Settleable and non-settleable suspended sediments in the Ogeechee River Estuary, Georgia, USA

Suspended particle dynamics were investigated in the Ogeechee River Estuary during neap tide in July 1996. Samples were operationally separated into ' truly suspended ' (settling velocity <0.006 cm s(-1)) and ' settleable ' (settling velocity >0.006 cm s(-1)) fractions over the course of a tidal cycle to determine whether these two fractions were comprised of particles with differing biological and chemical characteristics. Total suspended sediment, organic carbon and nitrogen, chlorophyll a and phaeopigment concentrations were measured in each fraction, as well as rates of bacterial hydrolytic enzyme activity [beta-1,4-glucosidase (beta Gase) and beta-xylosidase (beta Xase)]. The majority of the suspended sediment (by weight) was in the truly suspended fraction; all measured parameters were largely associated with this fraction as well. When compared to the settleable material, the truly suspended material was significantly higher in % POC (57 +/- 0.6 vs. 3.9 +/- 1.8), % chlorophyll (0.07 +/- 0.02 vs. 0.03 +/- 0.01), % phaeopigment (0.030 +/- 0.006 vs. 0.018 +/- 0.012), and weight-specific maximal uptake rates (V-max per mg suspended sediment) of both enzymes (1.8 +/- 0.4 vs. 0.7 +/- 0.2 nmol mg(-1) h(-1) beta Gase and 1.1 +/- 0.3 vs. 0.3 +/- 0.2 nmol mg(-1) h(-1) beta Xase), providing clear evidence for a qualitative distinction between the two fractions. These results are interpreted to mean that the more organic-rich, biologically active material associated with the suspended fraction is likely to have a different fate in this Estuary, as ' truly suspended ' sediments will be readily transported whereas ' settleable ' sediments will settle and be resuspended with each tide. These types of qualitative differences should be incorporated into models of particle dynamics in estuaries. (C) 2000 Academic Press.