A Stable Isotope Model Approach to Estimating the Contribution of Organic Matter from Marshes to Estuaries

Abstract

The impact of marsh carbon export (outwelling) on estuarine metabolism has been debated for the last three decades. Much of this controversy stems from interpretations of stable isotope data. Although the outwelling of marsh carbon to the estuaries can be substantial, the stable isotope signal (delta C-13) Of marsh material is generally not detected except in sediments and infauna at marsh fringes. However, most of these studies focus on the delta C-13 Of either particulate organic carbon (POC) or the delta C-13 Of estuarine organisms that depend on POC, even though this carbon pool may be the least likely to provide a marsh signal. A series of simple models are developed to show how marsh outwelling affects the delta C-13 of POC and the other major estuarine carbon pools, i.e., dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC). The models show that a marsh delta C-13 Signal will only be detected in estuarine POC or DIC when the marsh area is substantially larger than the estuary. However, because estuarine in-situ DOC production is only a fraction of POC production, our mixing models suggest that a marsh signal should be found in estuarine DOG, even when the marsh areas is smaller that the estuarine area. Finally, a transport model, incorporating a simplified bathymetry and hydrology for the Parker River, MA, is used to back calculate marsh outwelling from the estuarine delta C-13-DOC. The model estimate of marsh DOC outwelling is consistent with other estimates, and suggests that our parameterization of estuarine transport and degradation processes that regulate DOC isotope ratios is probably also correct.