Epifaunal community composition and nutrient addition alter sediment organic matter composition in a natural eelgrass Zostera marina bed: a field experiment

Eutrophication and fishing are common perturbations in aquatic ecosystems that have pervasive effects on community structure, including species diversity and abundance. While sedi- ment biogeochemical processes probably respond to these stressors, the linkages to ecosystem func- tioning remain poorly understood. To explore these linkages, we experimentally manipulated water column nutrient levels and food web composition (i.e. predator and grazer presence and absence) in a factorial design using field enclosures situated in a natural eelgrass Zostera marina bed. After 28 d, we quantified sediment organic matter (SOM) abundance and composition using measures of total organic carbon and nitrogen as well as fatty acid (FA) biomarkers. Nutrient enrichment led to a rapid increase of epiphytes and a decline in Z. marina biomass. Responding to the available algae, grazers reduced epiphytes and the abundance of microalgal FAs in the sediment. Predators reduced Z. marina abundance and possibly its ability to trap particulate organic matter (OM), leading to lower sediment organic carbon content and total FA abundance. There was evidence of a trophic cascade as FA contributions to sediments from epiphytes and diatoms were higher in treatments with both grazers and predators than in treatments with grazers only. Predators increased contributions of labile diatom-derived OM, which probably resulted in higher proportions of bacterial FA. Inter- actions between nutrient additions and food web composition indicated that SOM responses were complex and not predictable from single variables. Changes in SOM composition, combined with a rapid heterotrophic bacterial response, suggest that resource levels and aboveground community structure are important to sediment biogeochemistry in natural seagrass systems.