Food resources and water flow speeds vary on short-temporal and small-spatial scales important to feeding in natural populations of oysters (Crassosfrea uirginica). To better describe food availability to the oyster population, micro-current meters were used to measure flow 1-3 cm above the bottom and frequent water sampling was used to determine the quantity and composition of the,seston on Confederate Reef, Galveston Bay, Texas. The quantity and quality of food varied on time scales at least as short as 3 h and throughout the water column. Seston was high in detrital carbon, nitrogen poor, and had a high fraction of inorganic particles. The quantity of food at the bed was not predictable from samples taken near the surface. Water flow speeds were also highly variable. Whereas rapid flow speeds were measured, flow speed tended to cluster about a narrower range of slower speeds resulting in the mean speed being much higher than the median. Fluxes of food calculated from the quantity of food and water flow speeds were low and highly variable. Under most conditions, water flow speed was more important than food quantity in determining flux. Rapid flow rates could overcome low food quantity to result in relatively high fluxes, whereas slow flow resulted in small fluxes even if food was abundant. Changes in water flow rate associated with tidal changes and changes in speed are probably the primary agents controlling food availability on a daily basis. Decreased wind speed at night normally reduced flux in the early morning hours, for example. These results stress the importance of measuring food quantity and water flow speed on spatial and temporal scales important to oyster feeding. Model simulations of oyster populations at varying densities, flow rates and clump heights across the range of measured values show that flow rate is insufficient during most hours of the day and days of the year to prevent oysters from locally reducing food supply by their filtering efforts. Thus flow rate probably limits population density and adult size on Confederate Reef.
ProblemThe food available to natural populations of suspension-feeding bivalves is dependent on the amount of food present in the water column and the characteristics of the water flow delivering food to the population. If the quantity of food is low, or the rate of water flow is slow, food limitation may occur resulting in decreased U. S.
One component of the Gulf of Mexico Offshore Operations Monitoring Experiment (GOOMEX) was to assess the reproductive effort and health (defined as the prevalence and intensity of parasites or pathologies) of mobile macroepifauna collected near‐to and far‐from gas‐producing platforms. The stage of reproductive development as determined by visual inspection or histological analysis was rarely significantly different between near‐field and far‐field sites. Tissues were examined histologically to determine the prevalence and intensity of parasitism and disease. If a difference in parasitism or pathology existed, individuals collected far‐from the platforms typically had higher prevalences while individuals living near‐to the platforms usually had higher intensities. Parasitism and disease had little effect on stage of reproductive development except at MU‐A85 where individuals with increased disease and parasitism also had decreased reproductive development. Variations in infection intensity were not due to differences in age structure of populations. Larger, older individuals were not consistently more heavily parasitized.
Trend in reproductive stage and population health did not follow trends in environmental contamination; the fewest significant differences were observed at the most contaminated platforms. Patterns of differences in parasitism, disease and reproductive effort between the near and far‐field stations were platform specific, indicating that the physical and chemical characteristics unique to each platform probably control reproductive effort and levels of disease and parasitism in populations living there. The influence of structure, a variable common to all platforms, did not exert an overriding influence. Mobile taxa such as shrimp exhibited near‐field/far‐field differences as frequently as the less mobile species such as starfish. Thus, mobile epifauna maintain discrete populations over long‐enough time periods to develop different population histories. Shallow‐water sites were more likely to exhibit platform‐specific near‐field/far‐field differences in population structure suggesting that trends in water depth and the intensity of seasonality may explain the apparent uniqueness of each platform's effect on population structure and health.
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