Marsh:water Column Interactions in Two Louisiana Estuaries. Flux Measurements and Conceptual Implications.

Abstract

Many important ecosystem-level processes are integrated in hydrologically-forced marsh:water column interactions in estuaries. In this work, I quantified nutrient and sediment fluxes in two Louisiana estuaries using throughflow marsh flumes. The Barataria Basin estuary is in an later, deteriorating stage of the deltaic cycle. Brackish and saline marshes here exported dissolved inorganic nitrogen (DIN), total nitrogen, and dissolved organics (DOM); phosphorus fluxes were low and variable. Fourleague Bay marshes, in a earlier developmental stage of the deltaic cycle, imported DIN in the spring and released DIN in summer and fall. In this way, marshes here buffer open-bay N concentrations from high riverine N inputs. Fourleague Bay marshes exported phosphorus, but took up DOM on all tides sampled. Suspended sediment fluxes measured over individual tidal cycles were low and variable in both estuaries. Sediment accretion rates calculated from these fluxes were lower than actual measured accumulations. Apparently, most TSS flux onto marshes occurred during episodic climatological events. Wetland loss, a pervasive problem in Louisiana, has numerous implications to marsh:water column interactions. An extended period of low water levels in 1987-88 isolated marshes from the water column: a phenomenon I refer to as "ephemeral wetland loss". Multivariate analyses indicated that 37% (intermonthly) and 46% (interannual) of the variability in historical (1963-87) coastal water levels was explained by climatological parameters. Further, mean annual water levels explained 26% of the interannual variability in inshore shrimp harvest (a measure of marsh-dependent estuarine productivity). The relationship was nonlinear, with low shrimp harvest at low and high water levels. In both cases, low catch corresponded to El Nino events. Wetland loss represents a permanent removal of marsh area from the estuarine ecosystem. In Louisiana, most of this loss is a habitat change to open water. As marsh is converted to open water, the marsh:open water ratio changes and remaining marsh is "diluted" by increasing open water area. The functional loss of marsh, related to dilution, is greater than actual areal loss of marsh. The magnitude of functional loss depends on estuarine morphology, and is highest in marsh-dominated estuaries with high initial marsh:open water ratios.