Zooplankton: Pivotal role in the control of ocean production

Abstract

The article focuses on the nutrient-depleted (oligotrophic) areas of the oceans, including stratified temperate seas during summer, all showing little temporal change of phytoplankton concentrations. Zooplankton-mediated processes that lead from physically caused, nitrate-fueled new production to total primary production are emphasized. Under oligotrophic conditions, the rate of phytoplankton cell division in the light-saturated part of the euphotic zone depends directly on the rate of nutrient regeneration by the zooplankton; indirectly, it depends on the liberation of dissolved organic matter that provides the bacterial substrate, mainly from “sloppy” feeding of the zooplankton, excretion, and release from feces, and a small contribution from exudation by phytoplankton. The bacterial division rate is low and is inferred as being controlled by the supply rate of dissolved organic carbon. The possibility is discussed that, via nutrient regeneration, temperature-controlled zooplankton physiology largely controls the rate of phytoplankton production, given a physically set rate of new production. Phytoplankton-zooplankton interactions are treated briefly for the high nutrient-low chlorophyll regions that also have little temporal change of phytoplankton concentrations, and the open-sea regions with seasonal blooms. All available marine measurements >9°C of bulk phytoplankton division rates are plotted on temperature to improve upon Eppley's standard curve for modeling phytoplankton production.