Localized coral reef mortality event at East Flower Garden Bank, Gulf of Mexico

Flower Garden Banks National Marine Sanctuary contains the northernmost coral reef ecosystem in the Gulf of Mexico, with > 50% living coral cover. Historically, this reef system has harbored a healthy coral community, relatively free of disease with limited mortality from bleaching or other factors. In July 2016 an unprecedented localized mortality event was documented at East Flower Garden Bank affecting an area of approximately 5.6 ha (2.6% of the coral reef). Within this area, up to 82% of the coral colonies succumbed to partial or total mortality, along with invertebrates spanning numerous taxa, including cnidarians, poriferans, echinoderms, crustaceans, and mollusks. Within the affected area, the impact was highly stratified, with many coral colonies exhibiting a clear line of delineation with polyps above appearing healthy, and those below exhibiting paling, bleaching, tissue sloughing, and/or mortality. The mortality of sessile invertebrates across multiple taxa was similar to observations from coral mortality events in other regions associated with low dissolved oxygen (DO) concentrations. While instrumentation for measuring DO was not in place on East Flower Garden Bank, DO sensors at similar depths located 50-70 km northwest of East Flower Garden Bank confirmed low DO levels (< 3.5 mg L-1) at the time of the event. The mortality event likely resulted from a combination of stressors, but localized low DO is implicated as the primary contributing factor.