Grazing and iron limitation in the control of phytoplankton stock and nutrient concentration: a chemostat analogue of the Pacific equatorial upwelling zone

Abstract

Grazing control and iron limitation have been portrayed as mutually exclusive alternative explanations of the high nutrientflow phytoplankton stock condition in the nutnent-rich areas of the open sea. To contrast clearly the underlying assumptions and speciflc consequences of the 2 mechanisms, a simple mathematical model of a chemostat contaming a pelagic food c h a ~ n with 1 to 4 trophic levels is used as an analogue of the equatorial upwelling zone in the eastern Pacific. It shows that grazing control is essential to reproduce 3 conditions observed In the equatorial upwelling zone: low phytoplankton stocks, high concentrations of macronutrients, and phytoplankton specific growth rates that greatly exceed advective throughput rate in the mixed layer at the equator. Nevertheless, simultaneous grazing control and limitation by a trace nutrient such as iron could feas~bly account for observed phytoplankton specific growth rates that are less than the maximum rate possible under optimal growth conditions. In addition, release of an inefficiently grazed component of the phytoplankton assemblage from iron limitation seems to explain why escape from grazing control occurs under both experimental and natural iron ennchments.