The exchange of carbon, nitrogen, and phosphorus in dwarf and fringe mangroves of the oligotrophic southern Everglades

Water management has altered both the natural timing and volume of freshwater delivered to Everglades National Park. This is especially true for Taylor Slough and the C-111 basin, as hypersaline events in Florida Bay have been linked to reduced freshwater flow in this area. In light of recent efforts to restore historical flows to the eastern Everglades, an understanding of the impact of this hydrologic shift is needed in order to predict the trajectory of restoration. I conducted a study to assess the importance of season, water chemistry, and hydrologic conditions on the exchange of nutrients in dwarf and fringe mangrove wetlands along Taylor Slough. I also performed mangrove leaf decomposition studies to determine the contribution of biotic and abiotic processes to mass loss, the effect of salinity and season on degradation rates, and the importance of this litter component as a rapid source of nutrients. ^ Dwarf mangrove wetlands consistently imported total nutrients (C, N, and P) and released NO2− +NO3 −, with enhanced release during the dry season. Ammonium flux shifted from uptake to release over the study period. Dissolved phosphate activity was difficult to discern in either wetland, as concentrations were often below detection limits. Fluxes of dissolved inorganic nitrogen in the fringe wetland were positively related to DIN concentrations. The opposite was found for total nitrogen in the fringe wetland. A dynamic budget revealed a net annual export of TN to Florida Bay that was highest during the wet season. Simulated increases and decreases in freshwater flow yielded reduced exports of TN to Florida Bay as a result of changes in subsystem and water flux characteristics. Finally, abiotic processes yielded substantial nutrient and mass losses from senesced leaves with little influence of salinity. Dwarf mangrove leaf litter appeared to be a considerable source of nutrients to the water column of this highly oligotrophic wetland. To summarize, nutrient dynamics at the subsystem level were sensitive to short-term changes in hydrologic and seasonal conditions. These findings suggest that increased freshwater flow has the potential to lead to long-term, system-level changes that may reach as far as eastern Florida Bay. ^

Patterns of nutrient exchange in a riverine mangrove forest in the Shark River Estuary, Florida, USA

This study aimed to evaluate tidal and seasonal variations in concentrations and fluxes of nitrogen (NH4 +, NO2+NO3, total nitrogen) and phosphorus (soluble reactive phosphorus, total phosphorus) in a riverine mangrove forest using the flume technique during the dry (May, December 2003) and rainy (October 2003) seasons in the Shark River Estuary, Florida. Tidal water temperatures during the sampling period were on average 29.4 (± 0.4) oC in May and October declining to 20 oC (± 4) in December. Salinity values remained constant in May (28 ± 0.12 PSU), whereas salinity in October and December ranged from 6‒21 PSU and 9‒25 PSU, respectively. Nitrate + nitrite (N+N) and NH4+ concentrations ranged from 0.0 to 3.5 μM and from 0 to 4.8 μM throughout the study period, respectively. Mean TN concentrations in October and December were 39 (±0.8) μM and 37 (±1.5) μM, respectively. SRP and N+N concentrations in the flume increased with higher frequency in flooding tides. TP concentrations ranged between 0.2‒2.9 μM with higher concentrations in the dry season than in the rainy season. Mean concentrations were <1. 5 μM during the sampling period in October (0.75 ± 0.02) and December (0.76 ± 0.01), and were relatively constant in both upstream and downstream locations of the flume. Water residence time in the flume (25 m2) was relatively short for any nutrient exchange to occur between the water column and the forest floor. However, the distinct seasonality in nutrient concentrations in the flume and adjacent tidal creek indicate that the Gulf of Mexico is the main source of SRP and N+N into the mangrove forest.

Benthic Exchange of C, N, and P Along the Estuarine Ecotone of Lower Taylor Slough, Florida (USA): Effect of Seasonal Flows and Phosphorus Availability

The southern Everglades and Florida Bay have experienced a nearly 50 % reduction in freshwater flow resulting in increased salinity and landward expansion of mangrove forest. Given the marine end-member is a natural source of P to this region, it is necessary to understand the interactions between inflows and P availability in controlling the exchange of materials across the mangrove ecotone. From 2007 to 2008, we used sediment core incubations to quantify fluxes of dissolved inorganic N and P and dissolved organic carbon (DOC) in three ecotone areas (dwarf mangrove, pond, and bay). Experiments were repeated seasonally over 2 years involving P-enriched surface water as a factor. We saw consistent uptake of soluble reactive P (SRP), DOC, and nitrate + nitrite (N+N) by the soils/sediments and release of ammonium (NH4 +) from soils/sediments to the water column across all sites and seasons. P enrichment had no discernible effect on DIN or DOC flux, suggesting that rapid P uptake may have been more geochemically mediated. However, uptake of added P occurred across all sites and seasons, reflecting high uptake capacity in this carbonate system and the potential of the mangrove ecotone to sequester P as it becomes more available.