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Comparison of bottom-water chemistry in the marine-limnic habitat gradient shows greater phosphorus availability in marine waters, primarily because of enhanced iron sequestration by sulfide. In the oxidative hydrolysis of iron and the concomitant precipitation of phosphate, a minimum of two iron atoms are needed to precipitate one phosphate molecule (Fe : P ϭ 2). However, dissolved Fe : P Ͻ 2 predominates in anoxic marine waters, therefore leaving some phosphate in solution after oxygenation because of a shortage of dissolved iron for phosphate coprecipitation by iron oxyhydroxide. In contrast, anoxic bottom waters in most freshwater lakes show Fe : P Ͼ 2, allowing almost complete phosphate removal on oxygenation. This difference is a consequence of the high sulfate content of sea salt, and a main reason why nitrogen normally limits net primary production in temperate coastal waters, in contrast to the predominant phosphorus limitation of near-neutral lakes.

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