Generalized landscape-scale patterns of how ecosystem productivity
actually varied along the Shark River Slough (solid line) and southern
Everglades (TS/Ph; dashed line) transects, based on FCE I results.Childers et al. 2006
Initial research examined how DOM and associated organic N from upstream oligotrophic marshes interacts with a marine source of P, the limiting nutrient, to control productivity in the estuarine ecotone. We expected that productivity peaks would occur where communities are released from N or P limitation by the confluence of fresh and marine water. We found an increase in sawgrass and mangrove productivity in the oligohaline regions of SRS and two production peaks in the TS-Ph transect, one in the oligohaline zone and one at the terminal site close to the Gulf of Mexico. These productivity patterns conformed generally to our interpretation of water quality trends, with the unexpected oligohaline peak in the TS-Ph transect likely being driven by a supply of nutrients from the groundwater.
Research continued to explore how spatially distinct supplies of N and P interact to control primary productivity, with an added focus on how the relative supply of nutrients from the surface and groundwater interact to control biomass allocation and production in the ecotone. We found that variability in delivery of marine supplies of saltwater and nutrients, and their interaction with water residence time, primarily control production patterns across the coastal gradient.
In FCE II, we hypothesized that fresh water (FW) inflows resulting from fresh water restoration (light blue arrows) would shift the productive coastal zone toward the marine water (MW) of the Gulf of Mexico along the SRS transect during the wet season, due to a coastward movement of the severity of P-limitation. In TS/Ph, landward groundwater (GW) discharge of P during the dry season causes a productivity peak in the oligohaline ecotone that also shifts coastward when counteracted by seasonal or restored freshwater inflow. In the absence of restoration, there is significant variability in productivity (halos around lines) due to high inter-annual variance in wet and dry season freshwater flows and marine pulses, driven by tides and storm surge.
In upstream freshwater marshes, short inundation periods and severe P-limitation reduce sawgrass (Cladium) production in TS/Ph relative to SRS, resulting in low net ecosystem-atmosphere CO2 exchange rates indicative of oligotrophy. In the ecotone, Cladium production is declining with increased landward brackish groundwater intrusion, while commensurate delivery of P is stimulating mangrove root production and metabolism rates in the adjacent open water. Our long-term research revealed that groundwater supply of P is important throughout the coastal gradient, controlling biomass allocation in Florida Bay seagrass communities, where historic P subsidies leave a permanent legacy. The SRS ecotone is also susceptible to increasing brackish groundwater discharge, in addition to surface water pulses driven by tides and storms, which is increasing the rate of coastal plant community migration to the interior. 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 (SLR), stimulated mangrove belowground production, and eventually leached into the river water column to cause phytoplankton blooms across Florida Bay. Data from our mangrove eddy covariance tower enabled us to document recovery of this nearly completely defoliated forest, to one that sequesters CO2 at rates exceeding temperate and many tropical forests. In summary, the delays in rehabilitating freshwater flows to the Everglades have created a system where productivity in the coastal transition zone are largely driven by marine water supplies to the ecotone, confirming extraordinary sensitivity to saltwater encroachment associated with SLR and storm activity