The temporal and spatial variability of diatom distributions and chlorophyll a concentrations in the Savannah River estuary (Savannah, GA, USA)

摘要

The water quality of coastal environments can be monitored by the community structure of diatoms. One underlying purpose of this study was to identify diatom genera that could serve as ideal bio-indicators of water column properties in the Savannah River estuary. The other purpose was to determine the temporal and spatial variability of diatom distributions and chlorophyll a concentrations in the Savannah River estuary. Bi-monthly cruises were conducted along the Savannah River estuary from August 2009 to December 2012 aboard the R/V Savannah. Samples were taken at salinities ranging from >32 to <3 PSU. In October 2009 and October 2012, water samples were taken at ebb and flood tides to observe diatom and chlorophyll a concentrations in response to tidal and diel conditions. Diatom standing stocks and chlorophyll a concentrations were not consistently higher in flood/ebb, nor day/night. There were also no significant differences in diatom community diversity with tidal, nor diel conditions. Diatom community composition varied more with the sample date than between different tidal and diel conditions. In terms of diatom distributions across salinities, Skeletonema was the most dominant diatom genus throughout the estuary regardless of salinity. Rhizosolenia and Chaetoceros were most common in the lower estuary (30--32 PSU). Hence, Rhizosolenia and Chaetoceros could be good bio-indicators of salinity gradients in the estuary. Seasonally, diatom and chlorophyll a concentrations were highest when temperatures were warm and were lower in the winter. However, diatom diversity was lower in the summer than in the winter. Superimposed on this seasonal cycle was an overall decrease in diatom and chlorophyll a concentrations over the period of study. The maximum diatom stock was 18,092 cells L-1 in June 2010 and subsequent summer maxima were lower than this maximum. Seasonal diatom genera were Thalassionema , Coscinodiscus, and Ditylum that exhibited maxima only in the fall. These diatom genera could be ideal bio-indicators of fall conditions in the estuary. Changes extending vertically down the water column were also examined. Density stratification increased upriver. Diatom community composition was similar in surface and bottom waters despite stratification with greatest similarity in well mixed waters. Similarly, on average, diatom standing stocks in the bottom waters (768 cells L-1) were not significantly higher than stocks in the surface waters (547 cells L-1) were similar. Bottom waters consistently had higher diatom standing stocks and chlorophyll a concentrations than surface waters but, when averaged over the estuary, only chlorophyll a concentrations were statistically greater at the bottom.