Tracking sea surface salinity and dissolved oxygen on a river-influenced, seasonally stratified shelf, Mississippi Bight, northern Gulf of Mexico

Abstract

River discharge, and its resulting region of freshwater influence (ROFI) in the coastal ocean, has a critical influence on physical and biogeochemical processes in seasonally stratified shelf ecosystems. Multi-year (2010-2016) observations of satellite-derived sea surface salinity (SSS) and in situ water column hydrographic data during summer 2016 were used to investigate physical aspects of the ROFI east of the Mississippi River Delta to better assess regional susceptibility to hypoxia in the summer months. Time series of SSS data indicate that the shelf region impacted by the seasonal expansion of freshwater can be as extensive as the well-known "dead zone" region west of the Delta, and hydrographic observations from a shelf-wide survey indicate strong stratification associated with the ROFI. Peak buoyancy frequencies typically ranged between 0.15 and 0.25 s −1 and were concentrated in a 2-3 m layer around 4-10 m deep across much of the shelf. This ROFI is expected to be influenced by local freshwater sources which, while individually small, make a notable contribution in aggregate to the region (annually averaged daily discharge of approximately 2880 m 3 s −1). The dissolved oxygen (DO) conditions under this freshwater cap were spatially and temporally variable, with areas of hypoxia and nearhypoxic conditions over portions of the shelf, demonstrating the utility of satellite-derived SSS in identifying coastal areas potential vulnerability to hypoxia. These regions of low bottom dissolved oxygen persisted throughout the peak summer season at several sites on the shelf, with the northeastern corner of Mississippi Bight having the most intense and persistent hypoxia.

Contrasting fine‐scale distributional patterns of zooplankton driven by the formation of a diatom‐dominated thin layer

“…The Mississippi Bight region of the northern Gulf of Mexico (nGOM) is physically complex and biologically productive due to high amounts of nutrient-laden river discharge. During the summer, highly stratified conditions are favorable for mid-water thin layer formation of marine aggregates, also known as "marine snow" (Dzwonkowski et al 2018b;Greer et al 2018a). The spatial and temporal oceanographic variability in the nGOM system is driven primarily by winds and freshwater discharge plumes (Ohlmann and Niiler 2005;Walker et al 2005).…”

“…Despite conditions favorable for strong stratification, and the summer phenomenon of bottom-water hypoxia (dissolved O 2 < 2.0 mg L −1 , Diaz and Rosenberg 2008;Dzwonkowski et al 2018b), little is known about the ecological consequences of thin layers in the nGOM. If frequently occurring and generating overlapping high abundances among multiple trophic levels (Benoit-Bird and McManus 2012; Greer and Woodson 2016), these layers may contribute to the productive fisheries in this region (Grimes 2001).…”

Effects of Coastal Upwelling and Downwelling on Hydrographic Variability and Dissolved Oxygen in Mobile Bay

“…A growing body of work on dissolved oxygen changes in the northeast region of the Gulf of Mexico has shown that the region east of the Mississippi River Delta is susceptible to hypoxia development in the summer months (Brunner, 2006;Dzwonkowski et al, 2018;Milroy, 2016). A large focus of previous work has been on stratification and river nutrients driving low dissolved oxygen.…”

Limnology and Oceanography Bulletin Volume 28 Number 4 November 2019 117–152

“…The hypoxic water mass sometimes extends into upper Texas coastal waters [(maps of bottom oxygen are at https:// www.ncddc.noaa.gov/hypoxia/products/ and http://www.gulfhypoxia.net]. Hypoxia also occurs east of the Mississippi River and is becoming more frequent there (Dzwonkowski et al 2018).…”