WINTER AND SUMMER DIATOM COMMUNITY AND WATER COLUMN STRUCTURE OF THE SAVANNAH RIVER ESTUARY

This study characterized the diatom community and water column conditions of the Savannah River estuary during flood tide in winter (Feb. 26, 2010) and summer (Aug. 15, 2010). Eight stations were sampled from offshore of the river mouth (>33 PSU) upstream until a salinity of 3 PSU was reached. Surface waters had a mean temperature of 9.6ºC in February and 30.4ºC in August. Overall, the estuary was characterized by moderate salinity stratification and partially mixed conditions during flood tide. Well mixed conditions were present at the farthest offshore site in February. Fluorescence data revealed greater pigment concentrations within and beneath the halocline. At upstream sites near high tide, chlorophyll was highest very near the estuarine floor. The water of the Savannah River is very turbid with high CDOM concentrations (up to 49.4 PPB in February 2010). Light compensation depths occurred shallower than 5 m at the estuarine stations in winter, suggesting that the phytoplankton trapped within and beneath the halocline had limited light availability. Although diatom standing stocks were greater at depth than at the surface at most sites, the genera present were very similar in surface and bottom waters. In February, Skeletonema spp. dominated all surface water samples representing 49-87% of the diatoms, with secondary contributions from Asterionellopsis spp. (1-35%). In contrast, Skeletonema spp. increased upstream in August from 0 to 96% of the community with a coincident decrease in Rhizosolenia spp. (98 to 0%). The ratio of these two genera may serve as a good salinity indicator during summer conditions. Asterionellopsis spp. were absent in the August sample, suggesting that this genus may only be present seasonally in the estuary potentially favoring warm conditions.