Why does N-limitation persist in the world's marine waters?

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Primary production of vast regions of the world's estuarine, coastal and pelagic ocean waters is limited by availability of fixed nitrogen; this despite the fact that a highly diverse suite of microorganisms potentially capable of fixing N2 (eubacteria and cyanobacteria), inhabit these waters. Theoretically, diazotrophs should supply the N needs to balance the N required to support primary production, assuming other key limiting nutrients, phosphorus and iron, are available and energy requirements are met. In practice however, N2 fixation often does not meet ecosystem-scale N demands, even when these nutrients are replete. The problem lies with the fact that optimal rates of N2 fixation are often controlled by additional environmental factors, including light and organic matter availability, turbulence, and high levels of dissolved oxygen which can suppress this process in N-deplete surface waters. In addition, rates of N loss via denitrification and anammox can exceed N2 fixation and external N inputs on annual scales in coastal and pelagic waters, including those experiencing eutrophication. This creates a situation where chronic N limitation persists, even in the presence of anthropogenic nitrogen enrichment. Many aquatic ecosystems exhibit a perpetual “hunger” for fixed N to support primary and higher levels of production and this is likely to continue over forseeable biological and geologic timescales.

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