Hypotheses to explain high-nutrient conditions in the open sea

Abstract

Oceanic high-nutrient, low-chlorophyll waters are characterized principally by the persistence of major nutrients at the sea surface. This condition indicates control of autotrophic production by something other than NO3 or PO4, but the nature of this control is at present unresolved. The range of hypotheses to explain the high-nutrient condition is illustrated by the grazing hypothesis (specific growth rates of phytoplankton are maximal and environmental stability allows development of a balanced food web that maintains low standing crops of phytoplankton) and the iron hypothesis (standing crop of plankton is constrained by availability of Fe: if more Fe were available, the standing crop of phytoplankton would increase and NO3 would be depleted, despite grazing). The iron hypothesis has been examined experimentally in the subarctic and equatorial Pacific and in Antarctic waters. In each environment, Fe enrichment enhanced the final yield of phytoplankton biomass after incubations of many days. Interpretation of these experiments is contentious because containment in bottles is unnatural. Nonetheless, recent studies in the laboratory and in the field indicate that Fe and possibly other trace elements exert selective pressures on oceanic phytoplankton and that enrichment of high-nutrient waters with Fe would change the species composition of phytoplankton and food-web interactions, thereby enhancing utilization of NO3. The magnitude of this enhancement cannot be predicted with confidence. Results from the central equatorial Pacific indicate that the specific growth rates of phytoplankton are adequate to overcome physical forcing and to deplete ambient NO3 in the euphotic zone. It is suggested that grazing controls the populations of the dominant, small cells. However, the supply of Fe might ultimately regulate nutrient utilization by limiting the specific growth rates of larger cells that might otherwise escape grazing control and bloom. Observations from the subarctic Pacific are consistent with this view, but the regulation of phytoplankton growth and nutrient utilization might not be the same in cold, physically perturbed Antarctic waters.