BULLETIN OF MARINE SCIENCE. 30(3): 537-551. 1980



## THE ROLE OF SEAGRASS BIOMASS IN THE ORGANIZATION OF BENTHIC MACROFAUNAL ASSEMBLAGES
Allan W. Stoner

ABSTRACT

A one year survey of benthic macrofauna was conducted over a series of subtidal sites characterized by different standing crops of benthic macrophytes. Since the stations had similar granulometric properties, the role of seagrass biomass in regulating community organization of benthic macrofauna could be tested independently, unlike earlier studies. The density of macrobenthic animals (N/m") was directly related to mean macrofloral biomass as
was the number of species taken over the sampling period. The unvegetated site was characterized by the highest degree offaunal dominance and a species composition distinct from that found at vegetated sites. The relative abundances ofepifaunal amphipods and epifaunal polychaetes were directly related to macrophyte biomass. Abundance ofdeposit feeding and omnivorous polychaetes decreased as a function of macrophyte standing crop, whereas suspension feeding and carnivorous polychaetes increased with vegetation. Biomass ofbenthic macrophytes, independent of sediment granulometry and hydrodynamic effects, was an important regulator of species abundances, dominance, diversity, and trophic organization in macrofaunal assemblages. 

### Seagrass meadows have long been recognized as an important source offood and shelter for benthic invertebrates and a nursery ground for fishes (Phillips, 1960; den Hartog, 1967; Wood et aI., 1969; McRoy, 1977; and others). Consequently, several studies have been conducted to assess faunal differences among vegetated and unvegetated habitats. O'Gower and Wacasey (1967)found highest numbers ofmacrobenthic infauna and lowest faunal dominance associated with Dip/anthera (Ha/odule) wrightii 

### beds. The number of species and density of animals was significantly lower inbare sand habitats. Taylor and Lewis (1970), Orth (1977), and Brook (1978) also found differences among the faunas ofsites with different degrees of macrophyte cover. In all ofthese studies, however, major granulometric differences inthe sediments accompanied differences inseagrass biomass. Based on experimental manipulation of seagrass blades, Orth (1977)concluded that sediment stability created by seagrass rhizomes and blades
was responsible for the composition and diversity ofseagrass infauna. Only one study (Santos and Simon, 1974)tested for differences among stations characterizedbydifferent macrophyte species andbare sand where sediment granulometry
was not related to seagrass cover. Although faunal densities were higher insamples taken from vegetated sites, they found that polychaete species composition
was homogeneous through vegetated and unvegetated sediments when granulometry was constant. When preliminary analyses indicated macrofauna-macrophyte relationships different from those reported by Santos and Simon, a study was designed to decouple the role ofmacrophyte biomass from that ofsediment granulometry inthe regulation of species composition and abundance in Apalachee Bay, Florida. Emphasis was placed on species composition and life histories ofthe animals found at stations with different macrophyte biomass.

SAMPLE SITES

Based on long-term macrophyte data for Apalachee Bay, Florida (Zimmerman and Livingston, 1976; Livingston, unpublished data), four collecting sites were chosen from shallow subtidal regions

537538

• E 12

APALACHEE
BAY BULLETIN OF MARINE SCIENCE, VOL. 30. NO.3. 1980



## N 

# t

234 

### I I I

Figure 1. Locations ofcollecting sites offthe Econfina and Fenholloway drainage systems inApalachee Bay, Florida.

offshore from the mouths ofthe Econfina and Fenholloway Rivers (Econfina ]0and 12,Fenholloway 11and ]2; Fig. 1).The exact location ofeach station was identified with apermanent marker after SCUBA diving confirmed the location as representative ofabroad area, Fenholloway station 11had
a low total macrophyte standing crop with an annual mean of9.3 gdry wtJm2

, This station willbe referred toastheunvegetated station. Fenholloway station 12hadamean annual macrophyte biomass of 141gdry wtJm2 and mean biomasses at Econfina 10and Econfina 12were 215 and 320gdry wtJ
m2

, respectively (Livingston, unpublished data). The macrophytes at all sites were dominated by mixtures of Thalassia testudinum and Syringodium filiforme, except at Fenholloway 11where Halophila engelmanni comprised most ofthe very sparse vegetation.

Allofthe sampling sites werepolyhaline with salinities ranging from 17to34%0. Water temperatures, similar at all four stations, ranged from 7.2°C in December, 1976, to 32.2°C in July, 1977. On no sampling date was dissolved oxygen, atthe surface orbottom, less than ]00% ofthe saturation level.

METHODS
Benthic macrofauna were collected monthly (December, 1976to November, 1977)with hand held
cores (7.6 cm diameter, 45.36 cm2) from field quaduants sampled onthe same date forbenthic macrophytes. Although preliminary analysis showed that 98% ofthe macrofaunal numbers were in or above the top 5cm ofsediment andno animals were found below 10cm, cores were taken to adepth of 12em to insure adequate sampling. Based on 20replicates at two stations in the fall of 1976and the species accumulation analysis described by Livingston etal. (1976) ]2cores were found to sample 90%ofthebenthic macrofaunal species. Spring analysis confirmed theappropriateness ofthe sampling strategy.

Core samples were sieved inthe fieldusing a screen with 0.5 mm mesh. Use ofa 1.0mm screen, acommon practice inprior studies, allowed passage of 14%ofthe macrofaunal species and 47% of the macrofaunal numbers (Lewis and Stoner, 1980). Samples were stained with rose bengal (approx. 100mg/I)and preserved with a 10%formalin-seawater mixture.

The benthic samples were sorted by hand and all amphipods, isopods, decapods, molluscs, polychaetes, echinoderms, and mysids were identified to species. Less numerous taxa such as oligo-STONER: SEAGRASS AND ORGANIZATION OF MACROBENTHOS 539

Table I. Summary of sediment characteristics at four sampling sites in Apalachee Bay, Florida, December 1976,to November 1977(mean ± S.D., n for organics = 12,n for granulometry =4; * indicates that mean values were notdifferent, ANOVA and Duncan's Multiple RangeTest, P < 0.05)

CharJ.cter Fen II Fen 12 Eeon 10 Econ 12 Organics

(%ofdry wgt) 2.84 ± 0.84* 5.64 ± 0.70 4.4] ± 0.76 2.97 ± 0.50* Mean Grain Diam.

(Phi) 2.32 ± 0.13* 1.98 ± 0.16' 2.20 ± 0.3]* 2.33 ± 0.10* (mm) 0.201 ± 0.0]8* 0.254 ± 0.030* 0.221 ± 0.052* 0.200 ± 0.014' Sorting Coef.

(phi) 0.86 ± 0.15* 1.31 ± 0.12* 0.96 ± 0.39* 0.74 ± 0.06*

chaetes, nemerteans, and cumaceans were counted but not identified. Ash-free dry weights were determined for each ofthe benthic species (by season for the most numerous animals) by drying at 100°Cfor 24hr followed by ashing at 550°Cfor 4hr. Ash-free dry weight was estimated for a few rarely encountered species.

The top 5 em ofone core, unsieved and unpreserved, was taken for sediment analysis on each sampling date and station, aportion ofwhich was dried at 100°Cand ashed at 550°Cto determine the organic content ofthe sediment. After washing toremove salts anddrying, another sediment subsample ofapproximately 60gwas analyzed for granulometric properties using standard Ro-Tap procedures. Allgrain sizes less than 62 micrometer (4.0 phi) were lumped as the clay-silt fraction; however, this fraction never exceeded 4.0% ofthe total mass.

RESULTS 

### Sediments Granulometric properties ofthe sediments were statistically similar at allfour stations (ANOYA, P 

## < 0.05) with mean grain diameters inthe finesand category (Table I). Sorting coefficients less than 1.0phi at Econfina 10and 12and Fenholloway 11indicate that the sediments there are wellsorted. Although the sorting coefficient at Fenholloway 12was not statistically different from those found at the other stations, the sediments there were less well sorted. The presence of shell rubble at Fenholloway 12was responsible for a mean grain diameter and sorting coefficient slightly higher than other stations. Sediment organics were determined for each sampling date, but no seasonal trends occurred. The mean values forindividual stations were different; however, with the highest percentages oforganics at Fenholloway 12(5.64%) and lowest at Fenholloway 11(2.84%). Organic contents were 4.41% and 2.97% at Econfina 10and 12,respectively. Sediment organic contents were statistically similar at only Fenholloway 11and Econfina 12(ANOYA and Duncan's Multiple Range Test, P 

## <0.05).
Benthic Macrofauna Five hundred sixty-six core samples taken over 12months yielded 7271 macrobenthic animals, most ofwhich were amphipod crustaceans andpolychaetous annelids. One hundred seventy species were collected including 75polychaetes, 31amphipods, 21molluscs, 10shrimp, 6crabs, and 5isopods. The mean density for all stations and dates was 2827 animals/m2

, 

### but values ranged from 833 to 830l/m2 

### (Table 2). The unvegetated station, Fenholloway 11,

### had alower density ofanimals than vegetated sites (x 

## = I754/m2); 

### however, the three vegetated stations had similar mean densities (ANOYA and Duncan's Multiple Range Test,540 BULLETIN OF MARINE SCIENCE, VOL. 30, NO.3, 1980

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Table 3. Indices ofdominance for IOtalmacrofauna, amphipod fauna, and polychaete fauna at the four sampling stations (See text for definition ofthe indices)

Index Fen II Fen 12 Econ 10 Econ 12 Total Macrobenthos OI 0.301 0.175 0.198 0.200 Od 0.157 0.124 0.112 0.104 Amphipods
01 0.676 0.262 0.406 0.397 Od 0.393 0.138 0.236 0.220 Polychaetes
D1 0.546 0.378 0.355 0.333 Od 0.389 0.278 0.251 0.233



### P 

## < 0.05) ranging from 3107to 3293/m2

• 

### Also, Fenholloway II carried a lower macrofaunal biomass (7.81 gm/m2 ) 

### than the vegetated stations (10.00to 17.85gml


### m2); 

### however, the differences were not statistically significant due towide monthly variation. Because ofsensitivity ofthe biomass value to large, infrequently occurring species, primary emphasis inthis study was placed on the numbers of animals and species composition rather than biomass. Twelve cores produced amean of24species per month at station Fenholloway 11,42 species atFenholloway 12,and 38species atboth ofthe Econfina stations. Overall, however, atotal of82 species were collected at Fenholloway 11, 110at Fenholloway 12, 109at Econfina 10,and 117at Econfina 12.Regression analysis showed that the total number of species collected was directly related to the
mean macrophyte standing crop at acollecting site (R = 

### 0.917, P 

## <0.10).

Macrofaunal density peaked between April and May atallbut Econfina station 12which showed peak densities inFebruary and in May. Density generally declined during the late summer and fall. However, November collections on all stations showed very large increases in macrofauna following a sharp decline in fishpopulations inOctober and November, 1977(Stoner, 1979).Biomass showed less distinct seasonality, but the numbers ofspecies collected appeared to reflect the numerical density ofanimals at allfour stations. Macrofauna ofthe unvegetated station wascharacterized byagreater degree of dominance than other stations (DI 

## = 0.301, Dd

## = 0.157) (Table 3), where DI is the dominance index ofMcNaughton (1967)(DI = 

### nl 

## + n/N) and Ddisthe dominance index ofBerger and Parker (1970)(Dd = 

### nI/N). 

### n1 

### and n2 

### are the absolute abundances ofthe first and second most numerous species and N is the total count ofanimals. High dominance at Fenholloway 11 was largely due to high numbers ofthe polychaete Mediomastus californiensis 

### and the amphipod Ampelisca verrilli. 

### Lowest macrofaunal dominance wasfound atFenholloway station

12(DI = 

### 0.175, Dd = 

### 0.124).

Species composition atthe four stations was analyzed with the similarity coefficient rho (Van Belle and Ahmad, 1974)and aflexible grouping cluster strategy (beta 

# = -0.25). Based onthe 85most numerous taxa, the Econfina stations were most similar (rho 

# = 0.91), and Fenholloway 12was similar to Econfina stations (rho 

# = 0.83). The composition ofthe benthic community at Fenholloway II was distinct from that at the vegetated stations (rho 

## = 0.59).542 BULLETIN OF MARINE SCIENCE, VOL. 30. NO.3. 1980

Table 4, Relative abundances and microhabitats ofamphipod species collected at the four sampling sites; each value is the percentage of the total amphipod fauna for a given station which was represented by each species [Microhabitat information from Bousfield (1973) where: A

### = epifaunal freeliving, B

### = epifaunal domicolous, C 

### = infaunal tube-builder, D = infaunal free-burrower, E 

### = motile tube, F

### = commensal, and G 

### = both epi- and infaunalJ

MicroSpecies Fen II Fen 11 Econ 10 Econ 12 habitat Elasmoplls lel'is 2.2 ]3,8 23,6 12,6 A Ampelis('a \'errilli 39.3 1.2 3.9 1.0 C Ampelis('a wldorum 28.3 5,2 6.4 4.2 C Cymadusa sp A 2.0 9.0 17.0 10.0 B Lembos sp A 0.7 9.2 6.5 22.0 B Rlidilemboides naglei 0,7 4.3 9.4 17.7 B Lysianopsis alba 2.2 12.4 10.8 3.1 D POlltogellia sp A 1.7 6.1 3.9 4.8 G Ba/ea ('a/lwrinensis 5.9 5.4 1.9 4.1 A SYllchelidilim americallllm 4.7 5.2 1.9 2.5 G Carinobatea tricarinata 0.3 2.4 2.4 8.7 A Melita appendicl/lata 1.7 7.2 1.4 0.1 A Gitallopsis sp A 0.3 2.4 2.8 1.3 A? Parl/caprel/a tellais 1.0 2.3 1.1 1.6 A Corophium sp A 2.5 1.6 0.1 0.1 D Parl/phoxas sp A 0.3 1.7 1.7 0.8 D Tirolltropakis 0.5 3.4 0.1 0.2 ? Listriel/a sp B 1.0 0.1 0.1 0.2 F Stellethoe sp A 0.5 0.4 0.3 0.2 A Lysiallopsis hirsllta 2.1 1.4 3.8 D Ceraplls sp A 2.3 0.6 0.5 E Gammarlls macromllcrollallts 0.1 0.1 0.1 A Corophillm IOllisiallum 3.9 0.8 1.3 D Erichthollills sp A 0.2 1.2 1.0 B C%mas/ix sp A 0.2 0.1 F Gammarus mucrollatlls 0.1 0.1 A Listriel/a sp C 0.\ F Leucothoe sp A 0.2 F Heteroph/ias sec/usus 0.\ ? Lis/riel/a sp A 0.\ F Stella/hoe sp B 0.2 A



### Amphipoda Amphipods made up approximately 37%ofthe macrofaunal numbers taken at Fenholloway stations and 47% ofthe macrofauna at Econfina stations. Although 61%ofthe amphipod species collected were found atallfour stations, the relative proportions ofthe species differed (Table 4). Most striking was the rare occurrence ofElasmopus levis, Lembos sp A, and Rudilemboides naglei at the unvegetated station, Fenholloway 11. These species were abundant at vegetated stations. Two large, tube-building species, Ampelisca verrilli and A. vadorum, dominated the amphipod fauna at FenhoJloway 11 (DT 

## = 0.676, Dd

# = 0.393). Vegetated stations inthis study showed much lower numerical dominance than the unvegetated site (Table 3). Amphipods never associated withthe unvegetated station were Lysianopsis hirsuta, Cerapus sp A, and Colomastix sp A. Other species were collected too infrequently to determine ifdefinite associations occurred, but only one amphipod species taken at Fenholloway 11, Stenethoe sp B, was not collected at vegetated stations. In all, 22 amphipod species were collected at Fenholloway 11; and 25, 26, and 27 species were taken over theSTONER: SEAGRASS AND ORGANIZATION OF MACROBENTHOS 543 

### sampling period at stations Econfina 12, Fenholloway 12,and Econfina 10, respectively.

When applied to amphipod species composition cluster analysis similar to that employed forthe total macrobenthos showed the relative abundance ofamphipod species to be most similar at Econfina 10and Fenholloway 12(rho = 

### 0.94). The composition ofamphipod fauna at Econfina 12was also similar to that at the other vegetated sites (rho 

## = 0.88); however, the amphipod fauna at the unvegetated site was distinct from that at vegetated sites (rho = 

### 0.48).

The mean density ofamphipods was highest at Econftna station 10(1578/m2) 

### and lowest at Fenholloway 11(641/m2); 

### however, the mean densities ofamphipods were not different statistically at stations Fenholloway 11 and 12, and at stations Fenholloway 12, Econftna 10and Econfina 12(ANOVA and Duncan's Multiple Range Test, P 

## < 0.05). Seasonal patterns in amphipod density (Table 5) reflected the station patterns for total macrofauna. Greatest abundances of amphipods were found between April and May, with a peak density of 5047 amphipods/m2 

### atEconfina 10inMay. Densities and biomasses were lowest inthe fall. Based on Bousfield (1973), amphipods collected inthis study were categorized
as (1) epifaunal free-living, (2) epifaunal domicolous, (3) infaunal tube-building, (4)infaunal free-burrowing, (5)motile tube dwellers, (6)commensals, and(7)both epifaunal and infaunal (Table 4).Although the number ofspecies from each group is similar forthe four stations, the percentages ofthe total amphipod populations falling into the different microhabitat categories were different. Free-living epifaunal types made up between 28.9 and 34.2% ofthe amphipod fauna at the vegetated sites. Only 12.1% ofthe amphipods at the unvegetated site fit this classification. Likewise, amphipods categorized as domicolous epifauna were abundant at vegetated stations (23.5 to 49.7%) and were rare at the unvegetated station (3.6%). The relative abundance ofepifaunal amphipods (free-living and domicolous) was directly related to mean macrophyte biomass forastation (R

## = 0.959, P 

### <0.05). The reverse trend wastrue forinfaunal tube-building amphipods (namely Ampelisca verrilli and A. vadorum), where 67.6% ofallamphipods collected at Fenholloway 11were ofthis type. The highest representation by 

### these
same amphipods at a vegetated station was 10.2% at station Econfina 10. No clear pattern existed for the relative abundance offree-burrowing amphipods; however, the relative abundance of all infaunal amphipods (tube-building and free-burrowing) was inversely related tomean macrophyte biomass (R

## = -0.912, P 

## <0.10).

Polychaetes Polychaetous annelids made up40and 45% ofthe total number ofmacrofauna at Fenholloway stations 11and 12,respectively; and 35and 33% ofthe macrofauna atEconfina stations 10and 12.Of75species ofpolychaetes collected, only 34 were taken at Fenholloway 11. Fifty were taken at Fenholloway 12, 47 at Econftna 10,and 54species were collected at Econfina 12.The polychaete fauna at the unvegetated site was dominated by the capitellid Mediomastus californiensis and the arabellid Lumbrineris tenuis (38.9 and 15.6%ofthe polychaetes, respectively) (Table 6).Aricidea taylori was the only other worm which made up more than 5%ofthe total polychaete collection atFenholloway 11;

### however, this
was the most numerous worm at vegetated sites. As seen for the amphipod assemblages, the degree ofdominance inpolychaete fauna was less at vegetated stations than at the unvegetated station (Table 3). Syllids and serpulids were544 BULLETIN OF MARINE SCIENCE, VOL. 30, NO, 3, 1980

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### ~ ~STONER: SEAGRASS AND ORGANIZATION OF MACROBENTHOS 545

Table 6. Relative abundances, microhabitats, and trophic groups of polychaete species collected at the four sampling sites. Each value is the percentage ofthe total polychaete fauna for a given station which was represented by each species. Life history information was taken from: Sanders, 1960; Sanders et aI., 1962; Pettibone, 1963; Oay, 1967; Young and Rhoads, 197]; Bloom et aI., 1972; Santos and Simon, 1974; and Jumars and Fauchald, 1977. M

### = motile, OM 

### = discretely motile, S

### = sessile, B

### = burrower, E

### = epifaunal, SOF 

### = selective deposit feeder, NSOF 

### = nonselective deposit feeder, C 

### = carnivore, 0 

### = omnivore, H 

### = herbivore, SF 

### = suspension feeder

Species Fen II Fen 12 Econ 10 Econ 12 Life History
Aricidea tay/ori 11.0 27.8 25.1 23.3 M B SOF Priollospio heterohrallc/lia 3.5 3.3 6.7 7.7 OM B SOF P/atYllereis dlllllerilii 0.9 7.2 5.6 6.2 M B,E H EWRolle dispar 0.7 2.3 4.1 9.9 M B C,O Gyptis hrel'ipa/pa 2.9 4.5 3.7 2.0 M B C Don'i/lea sociahilis 0.2 9.6 0.3 1.5 M B SOF G/ycillde solitario 4.9 2.8 2.9 1.I M B C Fahricia sp A 0.9 0.6 3.9 5.6 OM B SF C/ymellel/a mllcosa 1.6 1.1 4.5 1.4 S B SOF Cirriformia jiliRera 1.3 2.4 2.7 1.8 OM B SOF Phyl/odoce fragi/is 0.4 1.7 2.3 2.5 M B,E C Hap/osc%p/os fragilis 0.2 J.3 2.3 2.6 M B NSOF Sahella sp A 0.9 1.0 4.5 1.5 S B SF G/ycera americalla 3.1 1.3 1.5 0.8 M B C,O MaRe/olla pettiholleae 1.1 1.0 2.2 0.2 OM B SOF Pectillaria gOIl/dii J.3 0.4 0.3 0.2 M B SOF Arahella iric%r 0.7 0.1 0.1 1.1 M B C Branchioa.l'ychis americana 0.2 0.1 0.1 0.2 S B SOF Capitellides jonesi 0.2 0.3 0.5 0.8 M B NSOF Diopatra cuprea 0.9 0.1 0.5 0.3 S B SOF,O Pselldellrythoe amhigua 0.2 1.5 0.6 M E C Notomastus /atericells 0.2 0.4 0.2 M B NSOF Phyl/odoce are/we 0.7 0.1 0.2 M B,E C The/epIIs .I·e/OSUS 0.4 0.1 0.9 S B SOF Parapriollospio pinnata 3.8 0.1 0.3 OM B SOF Asabel/ides sp A 0.4 0.3 0.2 S B SF Spio pettiboneae 0.9 0.6 0.2 S B SOF MediomaSI/IS ca/ifomiensis 38.9 10.0 3.0 M B NSOF Oorvilleid sp B 0.2 0.2 M B SDF Sc%p/os rubra 0.4 1.0 M B NSOF LIIII/brineris tenllis 15.6 8.4 M B 0 LIIII/brineris Iiarei/li 0.2 0.1 M B 0 Syllis sp B 0.5 10.4 10.0 M B,E C POII/atoceros caeru/escens 0.9 4.1 2.2 S E SF Syllis comllta 3.6 0.3 0.8 M B,E C Sabel/aria \'II/garis 0.4 0.1 1.4 S E SF Brallia c!al'{l/a 0.6 0.3 0.2 M B,E C Cabira incerta 0.4 0.1 0.2 M B C,O Hydroides proti/ico/a 0.5 0.5 0.2 S B SF Po/ydora /igni 0.1 0.3 0.2 S B SOF Scale worm 0.6 0.3 0.8 M B,E C Spirorhis spirillulll 0.8 0.1 0.6 S E SF Po/ydora sociahilis 0.1 0.8 S B SOF Samythella sp A 0.1 0.3 S B SF SCypllOproCIIIS sp A 0.1 0.9 M B NSOF Syllis sp E 1.5 2.0 M B,E C Spiop/wnes bombyx 0.3 3.2 S B SOF Heterol/lastlls jiliformis \.l 0.3 M B NSOF Me/inna maC/i1ata 0.1 0.5 S B SOF Notomasl/ls hemipodlls 0.1 0.5 M B NSOF Syl/is sp 0 0.5 0.2 M B,E C Gnllphi.l· neblliosa 0.1 0.1 S B SOF,O546 BULLETIN OF MARINE SCIENCE, VOL. 30. NO.3. 1980

Table 6. Continued

Species Fen II Fen 12 Econ 10 Econ 12 Life History
Pis/a cristata 0.1 0.1 M 

### B SOP Syllid sp 1 0.1 0.1 M 

### B,E C Armalldia agilis 0.3 M 

### B NSOF Brallia sp A 0.2 M 

### B,E C Capitella capitata 0.3 M 

### B NSOF Dorvillea sp A 0.2 M 

### B SOF Lysidice lIillelta 0.5 M 

### B C? Pista sp A 0.2 M 

### B SOF Syllid sp2 0.2 M 

### B,E C Nereid sp ] 0.1 M 

### B,E ? Dlluphis sp A 0.\ S B SOF,O Pherusa sp A 0.3 OM 

### B SOF Poecilochaetus johllstolli 0.3 OM 

### B ? Bhawallia f?oodei 1.3 M ? ? Aricidea sp A 0.1 M 

### B SOP Minuspio cirri/era 0.1 OM B SOF Sigambra bassi 0.1 M 6,E C,O Syllis hyalilla 0.3 M 

### B,E C Terrebellid sp A 0.3 S B SOP Nephtyid sp A 0.2 M B 0 Sthellelais boa 0.2 M B C



### limited in distribution to vegetated stations as were 41 other species. Although
common atFenholloway 11,M. californiensis andL. tenuis were rarely collected atEconfina stations. Occurrence ofthese species atFenholloway 12wasprobably
aresult ofoccasional sampling ofbare substrate at that site. Cluster analysis ofpolychaete species composition showed that stations Econfina 10and 12were similar (rho = 

### 0.92). Fenholloway stations were similar to each other (rho 

## = 0.79), but Econfina stations were distinct from those ofthe Fenholloway (rho = 

### 0.56).

The mean density ofpolychaetes was highest at Fenholloway station 12(1467/
m2) 

### and lowest at Fenholloway 11(705/m2) 

### (Table 7). Analysis ofvariance and Duncan's Multiple Range Test (P

## <0.05), however, suggest that two subsets exist. The mean densities ofpolychaetes were not different at Fenholloway II and Econfina 10and 12.Mean densities were also similar at Fenholloway station 12and Econfina 10and 12.The highest density was 2947polychaetes/m2 

### recorded atEconfina 10inNovember, 1977.The lowest density was l711m2

### atFenholloway 11 in December 1976. Seasonality ofpolychaete density and biomass generally reflected patterns found for total macrofauna, with peaks in the spring which dropped to fallminima. November collections, however, had very high numbers ofpolychaetes.

Polychaetes collected in this study were categorized according to motility, general microhabitat, and trophic group (Table 6). The relative abundance of epifaunal polychaetes was directly related to macrophyte abundance (R

## = 0.960, P 

## < 0.05), whereas an inverse relation was demonstrated by burrowers (R = 

### -0.956, P 

## <0.05). The relative abundance ofdeposit feeders (selective and nonselective) decreased with mean macrophyte biomass (R = 

### -0.978, P 

## <0.05) as did the relative abundance of omnivores (R

## = -0.944, P 

## < 0.10). Omnivores particularly associated withlowmacrophyte cover were twoLumbrineris species. Carnivores, most notably the syllids, increased with macrophyte biomass (R

## = 0.989, P 

## < 0.05). Suspension feeders were most closely associated withvegetated stations; these included sabellids, ampharetids, and serpulids.STONER: SEAGRASS AND ORGANIZATION OF MACROBENTHOS 547

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### 548 BULLETIN OF MARINE SCIENCE, VOL. 30, NO.3, 1980

DISCUSSION 

### The densities ofmacrofaunal organisms from allstations inthis study were low relative to those found inother investigations. Orth (1973)reported amean density of 14,284macrobenthic animals (> 

### 1.0mm)per square meter associated with Zostera marina inChesapeake Bay. Also for Chesapeake Bay Z. marina beds, Marsh (1973) reported densities ofmacrobenthic epifauna (> 

### 0.5 mm) ranging from slightly less than 1000to 25,000/m2

• 

### Santos and Simon (1974) reported a mean density of 13,425 polychaetes/m2 

### in Tampa Bay, Florida. Although they noted that this was amuch higher value than those found inmost other vegetated and unvegetated coastal and estuarine sites around the United States, the mean densities reported in this study are less than most ofthose reviewed by Santos and Simon. Reasons for low macrofaunal abundance inApalachee Bay are speculative at this point; however, Young et al. (1976) showed that inIndian River, Florida, density ofmacrofauna inHalodule wrightii beds was inversely related todegree oftidal flushing. Their sitewhich hadthe highest degree oftidal flushing demonstrated amean macrofaunal density of3994animals/m2

• 

### This value iscomparable to that for the vegetated sites of this study. Since the study area in Apalachee Bay also demonstrates ahigh degree oftidal flow, hydrodynamic effects probably explain the low macrofaunal abundances observed. Several authors have noted that macrofaunal densities at sites vegetated by seagrasses were higher than nearby unvegetated sites (O'Gower and Wacasey, 1967; Santos and Simon, 1974; Thayer et al. 1975; Orth, 1977); however, all except Santos and Simon found sediment granulometry correlated with macrophyte biomass making differentiation ofmacrophyte and granulometric effects impossible. Because the stations studied in Apalachee Bay included a gradient ofmacrophyte biomass ranging from 9to 320g dry wtlm2

, 

### while having similar granulometric properties, the role of seagrass density in regulating community structure ofbenthic macrofauna could be tested. Data presented here show that spatial pattern inthe abundance ofmacrobenthic animals isafunction ofmacrophyte cover and not fine sediments often associated with vegetation. However, abundances ofmacrofauna (particularly amphipods) did not follow temporal patterns in seagrass biomass. Stoner (1979) showed that for amphipods, seasonal abundance patterns were related to reproductive seasonalities and abundance of predatory fish. An obvious increase in macrobenthic animals occurred with fall emigration offishes from the grass beds. The number oftaxa collected inApalachee Bay (170)was comparable to that found in seagrass beds studied inother regions. Orth (1973)reported 117macrobenthic species collected from Chesapeake Bay Zostera marina beds, corresponding with the 117species collected on my most heavily vegetated site. He also noted a decrease in species richness with seagrass density as found inthis study (Orth, 1973, 1977). Young and Young (1977) found that the number of species outside their experimental cages ranged from 104to 115taxa inseagrass beds ofIndian River, Florida.

Unlike Kikuchi (1966),Kikuchi andPeres (1977),and Santos and Simon (1974), distinct differences inspecies composition and species richness ofepifaunal and infaunal groups found inthis investigation were related to macrophyte biomass. The number ofspecies (total macrofauna, amphipods andpolychaetes) washigher
on the three vegetated stations than at the unvegetated site, which showed the highest degree ofdominance. Similarly, Heck and Wetstone (1977) found that species composition and species richness oflarger (mostly decapod) grass bed macroinvertebrates in Panama were related to macrophyte biomass. They hy-STONER: SEAGRASS AND ORGANIZATION OF MACROBENTHOS 549


### pothesized that species richness was a function ofhabitat heterogeneity, food availability, increased living space, and protection from predators, all ofwhich
are associated with grass blade density. Similar hypotheses are appropriate for Apalachee Bay. Particularly high densities ofpinfish (Lagodon rhomboides) in Apalachee Bay which consume small epifaunal invertebrates probably have a large impact on both the abundance and species composition ofepifaunal components. However, predation is mediated by density ofseagrasses and species richness, abundance, and dominance isundoubtedly acomplex function ofboth habitat heterogeneity and predation (Stoner, 1979). Differences inspecies composition were related tothe microhabitat andtrophic characters ofthe animals. The relative abundances ofepifaunal amphipods and epifaunal polychaetes were closely related to the mean biomass ofmacrophytes. This, no doubt, isrelated to increased surface area and habitat complexity above the sediment provided by grass blades and epiphytes. The activities oftubebuilding and infaunal amphipods may be inhibited by heavy rhizome mats in seagrass beds and these species were inversely related to macrophyte biomass. The abundance ofdifferent polychaete trophic groups were also related tomacrophyte biomass. The abundances ofdeposit feeders and omnivores decreased with macrophyte standing crop, whereas carnivorous and suspension feeding polychaetes were most abundant invegetated habits. Differences inthe relative abundance ofpolychaete trophic groups has been attributed to differences insediment stability (Orth, 1977)and sediment granulometry (Sanders, 1958;Rhoads, 1974; Gray, 1974). Nagle (1965, 1968) suggested that epifaunal species composition associated with marine vegetation was a function ofkinetic energy and salinity ofthe surrounding water. Reports from Indian River (Young and Young, 1977) also suggest control over relative abundance ofvarious trophic groups ofpolychaetes and molluscs bydegree oftidal flushing. However, since tidal influence, sediment stability and granulometry were identical atallofthe sites examined in this study, species composition and trophic structure ofthe macrobenthos must be a function of other environmental variables, the most obvious ofwhich is macrophyte density. Low abundance ofdeposit feeders inheavily vegetated sediments may be due to heavy rhizome mats or abundance oflarge predatory poly

### chaetes, shrimp, and crabs found there, but abundances were notrelated toabundance of food in the sediments as measured by organic content. Increasing
earnivory associated with macrophyte biomass mayberelated to higher absolute abundances ofprey species such as protozoans, nematodes, and other small organisms; however, no data are available on these prey groups. The relative abundance and high degree of dominance by deposit feeding and omnivorous species suggests that opportunistic lifehistory strategies are advantageous atsites with low macrophyte biomass. On the other hand, the numerical importance of specialized and carnivorous polychaetes at sites with dense macrophyte cover suggests that opportunism is adaptively less important and that food webs, both above and within the sediment, are more complex in vegetated habitats. This study indicates that biomass ofbenthic vegetation, independent ofsediment granulometry, exerts a strong influence onthe abundance, dominance, diversity, and trophic organization of macrobenthic infauna and epifauna in Apalachee Bay, Florida.

ACKNOWLEDGMENTS

This research was supported bygrant number R-805288010fromthe U.S. Environmental Protection Agency to R. J. Livingston. Aid inidentification ofvarious macrofauna was generously provided by550 BULLETIN OF MARINE SCIENCE. VOL. 30. NO.3. 1980

E. L. Bousfield, H. Greening, F. G. Lewis, S. L. Santos, J. L. Simon, and P. F. Sheridan. M. S. Zimmerman and numerous graduate and undergraduate students aided inthe field work. Help with sediment analyses was provided by W. A. Tanner and J. Neal. K. Sheridan and K. Brady prepared computer cards and G. C. Woodsum assisted in computer analyses. Special thanks are due R. J. Livingston, D. Thistle, and F. G. Lewis forthoughtful reviews ofthe manuscript.

Contribution No. 171ofthe Harbor Branch Foundations, Inc., Fort Pierce, Florida.

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---. 1977. The importance ofsediment stability inseagrass communities. Pages 281-300 ill B. C. Coull, ed. Ecology ofmarine benthos. Univ. South Carolina Press, Columbia. Pettibone, M. H. 1963. Marine polychaete worms ofthe New England region. Bull. U.S. Natn. Mus. 227: 1-356. Phillips, R. C. 1960. Observations on the ecology and distribution ofthe Florida seagrasses. Fla. Bd. Conserv. Mar. Lab. Contr. No. 44. 72 pp. Rhoads, D. C. 1974. Organism-sediment relations on the muddy sea floor. Ann. Rev. Oceanogr. Mar. BioI. 12:263-300. Sanders, H. L. 1958. Benthic studies in Buzzards Bay. I. Animal-sediment relationships. Limnol. Oceanogr. 3: 245-258.

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Santos, S. L., and J. L. Simon. 1974. Distribution and abundance of the polychaetous annelids in

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DATE ACCEPTED: October 22, 1979.

ADDRESS: Departmelll ofBiological Science, Florida State University, Tallahassee, Florida 32306. PRESENT ADDRESS: Harbor Branch Institution, Inc., RR I, Box 196-A, Fort Pierce, Florida 33450.