Estuarine ecosystem dynamics during a climate-influenced, early-season Mississippi River flood diversion

Abstract:

Natural systems across a wide range of scales exhibit pulsing dynamics, with rivers providing a familiar example. Typically, Mississippi River water levels rise in the early spring, after snow melt in the watershed. In 2016, warm air temperatures in the central US coupled with high rainfall precipitation resulted in little storage as snow, leading to a historically early river flood pulse. The flood event of 2016 required the opening of the Bonnet Carré Spillway, a release valve designed to discharge up to approximately 17% of the design flood stage Mississippi River into the Lake Pontchartrain estuary, protecting downstream New Orleans, LA. Since completion in 1931, the spillway has been opened 11 times under extreme flood pressures, typically between March and May to match the spring river flood (8 of 11 times). The Jan 10^th, 2016 opening was the earliest on record and can serve as an analogue for temporally displaced estuarine flood pulses due to climate change. Nitrate concentrations are typically reduced to below detection within 1 month post spillway closure, accompanied by a moderate algal bloom of chlorophytes and/or diatoms. However, due to colder water temperatures and light limitation during the 2016 event, nitrate remained elevated for >2 months post-closure and Chl a values were low, indicating limited assimilation of nutrients into algal biomass. Consequently, much of the bioavailable N was discharged to the coastal ocean. Increasing warming trends in river watersheds may lead to earlier flood pulses as well as magnitude and timing of coastal nutrient transport and primary production.