『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- concentrations 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.

Keywords: Phosphorus; Salinity; Pore water; Iron; Estuaries; Nitrogen』

Introduction
Study sites
Methods
　Sediment core collection and processing
　Analytical methods
Results
　Pore water solute trends with salinity and depth in the sediment
　Salinity related trends in pore water Fe²⁺:PO4^3- and NH4⁺:PO4^3- ratios
Discussion
　Controls on pore water Fe²⁺ and PO4^3- concentrations along the salinity gradients
　Controls on pore water NH4⁺ concentrations along the salinity gradients
　Which nutrient exerts greater control over the NH4⁺:PO4^3- ratio switch?
　Effects on N and P availability in the water column
　Significance for nutrient limitation
　Unloading the iron conveyer belt
　Generality among estuaries
Acknowledgments
References