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High-nutrient , low chlorophyll -LRB- HNLC -RRB- is a term used in marine ecology to describe areas of the ocean where the number of phytoplankton -LRB- standing stock -RRB- are low and fairly constant in spite of high macro-nutrient concentrations -LRB- nitrate , phosphate , silicic acid -RRB- .. phytoplankton. phytoplankton. In general , some essential inorganic substances like nitrate may be present in oceanic waters in concentrations low enough to be limiting to plant production , but in HNLC regions the level of nitrate is never significantly depleted .. Instead , these regions are limited by low concentrations of metabolizable iron .. There are two popular explanations for the existence of HNLC regions  : the iron hypothesis and the ` grazing control hypothesis ' .. iron hypothesis. iron hypothesis. HNLC regions cover 20 % of the world 's oceans in three major areas  : equatorial Pacific Ocean , subarctic Pacific Ocean and the Southern Ocean .. Pacific Ocean. Pacific Ocean. Southern Ocean. Southern Ocean