Variability in herbivory in subtropical seagrass ecosystems and implications for seagrass transplanting

Herbivory structures ecosystems in multiple and complex ways and is capable of affecting the success of ecological restoration projects that involve reestablishing plant communities. Direct herbivory on experimental planting units assembled from leaves of the seagrasses Thalassia testudinum, Syringodium filiforme, and Halodule wrightii was assayed in 2009-2010 in south Florida, USA. Seagrass biomass loss to herbivory was compared with elemental composition of donor and ambient seagrass leaves as well as with fish communities at the assay sites. Seagrass herbivores were capable of removing substantial biomass from our experimental planting units, and this varied across seagrass species, location, and time. Seagrass biomass loss to herbivory ranged from 0% to 82%. More biomass was lost during the spring assays than in fall and winter assays. Biomass loss was greatest in S. filiforme and H. wrightii, and negligent in T. testudinum. The assay site closest to tidal cuts leading to the coral reef tract experienced the highest average levels of herbivory. C:N ratios and salinity were predictors of seagrass biomass loss, but not C:P ratios, temperature, or herbivorous fish abundance. Leaf loss to seagrass herbivores appears to be a spatially variable but critically important determinant of seagrass transplanting success. We recommend that local knowledge of herbivory pressure be considered during restoration planning. In our system, a conservative approach to seagrass transplanting, limiting projects to winter months, and refraining from seagrass transplanting all together in areas proximal to coral reefs, are likely to maximize the potential for success.