FLOODS AND EUTROPHICATION IN THE NORTHERN GULF OF MEXICO: AN EVALUATION OF LONG-TERM ENVIRONMENTAL CHANGE AND HUMAN IMPACTS ON COASTAL ESTUARIES

The Intergovernmental Panel on Climate Change projects 4 mm/yr sea level rise, with acceleration in this century. Impacts of sea level and climate on coastal environments remain uncertain, while the majority of the world population lives and relies upon the coast. In an effort to improve coastal resiliency of the Southeastern US, we investigate Holocene records of floods and algal blooms in response to different sea level rise rates and past climate in Weeks Bay and Mobile Bay, AL, which is the fourth largest estuary in the US and hosts key aquaculture and industrial infrastructure today.

Sediment cores were targeted in central bay stratigraphy to record the last ~9 kyrs of paleoenvironmental change in the linked bay systems. Piston cores WB-18-02 and MB-18-01 recovered 6 m and 9 m of undisturbed Holocene bay mud from Weeks Bay and Mobile Bay, respectively. Age models are constructed using Pb-210 and C-14 AMS dating, and integrated with published cores nearby. Multiple proxies are used to reconstruct environmental change during the Holocene and to elucidate relationships of extreme weather and algal bloom events with climate and sea level. Elemental composition, measured every cm with a handheld XRF, shows trends of increasing marine influence at the base of the core due to formation of the bays ~8.2 and 7 ka. Carbon and nitrogen isotopes from bulk sediment show phytoplankton source for organics, with trends of increasing TOC, decreasing δ¹³C, and increasing detrital elements in the Late Holocene, interpreted as increased terrestrial flux. Positive peaks in sand content, measured every cm with a Malvern Laser Particle Size Analyzer, are interpreted as riverine flood events with terrestrial sand influx and associated with increase in detrital elements. Ongoing diatom assemblage analysis will help identify algal blooms that may be triggered by eutrophication after floods.

Frequency and magnitude of floods and blooms during key intervals of the Holocene, especially ~7-9 ka when sea level rise in the Gulf of Mexico was 4 mm/yr like projected today, can help us better understand ecosystem response to future climate and sea level scenarios. Improved predictions, along with a pre-industrial record of environmental conditions, may help improve sustainability planning, policy, and restoration efforts.