Ecosystem Modulation of Dissolved Carbon Age in a Temperate Marsh-Dominated Estuary

Abstract

We measured the concentrations and isotopic values (Δ14C and δ13C) of dissolved inorganic, dissolved organic, and particulate organic carbon (DIC, DOC, and POC, respectively) in the Parker River watershed and estuary in Massachusetts, USA, to determine the age of carbon (C) entering the estuary and how estuarine processing affects the quantity and apparent age of C transported to the Gulf of Maine. The watershed measurements indicated the transport of Δ14C-enriched modern DIC and DOC and variably aged POC from the watershed to the estuary. The transport of organic matter from the watershed was dominated by DOC transport, with POC making up less than 10% of the total. Surveys within the watershed aimed at determining which land-use type dominated the DOC export indicated that wetlands, although they made up only around 20% of the land use, could be responsible for approximately 75% of the DOC export. We therefore conclude that the wetland land uses of the Parker River watershed are exporting mainly Δ14C-enriched modern DOC. DIC isotopes indicate that the source of DIC in the Parker River watershed is dominated by the weathering of noncarbonate parent material by Δ14C-enriched carbon dioxide (CO2) originating from the respiration of young organic matter in soils. Transects in the estuary displayed net additions of all C species. For DOC and DIC, the export of this internally added DOC and DIC was approximately equal to the amount being exported from the watershed, showing the importance of focusing on estuaries when estimating the export of C to the coastal ocean. With respect to DIC, the total input is even larger when the atmospheric exchange of excess pCO2 is calculated. The Δ14C-DOC and Δ14C-DIC transects indicate that the internally added DOC and DIC is Δ14C-enriched modern material. The source of this material is the fringing marshes and estuarine phytoplankton, with the relative importance of these two sources changing over time. Taken together, the bulk C and Δ14C measurements show that the estuary is adding significant quantities of young DOC despite the presence of vast quantities of old marsh peat flanking the entire estuary. Furthermore, the DIC data indicate that Δ14C-enriched modern material is what is fueling the majority of heterotrophic respiration within the system.