Limitation of Marine Primary Productivity and Control on Ecosystem Structure

IRON LIMITATION OF MARINE PRIMARY PRODUCTIVITY AND CONTROL ON ECOSYSTEM STRUCTURE 

Evidence for the Role of Iron in Regulating Productivity in High Nutrient Low Chlorophyll Regions 

Zero or near-zero nitrate concentrations and very low phosphate concentrations have been measured in summer in the subtropical gyres of all the major oceans and in the equatorial Atlantic and Indian Oceans, implying widespread N limitation of new production [156,188]. Other parts of the ocean, such as the North Atlantic above 50°N, the Arabian Sea, and various coastal upwelling regions, have elevated nutrient concentrations ( 2 pM NO ) but also support high chlorophyll concentrations ( 0.5 mg m ). In contrast to both the highly productive and the nutrient depleted ocean regions mentioned earlier, some 20-40% of the 

In vitro incubations differ from acmal ocean conditions in important ways that limit the applicability of their results to marine ecosystems. The fixed light intensity used in these experiments is a poor mimic of the variable conditions in the ocean mixed layer, while the walls of the incubation bottles themselves may provide unnatural growth surfaces. Bottle experiments cannot hope to accurately represent that nature or scale of the community response [151]. The artificial restriction on the abundance of grazers in bottle incubations, for instance, has been offered as an alternative (and confounding) explanation for the phytoplankton blooms observed in these experiments [221-223]. In an effort to overcome the limitations of bottle experiments, researchers have conducted a series of mesoscale in situ Fe enrichment experiments, in which large areas of the surface ocean were seeded with iron and the ecosystem response measured over many days. 

The ecosystem responses to iron enrichment in each of the experiments have been dramatic, and strikingly similar to one another. After seven such experiments, a classical response to iron addition can now be outlined  

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