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

International audienceLarge 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 Carr e 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