Extent of Mississippi River water in the Mississippi Bight and Louisiana Shelf based on water isotopes

Abstract

The northern Gulf of Mexico is a complex and very productive coastal river-dominated system that receives freshwater from numerous rivers including the Mississippi River. The dynamics of coastal ecosystems in the northern Gulf of Mexico are greatly influenced by the freshwater discharge but also by the high nutrient loads carried by the Mississippi River that lead to the seasonal development of one of the largest coastal hypoxic areas. Constraining the origin and fate of the freshwater inputs in the northern Gulf of Mexico will help increase understanding the physical and biogeochemical processes occurring in this region. Here, we focus on investigating the extent of the Mississippi River plume on both sides of the Mississippi River Delta: to the east in the Mississippi Bight, and to the west over the Louisiana Shelf. We determined the water isotopic signature (δ 18 O and δD) along with salinity of the different river plumes and performed a river mixing model on the coastal waters. Our findings provide useful information to better understand the functioning of the northern Gulf of Mexico ecosystem. In particular, the development of hypoxia is often attributed to the nutrient load of the Mississippi River, yet the Mississippi River seemed to have a limited influence on the Mississippi Bight. That is, the dominant source of freshwater in the Bight was supplied by local Mississippi/Alabama rivers. Furthermore, the water isotope mixing model showed that the source of freshwater to the Louisiana Shelf was dominated by the Atchafalaya River in summer, and by the Mississippi River during non-summer seasons. This pattern is consistent with the general shelf circulation that reverses in summer, but could not have been shown solely by the use of salinity.

Does a bottom-up mechanism promote hypoxia in the Mississippi Bight?

“…Dzwonkowski et al (2018) detailed hypoxia in the Mississippi Bight during summer 2016, pointing out that the region of freshwater influence east of the Mississippi River Delta was as extensive as the better-studied region to the west. However, despite this similarity to the Louisiana Shelf, Sanial et al (2019) used the stable isotopic composition of water to demonstrate a minimal influence of Mississippi River discharge in the Mississippi Bight as compared with the influence of other more local rivers. As pointed out by Sanial et al (2019), this is compatible with the regional circulation which most of the year causes Mississippi River discharge to flow west, away from the Mississippi Bight.…”

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

“…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). The Mississippi Bight, although primarily influenced by Mobile Bay outflow and numerous small rivers, occasionally receives pulses of Mississippi River water (Sanial et al 2019). Shelf waters can also become highly stratified due to the influence of buoyant freshwater discharge, and this, along with wind forcing, often drives circulation along coastal margins (Sanders and Garvine 2001;Whitney and Garvine 2005;Dzwonkowski and Park 2010).…”

Variations in Red Snapper oocyte development and spawning in relation to environmental and habitat parameters

“…In the nearby LA-TX dead zone, freshwater has a 3-month residence time over the shelf (Dinnel and Wiseman 1986) and a time lag of 2 months between nutrient delivery and hypoxia formation (Justić et al 1996;Turner et al 2005). The Mississippi River plume impacts the southern portion of our study area regularly with sporatic flows to the east rather than its typical advection pattern to the west; however, the dominant sources of freshwater to the Mississippi Bight are smaller local rivers (Sanial et al 2019). In addition, periodic openings of the Bonnet Carré Spillway divert a portion of the Mississippi River into Lake Pontchartrain before reaching the Mississippi Sound and ultimately the Mississippi Bight, drastically altering the hydrology of the region (Ho et al 2019;Parra et al 2020).…”