Influence of severe drought conditions on chromophoric dissolved organic matter dynamics in south Texas coastal waters

We investigated chromophoric dissolved organic matter (CDOM) dynamics in response to the severe drought conditions of 2009 in the Aransas–Copano (AC) Bay complex and adjacent nearshore Gulf of Mexico (GoM) along the southern Texas (USA) coast. Surface water absorption coefficients (a305 in m−1) were measured daily at the University of Texas at Austin Marine Science Institute (UTMSI) pier located at the interface between the AC complex and the GoM. From June to August 2009, a305 averaged 0.81 ± 0.31 m−1 compared to 1.20 ± 0.70 m−1 during summer of 2008 (June–August) and 1.84 ± 0.34 m−1 during summer 2007 (non-drought year). Despite negligible freshwater input to the AC system for most of 2009, mean a305 values for AC Bay sites were similar in 2008 and 2009, ranging from 3.24 ± 0.60 m−1 in the lower estuary to 6.22 ± 0.55 m−1 in the upper estuary during summer 2009, and 3.15 ± 0.38 m−1 to 6.81 ± 0.73 m−1 during summer 2008 (no data for 2007). CDOM photobleaching experiments were performed using a SunTest XLS + solar simulator with an irradiation spectrum that closely matches solar UV at subtropical latitudes. AC Bay samples exhibited a305 photobleaching half-lives of 33–48 h, while GoM samples exhibited a305 photobleaching half-lives of 36–89 h. We estimate that summertime photobleaching may reduce CDOM levels by >50% in the AC bay complex before discharge into the GoM and by up to 25% more throughout the nearshore waters along the western GoM shelf.