Cause and effects of hypoxia (low oxygen) in Corpus Christi Bay, Texas

Hypoxia (low oxygen conditions) has been documented in the southeastern region of Corpus Christi Bay, Texas every summer since 1988. Hypoxia is a serious water quality condition because all aerobic organisms require oxygen to live. Hypoxia in Corpus Christi Bay occurs only in bottom waters, within 1 - 2 meters above bottom, so effects are mostly on benthic organisms. The objectives of the current study were to determine the physical conditions related to the occurrence of hypoxia and ecological consequences of hypoxia in Corpus Christi Bay, Texas. Continuous oxygen recorders were deployed and water quality was surveyed during summer to measure spatial and temporal scales of hypoxia. Caging experiments were conducted to measure effects of hypoxia on trophic interactions in the benthic food chain. Water column profiles measured during spatial surveys revealed benthic oxygen depletion with and without salinity stratification, but only rarely captured hypoxia. Continuous oxygen monitoring captured numerous hypoxic events, many of which were of short duration (less than 1 hr) and low intensity (between 2 and 1 mg 1⁻¹). Continuous monitoring captured one event that lasted longer than 24 hrs and was intense (i.e., less than 1 mg 1⁻¹). The trends in the hydrographic data indicate that hypoxia is a transient event caused by a combination of biotic processes (i.e., respiration) and abiotic processes (i.e., low mixing potential because of sluggish currents and a small tidal range). Macrobenthic community characteristics (i.e., abundance, biomass, species richness, diversity, and evenness) were depressed in the hypoxic region compared to the normoxic region. Community structure was also different between the hypoxic and normoxic regions because of loss of species (presumably due to intolerance to low oxygen) in the hypoxic region. Although benthic invertebrates might be caused to move toward the surface during hypoxic conditions, there does not appear to be increased predation pressure on macrobenthos in Corpus Christi Bay due to hypoxia. The lack of increased predation pressure may be due to predator avoidance of the hypoxic zones, or lack of a prey behavior because of the short duration or low intensity of hypoxic events measured during this study. Hypoxia in Corpus Christi Bay is caused by a combination of biotic and abiotic processes and does have negative direct effects on benthic organisms. However, indirect ecological effects do not appear to be a problem in Corpus Christi Bay, at least during the period of this study.