Algal blooms and the nitrogen-enrichment hypothesis in Florida springs: evidence, alternatives, and adaptive management

Authors

  • James B. Heffernan,

    1. School of Forest Resources and Conservation, University of Florida, Gainesville, Florida 32611-0410 USA
    2. Water Institute, University of Florida, Gainesville, Florida 32611-6601 USA
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    •  Present address: Department of Biological Sciences, Florida International University, Miami, Florida 33199 USA. E-mail: jheffer@fiu.edu

  • Dina M. Liebowitz,

    1. School of Natural Resources and the Environment, University of Florida, Gainesville, Florida 32611-0230 USA
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  • Thomas K. Frazer,

    1. School of Forest Resources and Conservation, University of Florida, Gainesville, Florida 32611-0410 USA
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  • Jason M. Evans,

    1. School of Forest Resources and Conservation, University of Florida, Gainesville, Florida 32611-0410 USA
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  • Matthew J. Cohen

    1. School of Forest Resources and Conservation, University of Florida, Gainesville, Florida 32611-0410 USA
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  • Corresponding Editor: A. Ward.

Abstract

Contradictions between system-specific evidence and broader paradigms to explain ecosystem behavior present a challenge for natural resource management. In Florida (USA) springs, increasing nitrate (NO3) concentrations have been implicated as the cause of algal overgrowth via alleviation of N-limitation. As such, policy and management efforts have centered heavily on reduction of nitrogen (N) loads. While the N-limitation hypothesis appears well founded on broadly supported aquatic eutrophication models, several observations from Florida springs are inconsistent with this hypothesis in its present simplified form. First, NO3 concentration is not correlated with algal abundance across the broad population of springs and is weakly negatively correlated with primary productivity. Second, within individual spring runs, algal mats are largely confined to the headwater reaches within 250 m of spring vents, while elevated NO3 concentrations persist for several kilometers or more. Third, historic observations suggest that establishment of macroalgal mats often lags behind observed increases in NO3 by more than a decade. Fourth, although microcosm experiments indicate high thresholds for N-limitation of algae, experiments in situ have demonstrated only minimal response to N enrichment. These muted responses may reflect large nutrient fluxes in springs, which were sufficient to satisfy present demand even at historic concentrations. New analyses of existing data indicate that dissolved oxygen (DO) has declined dramatically in many Florida springs over the past 30 years, and that DO and grazer abundance are better predictors of algal abundance in springs than are nutrient concentrations. Although a precautionary N-reduction strategy for Florida springs is warranted given demonstrable effects of nutrient enrichment in a broad suite of aquatic systems worldwide, the DO–grazer hypothesis and other potential mechanisms merit increased scientific scrutiny. This case study illustrates the importance of an adaptive approach that explicitly evaluates paradigms as hypotheses and actively seeks alternative explanations.

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