Oxygen metabolism and water mass mixing in the northern Gulf of Mexico hypoxic zone in 2010

We evaluated O2 metabolism and mixing of river and seawater in the northern Gulf of Mexico (NGOMEX) in May and August of 2010. A strong correlation between the δ2H and δ18O of water and salinity indicated simple mixing of the two water sources. The isotope values for the freshwater source suggested that Mississippi River water predominated over the Atchafalaya River water in at our study sites in both May and August. Knowledge of the δ18O of water is required for determination of the ratio of primary production to respiration (P:R) from the δ18O and concentration of O2. We observed a strong correlation between δ18O of water and salinity, which indicates that subsequent studies of P:R may simply be able to predict the isotopic composition of water from salinity in this region. P:R values were determined from the concentration and isotopic composition of O2 and varied from 0.88 to 1.10. The range in P:R and progression from high values in May to net heterotrophy in August is consistent with enhanced primary production in spring followed by respiration in excess of production in summer. Averages of −5.8‰ and −3.4‰ were observed for the isotopic enrichment factor, εobs, for sub-pycnocline respiration in May and August, respectively. In contrast to previous studies we found that εobs is not an effective indicator of the proportion of water column to sediment respiration, but rather reflects the total rate of sub-pycnocline respiration (water column plus sediment respiration). Nonetheless, as εobs is an indication of total sub-pycnocline respiration insight into oxygen-removal can be obtained without the need for incubation. Even though our research cruises occurred approximately three weeks after the initiation and one month after cessation of the Deepwater Horizon oil spill, P:R and εobs values were each within ranges reported previously in NGOMEX. Thus, a direct link between the oil spill and O2 metabolism or hypoxia development was not apparent.