Seasonal hypoxia and models of benthic response in a Texas bay

Hypoxia occurs during summer in the southeastern region of Corpus Christi B ay, Texas. The objectives of this study were to identify potential causes o f recurrent hypoxic events, to determine hypoxic effects on benthic macroin fauna, and to develop models of benthic response. Long-term and short-term hydrographic surveys were performed, and macroinfaunal samples were collect ed from normoxic and hypoxic regions of the bay. Hypoxia occurred in seven of the nine summers sampled (1988 to 1996). In 1994, the hypoxic event pers isted for approximately 3 wk. Hypoxic events were associated with water col umn stratification where the difference between bottom and surface salinity was as high as 7.2 parts per thousand and averaged 4.1 parts per thousand. The salinity difference is surprising because water column stratification is not expected in shallow (< 4 m), windy (average 18.5 km h(-1)) bays. Str atification did occur-hypersaline bottom water in a relatively stagnant por tion of the bay-in spite of mixing forces (i.e., high winds), giving rise t o hypoxia. Benthic biomass decreased 12-fold, and abundance and diversity d ecreased 5-fold under hypoxic conditions. In addition, dominance patterns s hifted as oxygen levels declined from 5 mg O-2 l(-1) to < 1 mg O-2 l(-1). T he polychaete Streblospio benedicti and oligochaetes tolerated low oxygen b etter than other infauna. Community response to hypoxic disturbance was fit to a nonparametric categorical model and a parametric logistic model. Biom ass, abundance, and diversity exhibited a lag response at < 3 mg l(-1), and increased exponentially from 3 mg l(-1) to 6 mg l(-1). Based on both model s, 3 mg l(-1) appears to define the breakpoint between normoxic and hypoxic benthic communities in Corpus Christi Bay. This value is higher than tradi tional definitions of hypoxia, < 2 mg l(-1) or < 2 mi l(-1) (ca. 2.8 mg l(- 1)).