The distinctive karstic, freshwater wetlands of the northern Caribbean and Central American region support the prolific growth of calcite-rich periphyton mats. Aside from the Everglades, very little research has been conducted in these karstic wetlands, which are increasingly threatened by eutrophication. This study sought to (i) test the hypothesis that water depth and periphyton total phosphorus (TP) content are both drivers of periphyton biomass in karstic wetland habitats in Belize, Mexico and Jamaica, (ii) provide a taxonomic inventory of the periphytic diatom species in these wetlands and (iii) examine the relationship between periphyton mat TP concentration and diatom assemblage at Everglades and Caribbean locations. Periphyton biomass, nutrient and diatom assemblage data were generated from periphyton mat samples collected from shallow, marl-based wetlands in Belize, Mexico and Jamaica. These data were compared to a larger dataset collected from comparable sites within Everglades National Park. A diatom taxonomic inventory was conducted on the Caribbean samples and a combination of ordination and weighted-averaging modeling techniques were used to compare relationships between periphyton TP concentration, periphyton biomass and diatom assemblage composition among the locations. Within the Everglades, periphyton biomass showed a negative correlation with water depth and mat TP, while periphyton mat percent organic content was positively correlated with these two variables. These patterns were also exhibited within the Belize, Mexico and Jamaica locations, suggesting that water depth and periphyton TP content are both drivers of periphyton biomass in karstic wetland systems within the northern Caribbean region. A total of 146 diatom species representing 39 genera were recorded from the three Caribbean locations, including a distinct core group of species that may be endemic to this habitat type. Weighted averaging models were produced that effectively predicted mat TP concentration from diatom assemblages for both Everglades (R2=0.56) and Caribbean (R2=0.85) locations. There were, however, significant differences among Everglades and Caribbean locations with respect to species TP optima and indicator species. This suggests that although diatoms are effective indicators of water quality in these wetlands, differences in species response to water quality changes can reduce the predictive power of these indices when applied across systems.

Saltwater intrusion alters the natural salinity and phosphorus (P) gradients in the oligotrophic, freshwater wetlands located near coastlines of the Caribbean Basin with important consequences to the structure and function of key ecosystem components, including plants, soil microbes, and periphyton. Periphyton communities, particularly diatoms, are extremely sensitive to water quality changes and can serve as excellent bioindicators; however, little is known about their use in detecting novel rates of saltwater intrusion into coastal, freshwater wetlands. I examined the individual and combined effects of elevated salinity and P on periphyton functional processes and diatom composition by conducting transect surveys along salinity and P gradients in the southern Everglades, as well as through mesocosm studies in which salinity and P were experimental manipulated. I demonstrated that conductivity (a proxy for salinity) and P gradients drive spatial patterns in diatom assemblage structure in the southern Everglades and that these assemblages have relatively low conductivity (2 mS cm-1) and total P thresholds (82 µg g-1). These findings were supported by the experimental work which showed that monthly pulses of elevated salinity only ~1 ppt above ambient was sufficient to cause significant shifts in periphytic diatom assemblages along with reduced periphyton productivity, total carbon, and nutrient content. The addition of P to freshwater and salt-treated periphyton significantly elevated mat total P, underscoring the P-uptake efficiency of periphyton. Surprisingly, addition of P to freshwater periphyton did not elicit significant functional or compositional responses, although chlorophyll-aconcentrations and accumulation rates tended to be higher with P. Similar chlorophyll-atrends were observed for salt-treated mats with added P, but these mats also exhibited significantly higher gross primary productivity and net ecosystem productivity compared to all other treatments and a diatom assemblage distinct from any other treatment. This research provides new and valuable information regarding periphyton dynamics in response to changing water sources that will allow us to extend the use of periphyton, and their diatom assemblages, as tools for environmental assessments related to saltwater intrusion in the southern Everglades and other karstic, freshwater wetlands.