Seasonal dissolved organic carbon dynamics in estuarine and near-shore water

Dynamics of dissolved organic carbon (DOC) in near-shore and estuarine environments were examined in Massachusetts Bay and the Pawcatuck River estuary. Both field and laboratory studies were employed to identify important seasonal sources and sinks of DOC in these two ecosystems. Experimental and interlaboratory comparison studies were also conducted to assess the accuracy of the wet chemical oxidation (WCO) technique developed for DOC analyses.;As part of a large environmental monitoring program, DOC and other biological and chemical parameters were measured on 18 cruises conducted from 1992-1994 in Massachusetts and Cape Cod Bays. Regression analyses of DOC and ancillary parameters suggested that major DOC sources changed seasonally in the Bays. In spring months (February-April), surface DOC was negatively correlated with distance$sp{-1}$ from Boston Harbor (r$sp2$ = 0.26, p $<$ 0.0001) and positively correlated with phaeophytins (r$sp2$ = 0.23, p $<$ 0.0001). Together, these two parameters explained 42% of DOC variation and suggested that important spring DOC sources included wastewater-derived inputs entering via Boston Harbor, and in situ production associated with phytoplankton processes. Summer and fall months (June-October) showed a DOC pool associated with undersaturated dissolved oxygen and positively correlated with both ammonium and phosphate (r$sp2$ = 0.61 and 0.46, respectively; p $<$ 0.0001). These strong correlations suggest that microzooplankton grazing activity became an important DOC source during the summer/fall period.;In contrast to Massachusetts Bay, phytoplankton appeared to be the primary DOC source during the summer, dominating DOC dynamics in the Pawcatuck River estuary. Both field transects and incubation studies indicated large autochthonous DOC inputs (6.7-34.7 $mu$mol C liter$sp{-1}$ d$sp{-1}$) which were spatially associated with phytoplankton biomass and positively correlated with DCMU enhanced fluorescence (F$rmsb{DCMU}),$ a proxy for phytoplankton biomass (r$sp2$ = 0.555, p $<$ 0.001). While allochthonous DOC inputs were large, laboratory mixing experiments and field transects suggested that most of the riverine DOC mixed conservatively with only a small fraction ($sim$2-10%) potentially flocculating upon estuarine mixing. However, even the low percentage of flocculation could potentially provide 56 g C m$sp{-2}$ y$sp{-1}$ to the estuarine sediments. Riverine DOC may also indirectly influence DOC dynamics in this system through its control on light attenuation, and hence phytoplankton production in the Pawcatuck system.