Abstract
Phosphorus (P) availability in estuaries may increase with increasing salinity because sulfate from sea salt supports production of sulfide in sediments, which combines with iron (Fe) making it less available to sequester P. Increased P availability with increased salinity may promote the generally observed switch from P limitation of primary production in freshwater ecosystems to nitrogen (N) limitation in coastal marine waters. To investigate this hypothesis, we analyzed pore water from sediment cores collected along the salinity gradients of four Chesapeake Bay estuaries (the Patuxent, Potomac, Choptank, and Bush Rivers) with watersheds differing in land cover and physiography. At salinities of 1–4 in each estuary, abrupt decreases in pore water Fe2+ concentrations coincided with increases in sulfate depletion and PO4 3− concentrations. Peaks in water column PO4 3− concentrations also occur at about the same position along the salinity gradient of each estuary. Increases in pore water PO4 3− concentration with increasing salinity led to distinct shifts in molar NH4 +:PO4 3− ratios from >16 (the Redfield ratio characteristic of phytoplankton N:P) in the freshwater cores to <16 in the cores with salinities >1 to 4, suggesting that release of PO4 3− from Fe where sediments are first deposited in sulfate-rich waters could promote the commonly observed switch from P limitation in freshwater to N limitation in mesohaline waters. Finding this pattern at similar salinities in four estuaries with such different watersheds suggests that it may be a fundamental characteristic of estuaries generally.
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Acknowledgments
Funding was provided by National Science Foundation grant DEB-0235884 and the United States Environmental Protection Agency (EPA) Science to Achieve Results (STAR) Graduate Fellowship Program. Technical assistance was provided by Nancy Goff, Joseph Miklas, Marc Sigrist, and Kim Cone. Gregory Foster generously provided laboratory space and technical guidance. Jeffrey Cornwell was instrumental in helping obtain samples from the Choptank River. The suggestions of Donald Kelso, Gregory Foster, J. Patrick Megonigal, and two anonymous reviewers improved this paper.
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Hartzell, J.L., Jordan, T.E. Shifts in the relative availability of phosphorus and nitrogen along estuarine salinity gradients. Biogeochemistry 107, 489–500 (2012). https://doi.org/10.1007/s10533-010-9548-9
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DOI: https://doi.org/10.1007/s10533-010-9548-9