Seasonal hypoxia and models of benthic response in a Texas Bay

Hypoxia occurs during summer in the southeastern region of Corpus Christi Bay, Texas. The objectives of this study were to identify potential causes of recurrent hypoxic events, to determine hypoxic effects on benthic macroinfauna, and to develop models of benthic response. Long-term and short-term hydrographic surveys were performed, and macroinfaunal samples were collected 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 persisted for approximately 3 wk. Hypoxic events were associated with water column stratification where the difference between bottom and surface salinity was as high as 7.2‰ and averaged 4.1‰ The salinity difference is surprising because water column stratification is not expected in shallow (< 4 m), windy (average 18.5 km h⁻¹) bays. Stratification did occur—hypersaline bottom water in a relatively stagnant portion of the bay—in spite of mixing forces (i.e., high winds), giving rise to hypoxia. Benthic biomass decreased 12-fold, and abundance and diversity decreased 5-fold under hypoxic conditions. In addition, dominance patterns shifted as oxygen levels declined from 5 mg O₂ 1⁻¹ to <1 mg O₂ 1⁻¹. The polychaete Streblospio benedicti and oligochaetes tolerated low oxygen better than other infauna. Community response to hypoxic disturbance was fit to a nonparametric categorical model and a parametric logistic model. Biomass, abundance, and diversity exhibited a lag response at <3 mg l⁻¹, and increased exponentially from 3 mg 1⁻¹ to 6 mg 1⁻¹. Based on both models, 3 mg 1⁻¹ appears to define the breakpoint between normoxic and hypoxic benthic communities in Corpus Christi Bay. This value is higher than traditional definitions of hypoxia, <2 mg 1⁻¹ or <2 ml 1⁻¹ (ca. 2.8 mg 1⁻¹). *** DIRECT SUPPORT *** A01BY085 00002