1033  2002 Estuarine Research Federation Estuaries Vol. 25, No. 5, p. 1033–1044 October 2002 Impact of Habitat Edges on Density and Secondary Production of Seagrass-associated Fauna PAUL A. X. BOLOGNA* and KENNETH L. HECK,JR. University of South Alabama, Department of Marine Sciences, Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, Alabama 36528 ABSTRACT: Species richness and abundance of seagrass-associated fauna are often positively correlated with seagrass biomass and structural complexity of the habitat. We found that while shoot density and plant biomass were greater in interior portions of turtle grass (Thalassia testudinum) beds than at edges, mean faunal density was significantly greater at edges than interior sites during 1994. This pattern was also observed in 1995, although differences were not significant. The four numerically dominant taxonomic groups showed varying degrees of elevated densities at edges of T. testudinum beds. Peracarids and polychaetes had significantly greater densities at edges of T. testudinum beds, while both decapods and gastropods showed dramatic temporal variability in density, with reversals in density between edge and interior occurring during the course of the study. This within-habitat variability in abundance may reflect both active accumulation of fauna at edges and settlement shadows for species with pelagic larvae. Active accumulation of highly mobile taxa seeking refuge in seagrass beds may explain the differences in density between edge and interior of T. testudinum patches for peracarids in 1994 and in 1995. Active accumulation at edges may also explain differences in density for some decapod taxa. Changes in gastropod densities between habitats may reflect larval settlement patterns. Results showed a distinct settlement shadow for the gastropod Caecum nitidum whose densities (primarily second stage protoconch) in- creased by more than an order of magnitude in 1994. Settlement shadows and post-settlement processes may also explain density differences of polychaetes between the edge and interior of T. testudinum patches. The differences in faunal densities between edge and interior habitat resulted in habitat specific differences in secondary production among the major taxonomic groups. On four of five dates in 1994 and in 1995, secondary production was greater at edge than interior locations. These unexpected results suggest that differences in faunal densities and secondary production be- tween edges and interiors of seagrass patches represent a potentially vital link in seagrass trophic dynamics. If this elevated secondary production leads to increases in trophic transfer, then edges may serve as a significant trophic conduit to higher-level consumers in this system. Introduction Plant communities are frequently subjected to disturbance events that alter the coverage, species composition, biomass and functional characteris- tics of the community (Sousa 1979; White 1987; Holling 1992; Kruess and Tscharntke 1994). As a result, many plant communities are mosaics of hab- itat patches varying in shape and size (Forman and Godron 1981; Holt et al. 1995). The interface be- tween two different habitats (e.g., forest-meadow, seagrass-sand flat) can produce dramatic impacts on both physical and biological processes. In ter- restrial systems, vegetation areal extent and inter- face between differing habitats have been shown to affect the physical environment (e.g., wind, pre- cipitation, light), faunal species composition (Nils- son 1986), predation and foraging success (Don- ovan et al. 1997), and reproduction of plants and * Corresponding author; current address: Fairleigh Dickinson University, Department of Biological and Allied Health Scienc- es, 285 Madison Avenue, M-EC1-01, Madison, New Jersey 07940; tele: 973/443-8758; fax: 973/443-8766; e-mail: bologna@fdu. edu. insects (Didham et al. 1996). Proximity to an in- terface may determine the degree to which edge effects affect associated organisms (Holling 1992; Donovan et al. 1997). In aquatic communities, macrophytes can have dramatic effects on the physical environment (see Koehl 1986). Seagrass structure is important in dampening wave energy and deflecting and slow- ing water flow (Fonseca et al. 1982; Gambi et al. 1990). These effects have both geological and bi- ological impacts. The reduction of flow associated with grass beds increases particle deposition (Al- masi et al. 1987) and the extensive root-rhizome mat stabilizes the sediments (Thayer et al. 1984; Fonseca and Fisher 1986). Seagrass beds act as sed- iment traps and often contain finer sediments than unvegetated regions (Orth 1977). Initial reduction in flow at edges allows large particles to settle while finer particles are carried into the bed. When flow is substantially reduced, the concentration of fine particles will increase in the interior of grass beds (Fonseca et al. 1982; Ackerman and Okubo 1993). Since many marine larvae are small and possess poor swimming capabilities ( Jonsson et al. 1991),