Award: OCE-1234704

Spartina-dominated marshes are a dominant environment along the southeastern US coast. They perform a variety of very important environmental functions, from serving as habitat for the young of many fish species, as stopovers for migratory birds, and as an interface between inland river systems and the coastal ocean. Significant tides, as much as 3 meters in some regions, move waters back and forth between estuarine rivers and the marshes though intricate networks of channels. The continuous exchange of water between the deep river channels and the shallow marsh surface allows marshes efficiently create, modify and remove organic matter in the water column as it is repeatedly exposed to the marsh surface. The balances among these processes determine what is ultimately exported from the land into the coastal ocean. tHow environmental factors such as day/night cycles, temperature, rainfall, and the timing of tidal cycles, affect this balance are very much open questions. The goal of this project was to examine carbon and water balances within a small but typical spartina-dominated salt marsh near Savannah, Georgia. In order to do so, we instrumented the dominant tidal creek feeding the marsh with a variety of devices designed to measure salinity, temperature, water color and fluorescence (measures of dissolved organic matter concentrations), water current movements, and oxygen content. This was done over a period of 18 months, from July 2013 to January 2015. Because these tidally-influenced salt marshes are very active biologically, one of our challenges was to ensure that our instruments were faithfully recording their individual signals, which required swapping our many of them every 2-3 weeks for cleaning and re-calibration. Even with that effort, the data from our instruments required significant cleanup to remove dropouts (in one case, a fish took up residence in the intake port of an instrument). Calibration was also cross-checked with water samples taken during these instrument swapouts. In addition, we periodically collected water samples using an automated sampler for those types of measurements that cannot be measured optically or electronically in place, like particulate organic matter, chlorophyll and cell counts. And every 4 months we sampled the marsh waters by hand, for very time-sensitive samples like RNA and dissolved gasses. So in all we were able to gain a very detailed picture of how Groves Creek, and by extension the marshes dominating the Georgia and South Carolina Coastlines, function in terms of carbon, oxygen and other elements. With this data in hand, what did we learn? First, that this marsh is mostly ?net heterotrophic?, meaning that it burns more organic matter than it produces. We see that in a net overall consumption of oxygen, chlorophyll, and dissolved organic matter over the study period. However there are times of the year and even parts of a particular month, where this is reversed and the marsh becomes an exporter of dissolved organic matter. And even though the marsh as a whole may be a net importer, complex circulation of water on the marsh surface means that certain conduits and tributaries within the marsh may axt as net local exporters. Interestingly, although Spartina grass visually dominates the marsh ecosystem, it contributes relatively little towards carbon export. Instead, it is the very highly biologically active marsh muds that appear to strongly influence marsh particulate carbon exports, and possible also dissolved organics as well (that part will be resolved in the future as we finish sample analysis and data workup). We also observed several environmental events, including a pulse of water from an up-state dam release that left a pool of terrestrially-derived dissolved organic matter sloshing back and forth in the local estuaries for weeks. Also during our sampling period Georgia went from a dry-drought period to a wet period. These types of climatic shifts were recorded in both organic and inorganic carbon compositions and concentrations, as well as salinity and temperature records, and are the subjects of continued interpretation as we complete the latest rounds of manuscript writing. Last Modified: 12/02/2016 Submitted by: Jay Brandes