Inflow of shelf waters into the Mississippi Sound and Mobile Bay estuaries in October 2015

Abstract

Abstract. The exchange of coastal waters between the Mississippi Sound (MSS), Mobile Bay, and Mississippi Bight is an important pathway for oil and pollutants into coastal ecosystems. This study investigated an event of strong and persistent inflow of shelf waters into MSS and Mobile Bay during October 2015 by combining in situ measurements, satellite ocean color data, and ocean model predictions. Navy Coastal Ocean Model predicted high-salinity shelf waters continuously flowing into the estuarine system and forecasted low-salinity waters trapped inside the estuaries which did not flush out until the passage of tropical cyclone Patricia's remnants in late October. The October 2015 chlorophyll-a anomaly was significantly low inside and outside the MSS for the 2003 to 2015 time series. Similar low-chlorophyll-a anomalies were only seen in 2003. The October 2015 mean in situ salinities were up to 8 psu higher than mean from 2007 to 2015, and some estuarine stations showed persistent salinities above 30 psu for almost a month in agreement with model predictions. October 2015 was associated with low fall seasonal discharge, typical of fall season, and wind which was persistently out of the east to southeast °. These persistent wind conditions were linked to the observed anomalous conditions. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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

“…7) suggests that local rivers were the dominant source of fresh water to the Mississippi Bight. The spatial heterogeneity of salinity in the Mississippi Sound near the Mobile Bay estuary showed the dynamic mixing between freshwater from the Mississippi Sound and more saline water from the Mississippi Bight consistent with in situ measurement of currents and satellite chlorophyll analysis of this area (Cambazoglu et al, 2017). The river fraction analysis confirmed the dominant freshwater source was from local Mississippi/Alabama rivers to most of the Mississippi Bight area.…”

“…2). The exchange of water between the Mississippi Sound and the Mississippi Bight is tide-dominated and occurs frequently through several barrier island inlets (Cambazoglu et al, 2017). The total freshwater discharge from all the local rivers is approximately one order of magnitude lower than the Mississippi River (i.e., ∼7 ◊ 10 10 m 3 y −1 ; Water data USGS).…”

“…However, some smaller scale variations can also be observed. For example, the salinity at the mouth of the Mobile Bay varies on timescales of a few days between relatively fresh and saline water from the Bight, suggesting some frequent intrusion of Mississippi Bight waters towards the Bay depending on the wind direction (Cambazoglu et al, 2017). Eddies can also be formed due to the interaction of the Loop Current with the Louisiana Shelf break (Oey, 1995).…”

“…This could be the result of limited river discharge (Fig. 2) consistent with the high salinity, and a wellmixed water column that could be explained by the passage of Hurricane Patricia over the northern Gulf of Mexico few days before the fall 2015 cruise (Cambazoglu et al, 2017;Rogers et al, 2017). In summer 2016, the δ 18 O SW and δD SW were slightly more negative and covered a wider range (δ 18 O SW from −1.1‰ to +1.2‰, δD SW : 5.1‰ to +10.4‰) consistent with relatively larger freshwater river inputs than in fall (Fig.…”

Functioning of Coastal River-Dominated Ecosystems and Implications for Oil Spill Response: From Observations to Mechanisms and Models

“…During the fall, minimal freshwater input led to a well-mixed water column with relatively high salinity (Cambazoglu et al, 2017) and low biological productivity (Figure 6a,b), as measured by both primary production and zooplankton abundances . The water column underwent a dramatic change as spring rains led to increased river discharge from Mobile Bay, directly affecting large portions of the shelf and producing a large vertical salinity range (salinity of ~24 at surface and ~36 at depth).…”