Spatial patterns in water column respiration rates in the northern Gulf of Mexico hypoxic zone: results from four years of July shelfwide cruises

One of the largest hypoxic (low oxygen water conditions) areas in the world forms annually on the Louisiana-Texas continental shelf downstream of the Mississippi and Atchafalaya rivers as a result of strong and persistent stratification coupled with high rates of organic production fueled by river-derived nutrients. Despite growing interest in reducing the size and duration of hypoxia, there have been few respiration rate measurements in the region. We present spatial patterns in respiration rates across the LA-TX shelf during late July of 2012-2015. Surface and bottom water respiration rates are reported for stations in 3 depth classes (< 12, 12-24, > 24 m water depth) along inshore-offshore gradients from 6-14 transects per year. Across all years, respiration rates were higher in surface than bottom waters across the shelf and within each depth class. Respiration rates were highest at shallow stations (nearer to shore), intermediate at intermediate depth stations, and lowest at deeper offshore stations for both surface and bottom waters. There is some year to year variability in the spatial patterns that appears related to circulation patterns and plume dynamics at the time of sampling (e.g. July 2014 patterns were consistent with other years between the Mississippi and Atchafalaya deltas but rates were uniformly low at all western shelf stations where surface salinities were higher, chlorophyll lower and hypoxia less frequently detected). Additional comparisons (n =50) showed that R rates for bottom water collected with a single Niskin bottle (~0.5m off sediment) were significantly higher (p = 0.02) than for water collected using traditional approaches (Niskin bottle on rosette ~1.0-2.0m above sediments)—consistent with re-suspended materials near the sediment surface stimulating respiration rates.