FAU Institutional Repository http://purl.fcla.edu/fau/fauir This paper was submitted by the faculty of FAU’s Harbor Branch Oceanographic Institute. Notice: ©1983 Rosenstiel School of Marine and Atmospheric Science, University of Miami. This manuscript is available at http://www.rsmas.miami.edu/bms and may be cited as: Lewis, F. G., III., & Stoner, A. W. (1983). Distribution of macrofauna within seagrass beds: an explanation for patterns of abundance. Bulletin of Marine Science, 33(2), 296‐304. BULLETIN OF MARINE SCIENCE, 33(2): 296-304, 1983 DISTRIBUTION OF MACROFAUNA WITHIN SEAGRASS BEDS: AN EXPLANATION FOR PATTERNS OF ABUNDANCE F Graham Lewis, III and Allan W Stoner ABSTRACT An examination of macrofaunal microhabitats within a seagrass meadow was conducted in Apalachee Bay (north Florida). Core samples were taken from two substrata within the grassbed, Thalassia testudinum shoots and bare areas among the shoots, and compared with the fauna collected in randomly placed cores. Seagrass samples showed significantly greater numbers of individuals and species than the other two treatments. When compared with either bare substrate or random samples, four times the number ofindividuals and twice the number ofspecies were collected in cores containing seagrass shoots. Random samples were not significantly different from samples taken on the bare substrate. Many of the species undersampled in randomly placed cores were epifaunal and closely associated with the vege­ tation present. Macrobenthic species were classified according to preferred microhabitat (sea­ grass, bare substrate or no preference). It is suggested that macrofaunal density and species richness estimates may be greatly affected by the distribution of plants within the grassbed. This study points out potential difficulties in macrofaunal estimates when the preferred microhabitat of the species under examination is undersampled. Coastal seagrass meadows, although long recognized as potential sources of refuge, food and nursery grounds for a variety ofbenthic invertebrates and fishes (Kikuchi, 1966; Kikuchi and Peres, 1977; Thayer et aI., 1975; McRoy, 1977; and others), have seldom been the subject ofthe extensive quantitative investigations given to unvegetated, soft-bottom habitats. The distribution of benthic macro­ fauna within these seagrass beds has been restricted primarily to description of seagrass-faunal assemblages (Jackson, 1972; Marsh, 1973; 1976; Hooks et aI., 1976; Thorhaug and Roessler, 1977; Heck, 1977; 1979; Nelson, 1979a; 1980). Studies have focused on the relationships ofthe macrofauna to both macrophyte species composition (Ledoyer, 1962; O'Gower andWacasey, 1967; Moore et aI., 1968; Santos and Simon, 1974; Heck and Wetstone, 1977; Young, 1981) and biomass (Orth, 1973; 1977; Heck and Wetstone, 1977; Brook, 1978; Stoner, 1980a; Heck and Orth, 1980). Comparatively, little information exists on the small-scale distributions of macrobenthic invertebrates within a particular sea­ grass habitat (Kita and Harada, 1962; Nagle, 1968; Jacobs and Pierson, 1979). In a recent paper (Lewis and Stoner, 1981), the relative efficiency of three different-sized coring devices used for sampling macrobenthos in Thalassia tes­ tudinum beds in Apalachee Bay, Florida, was examined. The smallest corer (5.5­ em diameter) collected significantly greater numbers ofindividuals than either of the larger devices tested (7.6- and 10.5-cm diameters) when the total area (or volume) of sediment sampled by each was equal. From what is known about the life histories ofspecies collected in seagrass meadows, the majority ofindividuals undersampled by the larger corers were found to be epifaunal in habit and normally observed in close association with vegetation. The increased sampling efficiency of the smaller corer was attributed to the spacing of seagrass shoots and the increased probability of randomly sampling a shoot with the greater number of small cores taken per unit surface area. The present study was designed to examine the hypothesis that the majority of macrofauna (infauna and epifauna) within a seagrass bed is closely associated with the physical structures of the seagrasses present and that estimates of mac- 296 LEWIS AND STONER: ABUNDANCE PATTERNS OF SEAGRASS MACROFAUNA 297 rofaunal species composition and abundance may be greatly influenced by the sampling strategy employed. METHODS All benthic samples were collected on 26 September 1979 from shallow turtlegrass (Thalassia testudinum) beds in Apalachee Bay, Florida, USA. The collection site, as in the previous study (Lewis and Stoner, 1981), was located approximately 5 km southwest from the mouth of the Econfina River (permanent sampling station E12; Livingston, 1975) in approximately \.7-2.0 m of water. Total macrophyte biomass (above- and below-ground) at the time of faunal collection, estimated with the repetitive quadrat technique of Livingston et al. (1976), was 36\.4 g dry wt/rn-, dominated by T. testudinum (7\.0010). Salinity and temperature were 250/00 and 22.5°C, respectively. Sediment charac­ teristics of the sampling site were given by Stoner (1980a). To examine the distribution ofmacrofauna within the seagrass bed, three treatments were employed: (I) diver-operated cores were taken directly over and including one Thalassia shoot (bundle sheath and blades), hereafter called seagrass samples; (2) cores were taken between but not including the Thalassia shoots, hereafter called bare substrate samples; and (3) randomly placed cores were taken which mayor may not have included a seagrass shoot, hereafter called random samples. When taking a sample which included a Thalassia shoot, the corer was lowered while the blades were gently manipulated into the sampler. The spreading ofthe distal portions of the blades in the water column caused some difficulty in sampling and precluded the collection of the entire plant in some cases. However, the stalk, bundle sheath and basal portions of the blades were always sampled. Three 2 X 2-m grids were located adjacent to one another in a visually homogeneous area at the sampling site; each treatment was allocated a separate sampling grid. Twenty replicate cores were taken within each grid with placement of cores based on sets of randomly generated coordinates. In the first two treat­ ments, the nearest Thalassia shoot (seagrass sample) or nearest bare area between shoots (bare sub­ strate) to the random coordinates was chosen for core placement. Short sections of PVC pipe with an inside diameter of 5.1 em (surface area of 20.3 em- per core) were used for coring. Cores were taken to a depth of 10 em as initial investigations on this station (Stoner, 1980a) showed that 98010 of the macrofauna were found in the top 5 ern of a core. Core samples were sieved in the field with a 0.5-mm mesh screen and individually preserved in 10010 buffered Formalin and rose bengal stain. Samples were hand sorted in the laboratory and all amphipods, isopods, decapods, mysids, polychaetes, molluscs and echinoderms were identified to species. Less numerous taxa, such as oligochaetes, nemerteans, cumaceans and sipunculids, were counted but not identified. RESULTS Species composition and abundance (individualsilO cores) of macrofauna col­ lected in the three treatments are shown in Table 1. One hundred and one species (20 amphipods, 38 polychaetes and 43 miscellaneous taxa) were collected in the 60 core samples. Significant differences were found among the three treatments (ANaYA with log transformation and Kruskal-Wallis one-way analysis; P < 0.01) in both numbers of individuals and species. Seagrass samples contained over four times the number of individuals and twice the number of species than the numbers recorded for either bare substrate or random samples. No differences were observed in either number ofindividuals or species (Duncan's multiple range test and Kruskal-Wallis multiple comparison; P > 0.10) between the latter two treatments. Although an examination of the variances of the three treatments indicated a significant departure from homoscedasticity for both numbers ofspecies and individuals (Fmax = 6.0 and 10.1, respectively; P < 0.05), analysis ofvariance is robust enough to function well even with significant variance heterogeneity, if equal sample sizes are used (Box, 1954). Parametric analysis ofvariance and non­ parametric Kruskal-Wallis tests yielded identical results. Although the sampling design precluded an estimate ofvariability among sam­ ples leading to a potential confounding of treatment effects with location, prior sampling suggests that the observed differences in numbers of species and indi­ viduals are due to a treatment rather than a location effect. Twelve replicate cores 298 BULLETIN OF MARINE SCIENCE, VOL. 33, NO.2, 1983 Table I. Macrobenthic animals collected from three sampling treatments (Values represent number of individuals collected in 20 replicates) Treatments Species Seagrass Bare Substrate Random Amphipoda Ampelisca vadorum 8 Ampelisca verrilli I I Batea catharinensis 5 I Carinobatea carinata IS 3 5 Cerapus sp. (cf. C. tubularis) 3 I Cymadusa compta 49 3 10 Elasmopus levis 12 Erichthonius brasiliensis 17 2 Gitanopsis tortugae 4 I Grandidierella bonnieroides I Lembos unifasciatus 89 8 II Listriella barnardi 6 3 Luconacia incerta I 2 Lysianopsis alba 12 2 Melita appendiculata I Photis macromanus 3 Rudilemboides naglei 12 6 5 Stenothoe minuta 2 Synchelidium americanum 3 2 Tethygeneia longleyi 57 3 10 Number of individuals 297 35 50 Number of species 18 13