The effects of nutrient addition on seagrass beds in sub-tropical oligotrophic systems

Background/Question/Methods

Our research is part of the Florida Coastal Everglades (FCE) LTER.  The main inquiry being the interactions between an oligotrphic marsh with marine waters rich in the limiting nutrient P within an oligohaline ecotone, and how these interactions drive productivity of sub-tropical shallow coastal seagrass beds.

We more specifically asked the question how is seagrass community structure and calcareous green algae (CGA) productivity influenced by fresh and marine water balances and nutrient supply? We hypothesized 1) that CGA productivity would be greatest where salinity is stable and rich P marine waters are found, while lower P budgets associated with freshwater supplies will limit CGA productivity. And 2) that a shift from slower growing species dominance to faster growing species dominance will occur when nutrients increase in the system, resulting in a community dominated by rhizophytic algae rather than seagrasses.

We are using a long term monitoring survey coupled together with an enrichment experiment to compare community structure and CGA productivity between a marine versus an ecotone estuarine site.

Results/Conclusions

Our long-term monitoring shows a steady decreasing trend in CGA mass along the 4 years of survey in the marine site, which is rich in species. In contrast the ecotone species poor site did not show any decreasing trend. Nutrient tissue content was similar at both sites, which showed slight increasing trends were detected. Moreover, P was limited compared to N through all 4 years. After experimental enrichment, seagrass species increased and CGA decreased contrary to our hypothesis.  However, nutrient content depicted a complex pattern that varied between sites and species, but an increased trend was detected for CGA exposed to higher levels of P and N. These results demonstrate that sea grass communities are limited by P along the entire Florida Bay including marine sites.

Some valuable lessons that we learned from our experiments were that fresh and marine water supplies did determine the seagrass community species richness and production of CGA. However, unexpected shifts were observed as seagrass becoming dominant and competitively overgrew CGA when nutrients were experimentally increased. Our results suggest that we need to revisit our models of how physicochemical trends and conditions influence seagrass community structure and CGA productivity.