DOC dynamics in a small temperate estuary: Simultaneous addition and removal processes and implications on observed nonconservative behavior

Abstract

Dissolved organic carbon (DOC) dynamics in the Pawcatuck River estuary, a small temperate estuary in Rhode Island, United States, were examined through the use of field transect and in situ production studies. In late summer, when river discharge was minimal, phytoplankton blooms occurred in the upper reaches of the estuary and released large amounts of autochthonous DOC that accumulated in the middle reaches of the estuary. DOC production rates in August months, calculated both by mixing diagrams and in situ DOC incubations, ranged from 6.67 to 34.7 μmol C l−1 d−1 and were positively correlated with DCMU-enhanced fluorescence, an estimate of phytoplankton photosynthetic activity (r2=0.796, p<0.001). The percent extracellular release (PER) of DOC from phytoplankton, calculated from measured in situ DOC production and net phytoplankton production (NPP) rates, ranged from 5.8% to 40.6% and was negatively correlated with NPP (r2=0.80, p<0.01). Accumulated DOC was principally nonhumic in nature, and the humic DOC component behaved quite differently with either conservative mixing or significant removal at the head of the estuary. Humic removal at times amounted to approximately 50% of the humic material and 25% of the total incoming riverine DOC. These large humic losses were not observed in bulk DOC-salinity mixing diagrams but required distinct analyses of the humic and nonhumic components. DOC addition and removal processes co-occur in this system and observation of bulk DOC mixing diagrams may mask the true dynamic nature of the estuarine DOC pool. The net result of the DOC addition and removal processes is a seasonally variable transformation of a humic-rich incoming riverine DOC to a nonhumic enriched bulk DOC component that varies seasonally and with river discharge.