A major contribution by this working group was the generation of the "upside-down estuary" concept. Long-term nutrient data from the SRS and TS/Ph estuaries show how surface and groundwater supplies of phosphorus from the Gulf of Mexico subsidize productivity in the mangrove forest ecotone, rather than the classic estuary model where upstream sources provide the main source of nutrients to coasts. The SRS ecotone is largely subsidized by phosphorus delivered by tides and storms, whereas the tidally-dampened TS/Ph ecotone appears to respond to groundwater that moves into the surface water during the dry season.
Building on this "upside-down estuary" concept, subsequent research has tracked continued landward movement of brackish, P-enriched surface and groundwater resulting from reduced fresh water delivery and sea level rise. In the ecotone, this phosphorus subsidy enhances benthic algal uptake rates and microbial release of ammonium. Historic supplies of P to Florida Bay have left a permanent legacy in seagrass communities. In addition to this "press" (continuous) supply driven by encroachment, we have also observed the biogeochemical consequences of phosphorus delivered to the ecotone in storm surge. Hurricane Wilma (October 2005) delivered 3-4 cm of P-rich marine sediment into the fringing mangrove forest in SRS, which increased soil elevation relative to sea level rise, stimulated mangrove belowground production, and eventually leached into the river water column. Initial characterization of bacterial community structure revealed distinct gradients along the SRS and TS/Ph transects which were strongly related to underlying patterns in salinity and phosphorus availability. Long-term and continuous microbial abundance and production data are enabling these trends to be tracked over time in response to fluctuations in marine and fresh water nutrient supply. Microbial studies have stimulated a new area of research for FCE investigating the role of bacteria in mediating carbon and nutrient fluxes in the ecotone, under changing conditions of marine and fresh water supplies.
FCE scientists monitor total phosphorus concentrations in the water column in the mangrove ecotone. Their data suggest that sediment depositions after hurricanes, such as illustrated in the graph above, provide another source of marine phosphorous to the mangroves. Rather than a long-term increase in estuarine phosphorous concentrations, these changes show how storm surge delivers this nutrient to wetlands and fuels productivity there.
Source: Daniel Childers