The role grazing in structuring Southern Ocean pelagic ecosystem and biogeochemical cycles

This review examines the links between pelagic ecology and ocean biogeochemistry with emphasis on the role of the Southern Ocean in global cycling of carbon and silica. The structure and functioning of pelagic ecosystems is determined by the relationship between growth and mortality of its species populations. Whereas the key role of iron supply in conditioning the growth environment of land-remote oceans is now emerging, the factors shaping the mortality environment are still poorly understood. We address the role of grazing as a selective force operating on the structure and functioning of pelagic ecosytems within the larger conceptual framework of evolutionary ecology. That grazing pressure decreases with increasing cell size is widely taken for granted. We examine the impact of this principle across the range of size classes occupied by Southern Ocean plankton and show that relatively few species play crucial roles in the trophic structure and biogeochemical cycles of the Southern Ocean. High grazing pressure of small copepods and salps within the regenerating communities characteristic of the iron-limited Southern Ocean results in accumulation of large, heavily silicified diatoms that drive the silica pump. In contrast high growth rates of weakly silicified diatoms result in build-up of blooms under iron-sufficient conditions that fuel the food chain of the giants and drive the carbon pump. The hypotheses we derive from field observations can be tested with in situ iron fertilization experiments