Status and Trends of Dissolved Oxygen in Corpus Christi Bay, Texas, U.S.A.

The purpose of this studywas to determine status and long-term trends of dissolved oxygen concentrations (DO) in Corpus Christi Bay, Texas, U.S.A. A 20-year record of randomized stations was used to determine the trend of surface water DO, salinity, and temperature over space and time. A 13-year record of two fixed stations was used to determine the temporal nutrient trends. A 10-year record of fixed stations in the southeastern region of Corpus Christi Bay was used to determine the status of disturbance caused by low DO in bottom waters. From 1982 to 2002, there was a significant decrease in surface water DO at a rate of 0.06 mg L⁻¹ yr⁻¹ and a significant increase in surface water temperature at a rate of 0.07°C yr⁻¹. The southeastern region of Corpus Christi Bay had the lowest average DO, and during July and August, DO are steadily declining at a rate of 0.09 mg L⁻¹ yr⁻¹. It is not likely that eutrophication is causing hypoxia, because freshwater inflow rates have significantly decreased since 1941 and nutrient levels have not changed from 1987 to 2000. Even though long-term trends indicate that average surface DO is decreasing, disturbance by hypoxia appears to be stable, but this may be due to just eight years of data. In fact, if the current trend continues, surface water DO will not meet exceptional aquatic life standards (≤5 mgL⁻¹) in 2032.     

The digestive protease,chymotrypsin, as an indicator of nutritional condition in larval red drum (Sciaenops ocellatus)

Laboratory-reared red drum (Sciaenops ocellatus) larvae were used to evaluate the potential of chymotrypsin as an indicator of nutritional condition in marine fish larvae. The response of chymotrypsin activity to food deprivation and reductions in nutrient intake was determined. Enzyme activity declined rapidly to undetectable levels in food-deprived larvae 6–18 days old. Larvae fed poor-quality live prey (starved rotifers, Brachionus plicatilis) exhibited reductions in growth (18%) and enzyme activity (84%) relative to larvae fed high-quality prey (enriched rotifers). Potential sources of variation in chymotrypsin activity unrelated to nutritional status, including diel periodicity, and exogenous enzymes sources were examined. A diel pattern in chymotrypsin activity was detected with an 8.7-fold increase in activity occurring from low to high points during a 24-h period. Highest activity levels occurred late in the day (1600 hours) and lowest activity in the morning prior to feeding (0800 hours). The estimated contribution of exogenous enzymes from prey in the digestive tract to measurements of larval enzyme activity was small, reaching a maximum of 4.1% on day 18 in well-fed larvae. Results indicate that exogenous enzymes will not lead to the misclassification of larvae in poor condition. A relationship between chymotrypsin activity and standard length was established for well-fed and food-deprived larvae that could potentially be used to determine the nutritional condition of wild-caught larvae.