuals caught in eachbiweekly collection indicated that A. quadracus, M. menidia and P. americanus recruited in early spring, during the period of increasing water temperatures (Figure 2). Small juveniles of C. septemspinosa, S. fuscus and F. heteroclitus appeared in the estuarine habitats in June. For the remaining species,however, recruitment of newly settled juveniles did not begin until July or August, well after water temperatures had climbed above 20 “C (Figure 2), and abundancesdid not peak until late summer or early fall. Thus, although the potential period of estuarine utilization in this north temperate system is presumably limited, several species do not begin to exploit estuarine habitats until late in the summer. Densities of all of these species(except C. septemspinosa) decline to near zero in shallow water habitats during the winter (Wilson et aE., in review), and we can conjecture that successful use of estuarine resources during the summer is important to overwinter survival. Post and Evans (1989), for example, found that slowly growing young-of-theyear yellow perch (Percaflavescens) were more likely to succumb to starvation during the winter than individuals with fast growth rates during the preceding summer. Species that are not utilizing estuarine nurseries early in the seasonmay be somehow limited to late summer recruitment. Events during the spawning period and subsequent larval stagemay preclude earlier exploitation of nursery habitats. For example, if initiation of spawning requires increased water temperatures and larval development takes H weeks,newly settled juveniles would not appearuntil July, even though resource conditions in the nursery habitats appear suitable well before then.

Conclusions Our results provide another example of the importance of vegetated habitats for small fishes and decapods. Zostera is superior to Ulva as habitat for most fishes, but both vegetation types are preferable to unvegetated sand/mud substrates. Saltmarsh creeks can support very high densities, but only for a few habitat generalists that are common throughout the estuary. In southern New Jersey estuarieslacking Zostera, Wva provides a preferred habitat compared to unvegetated sand/mud substrates, but the lower fish densities indicate that Ulva does not provide an equivalent substitute for Zostera. The exception occurs with juvenile tautog, which appear to preferentially exploit Ulva patches and may be dependent on them for nursery habitat in this system. Decapod crustaceans (excluding xanthids and pagurids), in contrast, are not dependent on Zostera, with the exception of Hippolytepleuracanthus. For decapods, Ulva lactuca is equivalent or superior in habitat quality compared to Zostera.

Acknowledgements We thank Dan Roelke for his dedicated assistancein throw trap sampling and laboratory processing. Leslie Hartman, Tom Auletta, Nathan Able and other personnel of the Rutgers Marine Field Station provided additional support in the field. We thank Ken Heck and Kim Wilson for constructive ideas and suggestions.This study was funded in part by grants from the Electric Power ResearchInstitute, the National Audubon Society, the New Jersey Marine Sciences Consortium, the Leathem Fund and the Manasquan

518

S. M.

Sogard

& K. W. Able

Marlin and Tuna Club. This paper is Rutgers University Sciences contribution number 91-36.

Institute

of Marine and Coastal

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