Temporal and Spatial Dynamics of Particulate Organic Carbon in the Lake Pontchartrain Estuary, Southeast Louisiana, U.S.A

Abstract

The spatial and temporal distribution of particulate organic carbon (POC) was examined in the Lake Pontchartrain estuary, along with changes in freshwater and nutrient inputs. Increased turbidity due to increases in suspended particulate matter (SPM) concentrations during high freshwater discharge resulted in low inputs from autochthonous POC. For example, annual averages of chlorophylla(2·3±0·9μg l−1) and POC (0·9±0·6mg l−1) were found to be considerably lower than in other shallow turbid systems. A low N/P ratio was maintained throughout the study (from 2·3 to 8·3), primarily due to low inputs of dissolved inorganic nitrogen (DIN). Thus, phytoplankton communities in the Lake Pontchartrain estuary were primarily limited by light and nitrogen. Stations in the upper estuary were generally more light-limited as opposed to nitrogen-limited stations in the lower estuary. Based on chlorophylla/carotenoid ratios, the most dominant phytoplankton classes were cyanobacteria, cryptophytes, chrysophytes and chlorophytes—with episodic cyanobacterial blooms occurring in the summer months. High carbon/chlorophyllaratios suggested that much of the POC contained low concentrations of chlorophyllathat had degraded during its transport from adjacent wetlands. High C/N ratios (7·4 to 30·6) further suggested that allochtonous inputs of organic carbon from terrigenous sources were likely to be important. Lignin-phenol concentrations in sediments from the upper region of the estuary indicated that the Lake Pontchartrain estuary had received higher inputs of terrigenous organic carbon compared to other estuaries in the Gulf region.