ABSTRACT: Phytoplankton biomass in high-nitrate, low-chlorophyll (HNLC) ocean regions exhibits a pronounced stability: variation occurs only within a narrow range of values. The magnitude of this variation has profound ecological and geochemical consequences. While mechanisms believed to set the upper limits to HNLC phytoplankton biomass (iron limitation, microherbivore grazing) have received much recent attention, mechanisms setting the lower limits are largely unknown. The demonstrated importance of planktonic micrograzers, largely protists, in removing phytoplankton biomass in HNLC regions suggests that micrograzer behavioral and physiological capabilities may hold the key. This will be the case at any level of phytoplankton cell division greater than zero, regardless of the extent of growth rate limitation by resource (e.g. iron, light) availability. Indeed, HNLC plankton dynamics models almost universally include several biological responses that set lower phytoplankton biomass limits and confer temporal stability, including substantial feeding thresholds, zero micrograzer metabolic costs, and no micrograzer mortality at low food levels. Laboratory observations of these same biological responses in protist grazers are equivocal. There are no direct observations of substantive feeding thresholds, and many heterotrophic protists exhibit significant rates of respiration and mortality (cell lysis) at very low food levels. We present several candidate explanations for the discrepancy between laboratory observations and model biological 'requirements'. Firstly, laboratory-derived rate measurements may be biased by use of species and prey concentrations that are not representative of HNLC communities. Secondly, model micrograzer features may be a proxy for other stabilizing phenomena such as spatial heterogeneity ('patchiness') or carnivory (top-down control of microherbivores), though a logical analysis indicates that neither is likely to provide robust stabilization of lower phytoplankton biomass limits. Lastly, the highly plastic feeding capabilities of protist grazers, which include switching between phytoplankton and alternative prey such as bacteria, detritus, and other microherbivores, are a probable locus for stabilization of biomass limits. The extent to which such behavioral plasticity functions on the level of individuals or of species assemblages is unknown. We advocate a coupled modeling and experimental approach to further progress in understanding this key feature of HNLC ecosystems.

KEY WORDS: Planktonic food webs · Grazers · Feeding behavior · Plankton dynamics models · Microzooplankton · Phytoplankton

Published in MEPS Vol. 193 (2000) on February 28
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 2000