In Phase I of the FCE-LTER project, researchers devoted considerable resources to understanding the sources, fate, and transport of dissolved organic matter (DOM). The scientists developed methods for characterizing DOM so that they could identify major sources of DOM from key ecosystem components. These were then used in Phase II to examine the contribution of soil-derived DOM and groundwater-derived DOM to the overall pools of DOM in the estuarine ecotones.
Research tracking the quality and fate of OM through the SRS and TS/Ph estuaries showed how the ecotone receives OM subsidies from considerable distance upstream, particularly during large runoff events, supporting our hypothesis that increased fresh water inflows influence OM delivery to estuaries. Even particulate OM (floc
) is subject to long-distance transport at rates of just a few m/day, and floc
metabolism studies suggest a residence time of a month or more. The source materials for POM, or floc
, production, deposition and accumulation appear to be derived from periphyton
and macrophytes such as Cladium and Eleocharis that make up a significant portion of the freshwater marsh biomass. Litterbag experiments document significant DOM release from these materials over relatively short time scales, but the remaining refractory OM can be incorporated both into floc and surface soils. Interestingly, floc biodegradation is severely P-limited, but floc photo-dissolution is rapid and converts POM to DOM. Degradation kinetics suggest both labile and refractory pools for DOM, but surface water DOM in the FW marsh does not degrade significantly over the residence time of a conservative tracer.
Although OM in the estuarine zone of the FCE is mostly mangrove-derived, the oligohaline ecotone also generates OM via autochthonous production. The estuarine ecotone clearly reflects major differences in soil properties and organic matter dynamics, relative to FW and marine endmembers. Seagrass production controls much of the DOM quality in the central bay, particularly during the summer (June to August). However, as in the case of POM, DOM contributions from freshwater and estuarine ecotone environments can reach northern sections of the bay during high freshwater discharge periods in the fall. From paleoecological studies, temporal changes in hydroperiod and associated changes in vegetation cover are clearly reflected in profiles of molecular markers specific to SOM, i.e., these compounds can be utilized as paleoproxies. Biomarkers (i.e., Taraxerol) document the historical presence and persistence of mangrove vegetation in the SRS-5 and SRS-6 sites and dynamic changes in marsh/mangrove dominance at the SRS-4 location; probably reflecting the rapid changes in sea level rise during recent history. Although DOM dynamics at SRS and TS are well-understood, the next challenge will be assigning quantitative values to the different contributions in order to determine carbon fluxes and a mass balance of organic matter.