Nitrogen-rich groundwater intrusion affects productivity, but not herbivory, of the tropical seagrass Thalassia testudinum

Abstract

Discovery Bay, Jamaica, is a coastal lagoon with areas of substantial submarine groundwater discharge that supply terrestrially derived nutrients to the marine ecosystem. The present study demonstrates that groundwater nitrogen delivery through the sediments allows benthic primary producers to utilize this resource. Meadows of Thalassia testudi num located within the discharge zone of the groundwater seepage had significantly more aboveground biomass, higher rates of productivity and leaves enriched in nitrogen. However, there were no observable positive impacts of nitrogen enrichment on epiphytic algal biomass within these areas. While recent studies have shown that herbivores preferentially graze seagrass blades with elevated tissue nitrogen levels, reciprocal transplant experiments revealed that herbivores avoided the nitrogenen riched leaves and preferentially consumed leaves that were stoichiometrically enriched in phosphorus when transplanted within groundwater areas. This finding suggests that herbivore grazing was influenced more by the ecological stoichiometry of these plants than by the absolute nitrogen content in this phosphorus-limited tropical marine system.