Ecological Impacts of Macroalgal Blooms on Salt Marsh Communities

Despite excessive growth of macroalgae in estuarine systems, little research has been done to examine the impacts of increased algal biomass that drifts into nearby salt marshes and accumulates on intertidal flats. The accumulation of macroalgal mats and subsequent decomposition-related releases of limiting nutrients may potentially alter marsh communities and impact multiple trophic levels. We conducted a 2-year in situ study, as well as laboratory mesocosm experiments, to determine the fate of these nutrients and any bottom-up impacts from the blooms on the dominant salt marsh plant (Spartina alterniflora) and herbivores. Mesocosm results showed that macroalgal decomposition had a positive impact on sediment nitrogen concentrations, as well as S. alterniflora growth rates. In contrast, our in situ results suggested that S. alterniflora growth was hindered by the presence of macroalgal mats. From our results, we suggest that macroalgal accumulation and subsequent release of nitrogen during decomposition may be beneficial in nitrogen limited areas. However, as marshes are becoming increasingly eutrophic, releasing lower marsh plants from nitrogen limitation, this accumulation of macroalgal biomass may hinder S. alterniflora growth through smothering and breakage of culms. As macroalgal blooms are predicted to intensify with rising temperatures and increased eutrophication, the ecological impacts associated with these changes need to be continuously monitored in order to preserve these fragile ecosystems.