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 of faunal dominance and a species composition distinct from

that found at vegetated sites. The relative abundances of epifaunal amphipods and epifaunal

polychaetes were directly related to macrophyte biomass. Abundance of deposit feeding and

omnivorous polychaetes decreased as a function of macrophyte standing crop, whereas

suspension feeding and carnivorous polychaetes increased with vegetation. Biomass of benthic 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 of food

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 of macrobenthic infauna and lowest faunal dominance associated with

Dip/anthera (Ha/odule) wrightii beds. The number of species and density of

animals was significantly lower in bare sand habitats. Taylor and Lewis (1970),

Orth (1977), and Brook (1978) also found differences among the faunas of sites

with different degrees of macrophyte cover. In all of these studies, however,

major granulometric differences in the sediments accompanied differences in seagrass 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 of seagrass infauna. Only one

study (Santos and Simon, 1974) tested for differences among stations characterized by different macrophyte species and bare sand where sediment granulometry

was not related to seagrass cover. Although faunal densities were higher in samples 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 of macrophyte biomass from that of sediment granulometry in the

regulation of species composition and abundance in Apalachee Bay, Florida.

Emphasis was placed on species composition and life histories of the 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

537
538

• E 12

APALACHEE

BAY

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

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Figure 1. Locations of collecting sites off the Econfina and Fenholloway drainage systems in Apalachee Bay, Florida.

offshore from the mouths of the Econfina and Fenholloway Rivers (Econfina ]0 and 12, Fenholloway

11 and ]2; Fig. 1). The exact location of each station was identified with a permanent marker after

SCUBA diving confirmed the location as representative of a broad area, Fenholloway station 11 had

a low total macrophyte standing crop with an annual mean of 9.3 g dry wtJm2, This station will be

referred to as the unvegetated station. Fenholloway station 12had a mean annual macrophyte biomass

of 141 g dry wtJm2 and mean biomasses at Econfina 10 and Econfina 12 were 215 and 320 g dry wtJ

m2, respectively (Livingston, unpublished data). The macrophytes at all sites were dominated by

mixtures of Thalassia testudinum and Syringodium filiforme, except at Fenholloway 11 where Halophila engelmanni comprised most of the very sparse vegetation.

All of the sampling sites were polyhaline with salinities ranging from 17to 34%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, at the surface or bottom, less than ]00% of the saturation level.

METHODS

Benthic macrofauna were collected monthly (December, 1976 to November, 1977) with hand held

cores (7.6 cm diameter, 45.36 cm2) from field quaduants sampled on the same date for benthic macrophytes. Although preliminary analysis showed that 98% of the macrofaunal numbers were in or

above the top 5 cm of sediment and no animals were found below 10 cm, cores were taken to a depth

of 12 em to insure adequate sampling. Based on 20 replicates at two stations in the fall of 1976 and

the species accumulation analysis described by Livingston et al. (1976) ]2 cores were found to sample

90% of the benthic macrofaunal species. Spring analysis confirmed the appropriateness of the sampling

strategy.

Core samples were sieved in the field using a screen with 0.5 mm mesh. Use of a 1.0 mm screen,

a common practice in prior studies, allowed passage of 14% of the macrofaunal species and 47% of

the macrofaunal numbers (Lewis and Stoner, 1980). Samples were stained with rose bengal

(approx. 100 mg/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 not different, ANOVA and Duncan's Multiple Range Test, P < 0.05)

CharJ.cter Fen II Fen 12 Eeon 10 Econ 12

Organics

(% of dry 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 of the benthic species (by season for the most numerous animals) by drying at

100°C for 24 hr followed by ashing at 550°C for 4 hr. Ash-free dry weight was estimated for a few

rarely encountered species.

The top 5 em of one core, unsieved and unpreserved, was taken for sediment analysis on each

sampling date and station, a portion of which was dried at 100°Cand ashed at 550°C to determine the

organic content of the sediment. After washing to remove salts and drying, another sediment subsample

of approximately 60 g was analyzed for granulometric properties using standard Ro-Tap procedures.

All grain sizes less than 62 micrometer (4.0 phi) were lumped as the clay-silt fraction; however, this

fraction never exceeded 4.0% of the total mass.

RESULTS

Sediments

Granulometric properties of the sediments were statistically similar at all four

stations (ANOY A, P < 0.05) with mean grain diameters in the fine sand category

(Table I). Sorting coefficients less than 1.0 phi at Econfina 10 and 12 and Fenholloway 11indicate that the sediments there are well sorted. Although the sorting

coefficient at Fenholloway 12 was not statistically different from those found at

the other stations, the sediments there were less well sorted. The presence of

shell rubble at Fenholloway 12 was 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 for individual stations were different; however,

with the highest percentages of organics at Fenholloway 12 (5.64%) and lowest

at Fenholloway 11 (2.84%). Organic contents were 4.41% and 2.97% at Econfina

10 and 12, respectively. Sediment organic contents were statistically similar at

only Fenholloway 11 and Econfina 12 (ANOY A and Duncan's Multiple Range

Test, P < 0.05).

Benthic Macrofauna

Five hundred sixty-six core samples taken over 12 months yielded 7271 macrobenthic animals, most of which were amphipod crustaceans and polychaetous

annelids. One hundred seventy species were collected including 75 polychaetes,

31 amphipods, 21 molluscs, 10 shrimp, 6 crabs, and 5 isopods. 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 a lower density

of animals than vegetated sites (x = I754/m2); however, the three vegetated stations had similar mean densities (ANOY A and Duncan's Multiple Range Test,
540 BULLETIN OF MARINE SCIENCE, VOL. 30, NO.3, 1980

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STONER: SEAGRASS AND ORGANIZATION OF MACROBENTHOS 541

Table 3. Indices of dominance for IOtal macrofauna, amphipod fauna, and polychaete fauna at the

four sampling stations (See text for definition of the 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 3107 to 3293/m2• Also, Fenholloway II carried a lower

macrofaunal biomass (7.81 gm/m2) than the vegetated stations (10.00 to 17.85 gml

m2); however, the differences were not statistically significant due to wide monthly variation. Because of sensitivity of the biomass value to large, infrequently

occurring species, primary emphasis in this study was placed on the numbers of

animals and species composition rather than biomass.

Twelve cores produced a mean of 24 species per month at station Fenholloway

11,42 species at Fenholloway 12, and 38 species at both of the Econfina stations.

Overall, however, a total of 82 species were collected at Fenholloway 11, 110 at

Fenholloway 12, 109 at Econfina 10, and 117 at Econfina 12. Regression analysis

showed that the total number of species collected was directly related to the

mean macrophyte standing crop at a collecting site (R = 0.917, P < 0.10).

Macrofaunal density peaked between April and May at all but Econfina station

12 which showed peak densities in February 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

fish populations in October and November, 1977 (Stoner, 1979). Biomass showed

less distinct seasonality, but the numbers of species collected appeared to reflect

the numerical density of animals at all four stations.

Macrofauna of the unvegetated station was characterized by a greater degree of

dominance than other stations (DI = 0.301, Dd = 0.157) (Table 3), where DI is

the dominance index of McNaughton (1967) (DI = nl + n/N) and Dd is the dominance index of Berger and Parker (1970) (Dd = nI/N). n1 and n2 are the absolute

abundances of the first and second most numerous species and N is the total

count of animals. High dominance at Fenholloway 11 was largely due to high

numbers of the polychaete Mediomastus californiensis and the amphipod Ampelisca verrilli. Lowest macrofaunal dominance was found at Fenholloway station

12 (DI = 0.175, Dd = 0.124).

Species composition at the four stations was analyzed with the similarity coefficient rho (Van Belle and Ahmad, 1974) and a flexible grouping cluster strategy

(beta = -0.25). Based on the 85 most numerous taxa, the Econfina stations were

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

(rho = 0.83). The composition of the 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 of amphipod 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 nag lei 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

Tiroll tropakis 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% of the macrofaunal numbers taken at

Fenholloway stations and 47% of the macrofauna at Econfina stations. Although

61% of the amp hipod species collected were found at all four stations, the relative

proportions of the species differed (Table 4). Most striking was the rare occurrence of Elasmopus 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 in this study showed much lower numerical dominance than

the unvegetated site (Table 3). Amphipods never associated with the unvegetated

station were Lysianopsis hirsuta, Cerapus sp A, and Colomastix sp A. Other

species were collected too infrequently to determine if definite 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 the
STONER: 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 for the total macrobenthos showed the relative abundance of amphipod

species to be most similar at Econfina 10 and Fenholloway 12 (rho = 0.94). The

composition of amphipod fauna at Econfina 12 was 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 of amphipods was highest at Econftna station 10 (1578/m2)

and lowest at Fenholloway 11 (641/m2); however, the mean densities of amphipods were not different statistically at stations Fenholloway 11 and 12, and at

stations Fenholloway 12, Econftna 10 and 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 at Econfina 10 in May. Densities and biomasses were lowest in the

fall.

Based on Bousfield (1973), amphipods collected in this 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 of species from each group

is similar for the four stations, the percentages of the total amphipod populations

falling into the different microhabitat categories were different. Free-living epifaunal types made up between 28.9 and 34.2% of the amphipod fauna at the

vegetated sites. Only 12.1% of the 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 of epifaunal amphipods (free-living and

domicolous) was directly related to mean macrophyte biomass for a station (R = 0.959, P < 0.05). The reverse trend was true for infaunal tube-building amphipods

(namely Ampelisca verrilli and A. vadorum), where 67.6% of all amphipods collected at Fenholloway 11 were of this 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 of free-burrowing amphipods;

however, the relative abundance of all infaunal amphipods (tube-building and

free-burrowing) was inversely related to mean macrophyte biomass (R = -0.912,

P < 0.10).

Polychaetes

Polychaetous annelids made up 40 and 45% of the total number of macrofauna

at Fenholloway stations 11 and 12, respectively; and 35 and 33% of the macrofauna at Econfina stations 10 and 12. Of 75 species of polychaetes collected, only

34 were taken at Fenholloway 11. Fifty were taken at Fenholloway 12, 47 at

Econftna 10, and 54 species 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% of the polychaetes,

respectively) (Table 6). Aricidea taylori was the only other worm which made up

more than 5% of the total polychaete collection at Fenholloway 11; however, this

was the most numerous worm at vegetated sites. As seen for the amphipod assemblages, the degree of dominance in polychaete fauna was less at vegetated

stations than at the unvegetated station (Table 3). Syllids and serpulids were
544 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 of the 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

A rahella 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,O
546 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 sp 2 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 at Fenholloway 11, M. californiensis and L. tenuis were rarely collected

at Econfina stations. Occurrence of these species at Fenholloway 12 was probably

a result of occasional sampling of bare substrate at that site.

Cluster analysis of polychaete species composition showed that stations Econfina 10 and 12 were similar (rho = 0.92). Fenholloway stations were similar to

each other (rho = 0.79), but Econfina stations were distinct from those of the

Fenholloway (rho = 0.56).

The mean density of polychaetes was highest at Fenholloway station 12 (1467/

m2) and lowest at Fenholloway 11 (705/m2) (Table 7). Analysis of variance and

Duncan's Multiple Range Test (P < 0.05), however, suggest that two subsets

exist. The mean densities of polychaetes were not different at Fenholloway II

and Econfina 10 and 12. Mean densities were also similar at Fenholloway station

12and Econfina 10 and 12. The highest density was 2947 polychaetes/m2 recorded

at Econfina 10 in November, 1977. The lowest density was l711m2 at Fenholloway

11 in December 1976. Seasonality of polychaete density and biomass generally

reflected patterns found for total macrofauna, with peaks in the spring which

dropped to fall minima. November collections, however, had very high numbers

of polychaetes.

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 of deposit 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 with low macrophyte cover were two Lumbrineris species.

Carnivores, most notably the syllids, increased with macrophyte biomass (R = 0.989, P < 0.05). Suspension feeders were most closely associated with vegetated

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 of macrofaunal organisms from all stations in this study were low

relative to those found in other investigations. Orth (1973) reported a mean density of 14,284 macrobenthic animals (> 1.0 mm) per square meter associated with

Zostera marina in Chesapeake Bay. Also for Chesapeake Bay Z. marina beds,

Marsh (1973) reported densities of macrobenthic epifauna (> 0.5 mm) ranging

from slightly less than 1000 to 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 a much higher value than those found in most other vegetated

and unvegetated coastal and estuarine sites around the United States, the mean

densities reported in this study are less than most of those reviewed by Santos

and Simon. Reasons for low macrofaunal abundance in Apalachee Bay are speculative at this point; however, Young et al. (1976) showed that in Indian River,

Florida, density of macrofauna in Halodule wrightii beds was inversely related

to degree of tidal flushing. Their site which had the highest degree of tidal flushing

demonstrated a mean macrofaunal density of 3994 animals/m2• This value is comparable to that for the vegetated sites of this study. Since the study area in

Apalachee Bay also demonstrates a high degree of tidal 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 of macrophyte and granulometric effects

impossible. Because the stations studied in Apalachee Bay included a gradient

of macrophyte biomass ranging from 9 to 320 g dry wtlm2, while having similar

granulometric properties, the role of seagrass density in regulating community

structure of benthic macrofauna could be tested. Data presented here show that

spatial pattern in the abundance of macrobenthic animals is a function of macrophyte cover and not fine sediments often associated with vegetation. However,

abundances of macrofauna (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 of fishes from the grass beds.

The number of taxa collected in Apalachee Bay (170) was comparable to that

found in seagrass beds studied in other regions. Orth (1973) reported 117 macrobenthic species collected from Chesapeake Bay Zostera marina beds, corresponding with the 117 species collected on my most heavily vegetated site. He

also noted a decrease in species richness with seagrass density as found in this

study (Orth, 1973, 1977). Young and Young (1977) found that the number of

species outside their experimental cages ranged from 104 to 115 taxa in seagrass

beds of Indian River, Florida.

Unlike Kikuchi (1966), Kikuchi and Peres (1977), and Santos and Simon (1974),

distinct differences in species composition and species richness of epifaunal and

infaunal groups found in this investigation were related to macrophyte biomass.

The number of species (total macrofauna, amphipods and polychaetes) was higher

on the three vegetated stations than at the unvegetated site, which showed the

highest degree of dominance. Similarly, Heck and Wetstone (1977) found that

species composition and species richness of larger (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 of habitat heterogeneity, food

availability, increased living space, and protection from predators, all of which

are associated with grass blade density. Similar hypotheses are appropriate for

Apalachee Bay. Particularly high densities of pinfish (Lagodon rhomboides) in

Apalachee Bay which consume small epifaunal invertebrates probably have a

large impact on both the abundance and species composition of epifaunal components. However, predation is mediated by density of seagrasses and species

richness, abundance, and dominance is undoubtedly a complex function of both

habitat heterogeneity and predation (Stoner, 1979).

Differences in species composition were related to the microhabitat and trophic

characters of the animals. The relative abundances of epifaunal amphipods and

epifaunal polychaetes were closely related to the mean biomass of macrophytes.

This, no doubt, is related to increased surface area and habitat complexity above

the sediment provided by grass blades and epiphytes. The activities of tubebuilding and infaunal amphipods may be inhibited by heavy rhizome mats in

seagrass beds and these species were inversely related to macrophyte biomass.

The abundance of different polychaete trophic groups were also related to macrophyte biomass. The abundances of deposit feeders and omnivores decreased with

macrophyte standing crop, whereas carnivorous and suspension feeding polychaetes were most abundant in vegetated habits. Differences in the relative abundance of polychaete trophic groups has been attributed to differences in sediment

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 of kinetic energy and salinity

of the surrounding water. Reports from Indian River (Young and Young, 1977)

also suggest control over relative abundance of various trophic groups of polychaetes and molluscs by degree of tidal flushing. However, since tidal influence,

sediment stability and granulometry were identical at all of the sites examined in

this study, species composition and trophic structure of the macrobenthos must

be a function of other environmental variables, the most obvious of which is

macrophyte density. Low abundance of deposit feeders in heavily vegetated sediments may be due to heavy rhizome mats or abundance of large predatory polychaetes, shrimp, and crabs found there, but abundances were not related to abundance of food in the sediments as measured by organic content. Increasing

earn ivory associated with macrophyte biomass may be related to higher absolute

abundances of prey 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 life history strategies are advantageous at sites

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 of benthic vegetation, independent of sediment granulometry, exerts a strong influence on the abundance, dominance, diversity, and

trophic organization of macrobenthic infauna and epifauna in Apalachee Bay,

Florida.

ACKNOWLEDGMENTS

This research was supported by grant number R-805288010 from the U.S. Environmental Protection

Agency to R. J. Livingston. Aid in identification of various macrofauna was generously provided by
550 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 in the 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 for thoughtful reviews of the manuscript.

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

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DATE ACCEPTED: October 22, 1979.

ADDRESS: Departmelll of Biological Science, Florida State University, Tallahassee, Florida 32306.

PRESENT ADDRESS: Harbor Branch Institution, Inc., RR I, Box 196-A, Fort Pierce, Florida 33450.