Temporally-displaced Mississippi River spring flood pulse shows muted aquatic ecosystem response in estuarine waters: A climate change warning for coastal foodwebs

Yadav Sapkota; Sibel Bargu; John R White

doi:10.1016/j.scitotenv.2023.162623

Temporally-displaced Mississippi River spring flood pulse shows muted aquatic ecosystem response in estuarine waters: A climate change warning for coastal foodwebs

Sci Total Environ. 2023 May 20:874:162623. doi: 10.1016/j.scitotenv.2023.162623. Epub 2023 Mar 4.

Authors

Yadav Sapkota¹, Sibel Bargu¹, John R White²

Affiliations

¹ Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA, USA.
² Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA, USA. Electronic address: jrwhite@lsu.edu.

PMID: 36871708
DOI: 10.1016/j.scitotenv.2023.162623

Abstract

Mississippi River water levels typically rise in the early spring after snow melt in the extensive watershed. However, in 2016, warm air temperatures coupled with high precipitation led to a historically early river flood pulse, resulting in the opening of a flood release valve (Bonnet Carré Spillway) in early January to protect the city of New Orleans, Louisiana. The goal of this research was to determine the ecosystem response of this wintertime nutrient flood pulse on the receiving estuarine system and compare it to historical opening responses, which are generally several months later. Nutrients, TSS, and Chl a were measured along a 30 km transect in the Lake Pontchartrain estuary, before, during, and after the river diversion event. In the past, NO_x concentrations were quickly reduced to below detection in the estuary in <4 weeks post-event accompanied by a moderate phytoplankton bloom. However, due to seasonal limitations (cold water temperatures and light limitation) during the 2016 event, NO_x remained elevated for >2 months post-closure and Chl a values were low, indicating limited assimilation of nutrients into phytoplankton biomass. Consequently, much of the bioavailable nitrogen was denitrified by sediments and dispersed to the coastal ocean over time, limiting the transfer of nutrients into the food web by means of a spring phytoplankton bloom. An increasing warming trend in temperate and polar river watersheds is leading to earlier spring flood pulses, altering the timing of coastal nutrient transport, decoupled from conditions supporting primary production, which could significantly affect coastal food webs.

Keywords: Estuary; Food web; Nutrients; Water quality; Wetland.