Impact of Water Column Structure on Oxygen and Carbon Distributions on the Texas-Louisiana Continental Shelf

Physical and chemical observations were performed on the water column over the inner Texas-Louisiana continental shelf during the summers of 2016-2019 aboard the R/V Pelican. These observations include conductivity, temperature, density, dissolved oxygen, CDOM (Colored Dissolved Organic Matter) fluorescence, and chlorophyll fluorescence. Observation stations were visited along the cruise track from Galveston Bay towards the Flower Garden Banks for 2016. For 2017-2019, a triangular track connecting Galveston Bay, the Flower Garden Banks, and Terrebonne Bay was followed. Station spacing along the cruise track was about 25km. Warmer and fresher surface waters were present in 2016 and 2019, with 2017 and 2019 having warmer water throughout the column. The coldest and saltiest surface waters occurred in 2017. Hypoxic conditions (dissolved oxygen < 1.4 mL/L) were observed in bottom waters for the years 2016, 2018, and 2019. These conditions coincided with a sharp water column pycnocline, which indicated high stratification that inhibits oxygen ventilation to bottom waters. This stratification persists due to low wind speeds and the presence of fresh surface waters. The absence of hypoxia in bottom waters in 2017 coincided with well mixed waters due to higher wind speed events, promoting oxygen ventilation. A towed instrument was able to observe the presence of two interacting water masses in shallow waters (7-14 m) that was overlaying a dense hypoxia structure. The results of this project reinforce the importance of high stratification on the development of hypoxic conditions in the Texas-Louisiana continental shelf.