Transport and fate of dissolved organic carbon in the Lake Pontchartrain estuary, Louisiana, U.S.A.

Abstract

Cycling dynamics of dissolved organic carbon (DOC) were examined in Lake Pontchartrain estuary, Louisiana, in relation to changes in freshwater inputs. DOC concen- trations ranged from 5.3 to 8.5 mg C L-' reaching their highest during high river inflow. The percentage of DOC represented by HMW DOC (or colloidal material) was greatest (ca. 11%) at stations where freshwater discharge from rivers and surrounding wetlands was most significant. Moreover, the lignin-phenol content of this material (A ranged from 0.09 to 0.33 and A from 0.11 to 0.39) confirmed that a significant fraction of colloidal organic carbon was derived from terrestrial sources. Riverine and benthic fluxes represented the dominant sources of DOC to the estuary. On an annual basis, riverine and benthic DOC concentrations were estimated to be 2.8 x 1010 g C yr-' and 8.8 x 1010 g C yr-', respectively, while the total DOC pool in the estuary was 3.8 x 1010 g C. Annual average concentrations of dissolved inorganic carbon (DIC) (1298 /iM) and pCO2 (5774 /iatm) were comparable to those found in other freshwater systems that reached CO2 saturation levels. Net losses of DOC in the Lake Pontchartrain estuary appeared to be primarily controlled by heterotrophic consumption (con- version of CO2) - which may have been amplified by the long residence time (approximately 120 days) of DOC in this system.