A mass- and isotope-balance model of DOC mixing in estuaries

Abstract

Expanding knowledge of degradation kinetics coupled with significant improvements in measurement precision of dissolved organic carbon (DOC) necessitates a review of factors that influence its cycling in estuaries. A onedimensional, time-dependent estuarine model was used to ascertain how sinks and sources of DOC produced conservative and nonconservative property-salinity distributions. DOC from riverine and coastal mixing members was assumed to have labile (k = 0.1 d-1) and refractory (k = 0.001 d-1) components. The magnitude of other inputs (i.e., marshes and phytoplankton) varied with respect to riverine loading (20 mol C s-1). Also, marsh (δ13C = -12) and phytoplankton (δ13C = -20) DOC had distinct isotopic ratios compared with riverine (δ13C = -28) and coastal (δ13C = -23) DOC. Diffusive mixing times were modified by adjusting the dispersion coefficient (velocity was constant). As expected, conservative profiles were obtained when time-scales (half-life, turnover time) associated with sources and sinks were significantly greater than estuarine mixing times. Degradation of coastal DOC, however, had little impact on property-salinity relationships at all mixing times studied. Moreover, nonconservative profiles were detected when at least 10% of riverine DOC was labile. In turn, the magnitude of marsh or phytoplankton inputs compared to riverine loading had a greater impact on property-salinity distributions than estuarine residence time. Reexamination of literature data (Parker River and Ipswich River estuaries, Massachusetts) indicated that DOC concentration and isotope data can, coupled with this modeling effort, be used to estimate the magnitude and origin of DOC in estuaries.