The High Nutrient Low Chlorophyll (HNLC) Phenomenon and the Iron Hypothesis
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Date
1996-04
Authors
Stenn, Erik Stricker
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Abstract
With rising CO2 levels in the atmosphere it becomes increasingly more important to understand the nature of the oceans as a sink for CO2 as well as the mechanisms that transport carbon from the atmosphere to the oceans. Regions of ocean in the subarctic Pacific, eastern equatorial Pacific and the southern Ocean have been recognized as being abnormally low in total biomass and yet they maintain high levels of available macronutrients. Due to the characteristic high nutrient low chlorophyll content of these regions, they have been dubbed HNLC. The 'biological pump' concept is a proposed mechanism serving as a carbon sink and was assumed to be limited by nitrogen. Early shipboard container experiments demonstrated that iron might be the limiting nutrient and not nitrogen. The data from the early experiments proved to be inconclusive due to imprecise methodology. In the late 1980' s there was renewed interest in HNLC areas championed by J.H. Martin. He maintained that HNLC regions exhibit Leibig limitation by iron, where, standing crops of phytoplankton are constrained by availability of iron: if iron were available, the standing crops of phytoplankton would increase and nitrate would be depleted despite grazing. Others argue that HNLC regions are a manifestation of active grazing in a steady state ecosystem. An intermediate camp claims that HNLC regions are a result of combined physical and biological processes that prevent the utilization of the surface macronutrients. The debate surrounding this issue prompted Martin and colleagues to perform the IronEx experiment: the in situ use of iron to enhance an HNLC patch in the Galapagos region of the Pacific. The ecosystem demonstrated an unequivocal response to iron; however, macronutrients were still relatively abundant after the experiment. Subsequent studies have revealed that iron impacts all cell size groups of phytoplankton and constrains new production in HNLC areas. What remains unclear is the effect of grazing within these ecosystems.
Description
47 pages
Keywords
Marine Biology, CO2 Cycling, Iron Hypothesis, High Nutrient Low Chlorophyll Phenomenon, HNLC Phenomenon