Bonnet Carré Spillway freshwater transport and corresponding biochemical properties in the Mississippi Bight

Sabrina M Parra,Virginie Sanial,Adam D Boyette,M Kemal Cambazoglu,Inia M Soto,Adam T Greer,Luciano M Chiaverano,Angie Hoover,Michael S Dinniman

doi:10.1016/j.csr.2020.104114

Abstract

Large freshwater pulses to coastal ecosystems change local hydrologic regimes and alter biogeochemical processes. The Mississippi Bight coastal ecosystem, located in the northern Gulf of Mexico shelf, is influenced by extensive freshwater inputs: the Mississippi River (MSR) and several smaller rivers to the east. Under river flood conditions, MSR waters flow through the Bonnet Carré Spillway (BCS) to relieve pressure on levees in New Orleans, Louisiana. In 2015, mild wintertime temperatures and heavy rainfall throughout the MSR watershed led to extreme flooding and prompted an unusually early BCS opening on January 10, 2016 for 23 days. This study examines the effects of such intermittent freshwater diversions on local shelf circulation, planktonic distributions, and potential contaminant transport pathways. Physical, chemical, and remote sensing data collected one month after the BCS opening suggested the region was comprised of three water masses: shelf saltwater, MSR waters, and local river waters. Observations and circulation model results showed the BCS waters remained within the estuarine lakes and sounds, where winter wind patterns mixed the waters and prevented BCS waters from flowing onto the shelf. Freshwater within the Mississippi Bight was primarily from concurrent flooding of local rivers. Two distinct clusters of microplankton (offshore versus nearshore stations) and zooplankton (Chandeleur Sound versus other stations) community compositions were detected. No algal blooms were observed during this BCS opening. The 2016 wintertime BCS opening resulted in muted effects on the sounds and shelf because of its short duration and uncharacteristically early release.

Highlights

River diversions and spillway openings are common water management practices for alleviating rising river waters, and can result in large freshwater pulses that may have downstream biogeochemical effects depending on the volume of water, nutrient and sediment content, and where the water is transported
Remote sensing optical backscatter indicated that the Bonnet Carre Spillway (BCS) plume hugged the southern coast of Lake Pontchartrain Estuary, and remained confined within the lake until after January 19 (Fig. 3)
For the primary sources of freshwater (MSR and local river watersheds) to the Mississippi Bight, which is located within the northern Gulf of Mexico, total discharge was anomalously high during winter and resulted in an unprecedented early opening of the BCS

Summary

Introduction

River diversions and spillway openings are common water management practices for alleviating rising river waters, and can result in large freshwater pulses that may have downstream biogeochemical effects depending on the volume of water, nutrient and sediment content, and where the water is transported. The Mississippi Bight, which is located within the northern Gulf of Mexico, is a managed coastal ecosystem that includes the heavily managed Mississippi River (MSR) with levees and dams all along its extent (Templet and Meyer-Arendt 1988; Kesel, 1988; Mossa 1996). The levees and dams control water, sediment, and nutrient fluxes down the river and into the Gulf. The strong confinement of the river leads to a control on the discharge, which becomes problematic during uncharacteristic spring floods. River floods require additional management when river discharges exceed design criteria for the levees.

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