Effects of grass shrimp (Palaemonetes spp.) density manipulations and nutrient enrichment on widgeongrass (Ruppia maritima) condition, epiphyte load, and epiphyte functional groups
Estuaries are dependent upon the critical ecosystem services that submerged aquatic vegetated habitats provide such as shoreline buffering, nursery habitat for nekton, and retention and cycling of benthic and water column nutrients. Anthropogenic nutrient input can cause increased epiphytic algal loads and indirectly lead to declines in submerged aquatic vegetation (SAV) biomass and areal coverage in estuarine ecosystems. Epiphyte grazers can deter the detrimental effects of nutrient enrichment by reducing overgrowth of epiphytes on SAV. Few studies have investigated how in situ SAV condition and the composition of SAV epiphyte functional groups respond across a gradient of grazing pressure under nutrient enrichment. The combined effects of grass shrimp, Palaemonetes spp., density and nutrient enrichment on: (1) the condition responses of three growth stages of widgeongrass, Ruppia maritima , and (2) R. maritima epiphyte functional groups were investigated. Widgeongrass was enclosed in acrylic microcosms with three levels of grass-shrimp density under ambient or nutrient enriched conditions within a Split-Plot experimental design. Three experiments were conducted for 28 d each in July 2001, August 2001, and June 2002 on widgeongrass in mid-stage, late-stage, and early growth, respectively. Significant grass-shrimp effects elucidated a positive influence of grass shrimp on SAV condition in all three experiments, however condition responses to grass shrimp were mediated by SAV growth stage, water temperature, and background water column nutrient concentrations at the time each study was conducted. Nutrient enrichment did not have a significant effect on widgeongrass condition, however, significantly higher epiphyte mass as indicated by [chl a ] was observed under nutrient enriched conditions in experiment three. Among epiphyte groups, red algae exhibited the most dynamic responses to grass shrimp and nutrient enrichment. Results fail to support the contention that eutrophication is a primary cause of seagrass decline in estuarine ecosystems, but do demonstrate the critical role epiphyte grazers play in seagrass food webs. The maintenance and enhancement of estuarine SAV by resident fauna is critical for maintaining the ecosystem services provided by SAV habitats.