<P> A secondary variable is nutrient availability . Although large areas of the tropical and sub-tropical oceans have abundant light, they experience relatively low primary production because they offer limited nutrients such as nitrate, phosphate and silicate . This results from large - scale ocean circulation and water column stratification . In such regions, primary production usually occurs at greater depth, although at a reduced level (because of reduced light). </P> <P> Despite significant macronutrient concentrations, some ocean regions are unproductive (so - called HNLC regions). The micronutrient iron is deficient in these regions, and adding it can lead to the formation of phytoplankton blooms . Iron primarily reaches the ocean through the deposition of dust on the sea surface . Paradoxically, oceanic areas adjacent to unproductive, arid land thus typically have abundant phytoplankton (e.g., the eastern Atlantic Ocean, where trade winds bring dust from the Sahara Desert in north Africa). </P> <P> While plankton are most abundant in surface waters, they live throughout the water column . At depths where no primary production occurs, zooplankton and bacterioplankton instead consume organic material sinking from more productive surface waters above . This flux of sinking material, so - called marine snow, can be especially high following the termination of spring blooms . </P> <P> Aside from representing the bottom few levels of a food chain that supports commercially important fisheries, plankton ecosystems play a role in the biogeochemical cycles of many important chemical elements, including the ocean's carbon cycle . </P>

N which marine zone would you find the majority of plankton