Estuarine, Coastal and Shelf Science (1999) 48, 757–766 Article No. ecss.1999.0477, available online at http://www.idealibrary.com on An Experimental Test of the Capacity of Food Web Interactions (Fish–Epiphytes–Seagrasses) to Offset the Negative Consequences of Eutrophication on Seagrass Communities E. Gacia a,d , M. M. Littler b and D. S. Littler b,c a Smithsonian Marine Station at Link Port, 5612 Old Dixie Highway, Fort Pierce, FL 34946, U.S.A. b Department of Botany, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, U.S.A. c Division of Marine Science, Harbor Branch Oceanographic Institution, 5600 U.S. 1 North, Fort Pierce, FL 34946, U.S.A. Received 13 July 1998 and accepted in revised form 8 February 1999 A mechanism of competition between epiphytes and seagrasses potentially modulated by grazers was studied in a high-nutrient Thalassia testudinum meadow in the Indian River Lagoon (Florida, U.S.A.). The effects of fish grazing on epiphytes, and likely enhancing T. testudinum growth, was tested through an exclusion experiment. Twelve (2 2 m) independent experimental plots were selected within a shallow monospecific bed to which three randomized treatments (exclusion fences, open fences and controls) with four replicates each were assigned. The epiphyte load was monitored on T. testudinum leaves inside the plots from January 1995 to March 1996. Treatment effects occurred during a chlorophyte bloom in March 1995, when the epiphyte biomass was significantly higher inside the exclusion cages than in either of the controls. The composition of the epiphytic community in March 1995 was dominated by sheet-like Enteromorpha and filamentous algae such as Cladophora, which are less resistant to herbivory than the coarsely-branched forms of red algae (e.g. Hypnea, Chondria and Acanthophora) that bloomed subsequently. These results suggest that herbivory change seasonally depending on the availability of different prey species to fish-grazers, which preferentially utilize the fleshy green algae typical of bloom conditions over the thicker coarsely-branched red algae. In the nutrient-rich lagoon the role of top-down interactions in enhancing T. testudinum growth is limited to the reduction of shading by green macroalgae.  1999 Academic Press Keywords: seagrasses; epiphytes; competition; fish-grazers; functional morphology; macroalgae; Thalassia testudinum; Indian River Lagoon Introduction Plant communities are regulated by a combination of bottom-up factors, involving a dependence on resource availability, and top-down factors, involving the activities of higher trophic levels (McQueen, 1989). Changes in trophic levels above plant commu- nities can cascade down through the food web (Carpenter et al., 1985), triggering complex inter- actions whose outcome will depend on the quantity (Threlkeld, 1988) and quality (Levitan, 1987) of the processes involved. In seagrass systems, an increase in nutrient supply to the water column leads to increased epiphyte loads on seagrass blades (Sand-Jensen, 1977; Cambridge & McComb, 1984; Twilley et al., 1985; Silberstein et al., 1986; Tomasko & Lapointe, 1991) causing a shift in the primary production of the community. Over- growth of epiphytes reduces nutrient and gaseous exchange as well as light supply to the seagrass leaves, suppressing seagrass growth (Kiorbøe, 1980; Kemp et al., 1983; Short & Short, 1984) and reproduction (Orth & Van Montfrans, 1984). Grazing by inverte- brate mesofauna can regulate epiphyte biomass and, therefore, seagrass productivity (Orth & Van Montfrans, 1984; Klumpp et al., 1992). Conse- quently, grazing activity may act to buffer the negative consequences of the enhanced nutrient load on seagrass communities associated with epiphyte over- growth. However, the importance of these interactions have not been thoroughly tested, partially because of the complexity of the interactions triggered by changes in food webs (Hunter & Price, 1992). d Present address: Centre d’Estudis Avanc ¸ats de Blanes, Ctra. Santa Ba `rbara s/n, 17300 Blanes, Spain; Tel: 34—972 336101; Fax: 34—972 337806; E-mail: gacia@ceab.csic.es 0272–7714/99/060757+10 $30.00/0  1999 Academic Press