| Abstract: | Field studies were conducted at Confederate Reef, Galveston Bay, Texas, to assess the variability of food resources and water flow speeds on the short temporal and small spatial scales relevant to feeding in natural oyster populations. The quantity and quality of food resources varied on time scales as short as 3 hr and throughout the levels of the water column. Food resources were plentiful, but were of relatively low quality, being high in detrital carbon. Both chlorophyll a and organic nitrogen were present in sufficient quantities throughout the year to meet the nutritional needs of the population. No one level of the water column was consistently higher or lower in food than any other, so the quantity of food at the bed was not predictable from samples taken near the surface. Water flow speeds were also highly variable, but tended to cluster about a narrower range of slower values. Fluxes of food resources calculated from the quantity of food present and the water flow speeds were small and also highly variable. Under most conditions, water flow speed was more important than food quantity in determining flux. Rapid flow rates can overcome low food quantity to result in relatively high fluxes, while even if food is abundant, slow flow results in low fluxes. Because oysters require large amounts of food, water flow rate is very important in maintaining the supply. Increases in water flow speed associated with tidal changes is probably the control on food availability on a day to day basis. Simulations run utilizing an energy flow model suggested that productivity was overestimated when using average conditions of food and flow as compared to the more variable data obtained. Results of the simulations suggested that under some conditions of food and flow, the population at Confederate Reef was food limited. The population exhibited a net loss of somatic tissue during the winter. Changes in size class distribution suggested that somatic energy was lost preferentially from large individuals, while smaller oysters grew well. The model also indicated that water flow is very important in determining productivity because of its control on food supply, and emphasizes the necessity of measuring both food availability and flow speed. |
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