Historical Data Review of Dissolved Oxygen and Related Parameters for the Lavaca Bay TMDL Project

"This historical data review is part of the Lavaca Bay/Chocolate Bay (Segment 2453) Total Maximum Daily Load (TMDL) Project. The data generated will be used to identify areas within the segment for further investigation, assess the segment for Dissolved Oxygen (DO) exceedences, and support the development of a TMDL for DO. Segment 2453 was listed on the 305(b) list because DO concentrations are occasionally lower than the criterion established to assure optimum conditions for aquatic life in a 13.7-square mile area near the Alcoa Ship channel. “Aquatic life use” is a term used in Texas water resource management to characterize water bodies and specify water quality criteria for those bodies. The Texas Natural Resource Conservation Commission (TNRCC) has identified four aquatic life use categories: exceptional, high, intermediate, and limited. The TNRCC criteria for DO varies with the Aquatic life use designation of a water body (Table 1; Texas Natural Resource Conservation Commission, 2000). For Texas estuaries designated with exceptional aquatic life use, DO criteria are 24-hr mean < 5.0 mg 1⁻¹ and 24-hr minimum < 4.0 mg 1⁻¹. Lavaca Bay (segment 2453) is designated for exceptional aquatic life use. Estuaries designated for high aquatic use must meet less strenuous criteria of 24-hr mean < 4.0 mg 1⁻¹ and 24-hr minimum < 3.0 mg 1⁻¹. Dissolved oxygen is a parameter used to evaluate aquatic ecosystem health. Dissolved oxygen is a useful indicator because it is required for organism respiration and microbial sediment decomposition processes (Strobel and Heltshe 2000). In addition, low DO (i.e., hypoxia) can be caused by excess nutrient inputs into the aquatic system (e.g., sewage outfalls, non-point source pollution, etc.)"--Introduction.