Interactions on Minerals and Organic Materials Along the Salinity Gradient in Barataria Basin, Louisiana.

Changes in the associations among mineral and organic materials in estuaries may control the nature and quantity of organic material (OM) preserved in sediment deposits or cycled in the water column. Estuarine sediment-OM attachments were examined using descriptive analyses and experiments to assess their response to salinity changes and the bioavailability of sedimentary OM. Desorption experiments compared the amount of organic carbon extracted from sediment in rinses of different salinity. Uptake experiments monitored changes in carbon and bacteria concentrations to compare the bioavailability of dissolved and sedimentary OM. Clay mineralogy, surface area and elemental analysis of suspended and deposited sediment were examined along the salinity gradient. Salinity, suspended sediment, dissolved and particulate organic carbon, and chlorophyll concentrations were used to describe water quality. Experimental sediment OM releases of ∼ 1--5% (deposit) up to 30% (suspension) were controlled primarily by sediment organic content and secondarily by salinity, pH and DOC. Bacteria consumed both DOM ( ∼ 11%) and sedimentary OM (43--79%). The largest losses were from suspended sediment OM (65--79%), and corresponded to growth of the largest bacterial populations (30 x 106 cells mL--1 ). Fine sediment was composed of smectite, illite, kaolinite and chlorite. Mineralogical variation was attributed to resuspension effects. Sediment surface area averaged 28.5 m2 g--1 (deposit) to 55.0 m2 g--1 (suspension). Deposited sediment averaged 4.1% organic carbon and 0.4% nitrogen; with values increasing upstream. Concentrations of both were higher in suspended sediment (OC = 8.3%, TN = 0.9%). Carbon to nitrogen ratios were lower in deposited than suspended sediment ( ∼ 11.3 and 13.5, respectively), suggesting that suspended sediment was enriched in labile organic components. Physical characteristics, including clay mineralogy and surface area, control the amount of organic carbon attached to sediments. Environmental factors, such as distance from the mouth of the bay and chlorophyll concentration, determine the nitrogen content and thus the lability of attached organic material. A conceptual model supported by the experimental results and descriptive data, suggests that labile OM adsorbs to sediment in productive regions and carries it to poorer regions where it can be released. Through this mechanism, suspended sediment influences the distribution and consumption of OM in estuaries