Response of seagrass (Thalassia testudinum) metrics to short-term nutrient enrichment and grazing manipulations

The relative influence of top-down and bottom-up factors in seagrass communities has received considerable attention; however, individual seagrass attributes do not always respond in consistent fashion across studies. Because these metrics have been used as indicators of ecosystem stress, it is necessary to examine whether seagrass response to nutrient enrichment and grazing is persistent. To do so, we examined seagrass and epiphyte response to field manipulations of nutrients and grazing in Big Lagoon, FL, USA where previous studies were conducted with similar methodologies. Our manipulations were successful in elevating water column and porewater nutrient levels. Nutrient additions caused significant increases in tissue nitrogen contents (p = 0.01), but they did not affect seagrass or epiphyte biomass, shoot density, or percent cover. Aboveground biomass was significantly reduced in both treatments involving grazing simulations when compared to the fertilizer addition only treatment (p = 0.001). Fish counts revealed greater numbers of pinfish in fertilized plots compared to all other treatments (p = 0.000), suggesting the fish were attracted to plots in which fertilization resulted in elevated tissue nitrogen contents. Overall, effects of fertilization and grazing on various seagrass attributes showed both similarities and differences in comparison to other studies from the same and different locations, raising the question about how predictable and informative the responses of individual seagrass characteristics for discerning the relative, long-term influence of these factors on seagrass populations.