Response of the diatom Encyonema evergladianum to environmental changes associated with sea level rise in the Caribbean Basin

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The karstic, freshwater wetlands of the Everglades and Caribbean are threatened by saltwater intrusion due to sea level rise. Saltwater intrusion not only elevates conductivity in these environments but has been shown to elevate phosphorus (P) levels as well. One of the main areas of FCE III research is to understand the biogeochemical and compositional modifications caused by exposure to elevated salinity and P in the naturally low phosphorus, freshwater Everglades and the effectiveness of freshwater restoration in thwarting these changes. Karstic wetlands in the Caribbean face similar pressures and possibly respond in similar ways as the Everglades allowing FCE research informing management and restoration to be applied to the wider Caribbean region.

Calcareous periphyton mats are a ubiquitous and important component of karstic wetlands found throughout the Caribbean Basin and contain a characteristic diatom community that is reacting in diagnostic ways to these changes in water quality. Encyonema evergladianum is a dominant diatom species in these mats and may provide insight into the relationship between periphyton abundance and shifting water sources. We propose that this species is a potentially powerful indicator of water quality changes associated with saltwater intrusion into these wetlands and responds to these changes in ways that reflect whole periphyton community production trends. In this study, we explore the response of E. evergladianum abundance to periphyton mat TP, mat inorganic carbon content (calcareousness), and conductivity in the Florida Everglades and three Caribbean wetlands.

We found that in both the Everglades and the Caribbean, E. evergladianum is highly sensitive to changes in the inorganic carbon content of the periphyton mats in which they live and decreases in inorganic carbon are strongly correlated to TP concentration. Furthermore conductivity was positively correlated with TP and negatively correlated to mat inorganic carbon in the Everglades, but these relationships were not seen in the Caribbean. These results suggest that calcareous periphyton mats are the preferred niche of E. evergladianum and that loss of this mat structure is correlated to elevated TP which, in the Everglades, is coupled with elevated conductivity.

This study provides preliminary support for the use of E. evergladianum as an indicator species in karstic, freshwater wetlands threatened by elevated salinity and phosphorus associated with saltwater intrusion. Mesocosm experiments are underway to test the individual and synergistic effects of increased salinity and phosphorus on E. evergladianum abundance and periphyton mat calcareousness in order to disentangle the direct and indirect drivers of changes in E. evergladianum abundance.