Award: OCE-0961894

Scientific Merit. Every summer in many estuaries and coastal margins, eutrophication-elevated phytoplankton production drives rapid bacterial growth, which consumes oxygen and creates hypoxic and anoxic bottom waters. These so-called "dead zones" exclude fish, kill benthic organisms, and eliminate habitat. Despite their popular name, anoxic/hypoxic zones are not really dead, but rather are populated with living and active microbial communities. Once oxygen is depleted, microbes switch to anaerobic metabolisms to respire organic matter, and they follow a succession based on the energetics and availability of the compound they use in place of oxygen (e.g., O2, NO3-, Mn(IV), Fe(III), and SO42-). Anaerobic respiration and anaerobic bacterial production had never been measured simultaneously in the water column of seasonally anoxic estuarine waters, and thus the impact of these zones on estuarine carbon cycles are either unknown or approximated from indirect measurements (e.g., sulfate respiration rate). Our detailed surveys of anoxic bottom waters applied new techniques for measuring bacterial growth, respiration, diversity, and gene expression to better understand the Life in the Dead Zone. We mapped patterns in redox chemistry and demonstrated parallel space-for-time development in which changes associated with aging bottom waters as they moved up-estuary matched changes at a single station over time. From April to October, redox conditions shifted from oxic to hypoxic to sub-oxic to sulfidic, and then shifted back to oxic either gradually (2010) or quickly (2011) following hurricane-associated mixing of the Bay. These changes in redox chemistry paralleled changes in the phylogenetic diversity of bacterial communities, in microbial gene expression patterns, and in heterotrophic activity. Spatially, bottom waters moving up-estuary changed from oxic to hypoxic to sub-oxic to sulfidic, with comparable shifts in phylogenetic diversity and heterotrophic activity. A third formation of this coordinated shift in redox chemistry and biology existed at the pycnocline/oxycline overlying anoxic bottom waters. This gradient varied in thickness with stratification strength, and was often thin and difficult to sample. Our results suggest that this redox gradient hosts a gradient in microbial diversity, heterotrophic activity, and thin layers of intense chemoautotrophic metabolism. Thus, we identified spatial and temporal transitions between oxic and sulfidic waters each of which hosted coordinated shifts in microbial activity, diversity and gene expression. Biological communities were surprisingly active under all redox states, including sulfidic waters where somewhat reduced rates of heterotrophic production were paired with elevated rates of chemoautotrophic production. Broader Impacts. Estuaries are centers of human population, and they provide society with innumerable recreational and economic benefits. Recently, anoxic and hypoxic zones in estuaries came to the publicÆs attention as indicators of anthropogenic eutrophication. Anoxia/hypoxia is now considered a bellwether of eutrophication in coastal waters, and consequently, most anoxia-related estuarine research is aimed at managing the problem. In contrast, relatively little effort has gone into studying the basic biogeochemistry, microbial ecology and diversity of these waters and the impact of anaerobic respiration on estuarine ecosystems. This research advanced estuarine science by providing (1) reliable measurements of production and respiration in anoxic/hypoxic waters, (2) techniques and protocols to make these measurements in other systems, (3) ecological interpretation for predicting future changes with ongoing restoration efforts in anoxia-impacted estuaries, and (4) novel insight into the diversity and genomics of microorganisms in seasonal anoxic zones using the most current molecular techniques. Mitigation of anoxia/hypoxia is a major goal of many estuar...