Saltwater intrusion into freshwater coastal wetlands is an urgent problem caused by climate change and water management practices. In the Florida Everglades marine water intrusion, caused by sea level rise and reduced freshwater flows from upstream, is the major contributor to elevated salinity in this freshwater wetland. Intrusion of marine water also delivers excess phosphorus (P) into this naturally oligotrophic landscape, which, in combination with elevated salinity, results in biogeochemical and ecological consequences that are expected to greatly affect the carbon storage capacity of this wetland.
Periphyton, mat-forming benthic algal communities, are a ubiquitous component of the Everglades ecosystem and play an important role in carbon storage, as well other ecological processes, but are extremely sensitive to changing biogeochemical conditions such as salinity and excess nutrients. In this study, we experimentally test the effects of elevated salinity on periphyton in a freshwater (FW), naturally low P site and a brackish-water (BW), high P site. We hypothesize that increased salinity, and accompanying elevated P, will change the species composition of periphyton resulting in a replacement of calcareous, cohesive mats, with a high carbon storage capacity, to filamentous-film mats with low carbon storage capacity.
Following the first salinity dosing, treatments experienced decreased periphyton NPP relative to controls at both the FW (treatment =1.76, control= 3.45 mgO2/gAFDM/hr) and BW (treatment = 0.18, control = 1.11 mgO2/gAFDM/hr) sites. After one year of dosing, NPP remained lower in treatments (2.24 mgO2/gAFDM/hr) versus controls (3.16 mgO2/gAFDM/hr) at the FW site but not the BW site. Similarly, experimentally elevated salinity decreased the carbon content of the periphyton mats at the FW site after 1 month (treatment = 252.3, control = 331.0 g C/g) and 12 months (treatment = 242.1, control = 294.6 g C/g) of dosing. The total phosphorus (P) and nitrogen (N) content of the mats were lower in the elevated salinity treatments at the FW site at both sampling times (T1: 179.3 μgP/g, 11.5 gN/g, T12: 139.7 μgP/g, 10.3 gN/g) compared to controls (T1: 264.2 μgP/g, 21.7 gN/g, T12: 178.7 μgP/g, 14.7 gN/g). These results suggest that the periphyton community at the BW site has a higher tolerance to elevated salinity than the calcareous periphyton mats at the FW site, which respond to simulated saltwater intrusion with decreased production, carbon storage capacity, as well as N and P content as a result of salt stress.