Ecological nutrient thresholds in estuaries:
Moving targets influenced by human and climate change

Hans W. Paerl
Institute of Marine Sciences
University of North Carolina at Chapel Hill

Ecological Thresholds Meeting
November 4-5, 2002

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Abstract

Estuaries are physically, chemically and biologically complex and dynamic waterbodies that support key fisheries nurseries and a play vital role in the ecological and economic function and welfare of the coastal zone. Establishing thresholds as guidelines for environmental protection is a critical management need. Human nutrient (especially nitrogen) pollution has altered estuarine primary production, nutrient cycling, and trophodynamics. The generation, delivery, and fates of nutrients are strongly influenced by climatic factors, including hurricanes, floods and droughts, which are also undergoing change. Setting nutrient thresholds aimed at reversing declining water quality (algal blooms, hypoxia) should integrate the impacts of nutrient enrichment over a range of hydrologic (residence time) conditions. Selecting bioindicators that meaningfully gauge ecological condition and change under the influence of multiple stressors is of central importance. Ideally, these indicators should be integrated in monitoring (including remote sensing), experimental, stoichiometric, microscopic and molecular analyses of ecosystem response to nutrient loading. They should also serve as conceptual and empirical bases for understanding and predicting relationships between nutrient loading, primary production, biogeochemical and higher trophic level (i.e., fish) responses. Examples are provided for the Neuse River Estuary, NC, where research and management needs have converged to develop indices of nutrient limitation, nutrient sensitivity designations, total mean daily (nitrogen) loads (TMDL's), and mandated nutrient (nitrogen) reduction strategies aimed at reducing unwanted symptoms of eutrophication. Nutrient management efforts are complicated by elevated hurricane activity and associated hydrologic perturbations, which strongly interact with nutrient-productivity dynamics. These features must be incorporated in threshold development and interpretation.