Temporal and spatial variation in seston available to oysters and the contribution of benthic diatoms to their diet in the Duplin River, Georgia
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This study examined the quantity and quality of seston available to oysters and the contribution of benthic diatoms to their diets in the Duplin River, Georgia, and how it varied over different temporal and spatial scales. Average suspended particulate material (SPM), particulate organic carbon (POC), and chlorophyll a concentrations were significantly higher at the mouth (327.6 mg L-1, 5.0 mg L-1, 19.9 µg L-1) than at the headwaters (93.4 mg L-1, 1.6 mg L-1, 9.4 mg L-1). The quality of the seston was lower at the mouth with significantly higher carbon to nitrogen and POC to chlorophyll a ratios, but the seston was detrital-dominated throughout the inlet. Significant differences in seston characteristics were observed over tidal cycles at the mouth only, but all sites exhibited differences over lunar (spring tides greater than neap) and seasonal (spring and summer maxima) cycles. These differences were most consistent for SPM but were observed at some stations for other characteristics as well. Seston variability was greatest at the mouth at any temporal scale, but when compared within each site, the two sites closest to the mouth exhibited variability on tidal and lunar scales that was comparable to or even greater than seasonal scales, whereas at the up-river sites seasonal variability was the greatest. A significantly higher proportion of the diatoms were classified as benthic (pennate) at the headwaters as compared to the mouth in both seston (37% vs. 17%) and oysters (33% vs. 21%), although there was no evidence of selection for benthic forms. Microscopic observations of diatoms in seston and oysters were used to calculate the 13C and 34S values of oysters, assuming a diet of strictly diatoms, and these agreed well with observed values. This suggests that diatoms are a major food resource of oysters in this system and that benthic diatoms contribute to this pool. Oyster performance was evaluated at the study sites using shell height, dry tissue weight and condition index values. Oysters were significantly larger at the three up-river sites, which could be related to less than optimal feeding conditions and the high-energy physical regime near the mouth.