Interacting growth and loss rates: The balance of top-down and bottom-up controls in plankton communities

Abstract

Application of resource-based competition theory to high-nutrient, low-chlorophyll regions of the ocean suggests that single-factor controls on vertical export rates of carbon from euphotic zones are unlikely. High specific rates of grazing or sinking losses interact with growth physiology to produce nutrient requirements in situ that are much higher than those required for the growth of populations held in bottle bioassays. The efficiency of vertical export of carbon by sinking particulates can vary with species composition of the plankton, which in turn can be altered by nutrient manipulation. A simulation model explores possible changes to species composition and vertical carbon flux which might result from addition of Fe to Southern Ocean plankton communities. Nutrient manipulation permits invasion of plankton communities by taxa not originally present and does not necessarily increase the biomass or metabolism of resident species. This makes a priori prediction of fluxes associated with an enriched and altered community fundamentally uncertain if predictions are based on stoichiometries and physiologies of the original resident taxa. Vertical carbon flux could either increase or decrease in response to single-element addition, depending on the attributes of the invading species.