The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles

TLDR

Pelagic ecosystem structure depends on the balance of growth and mortality, with iron supply influencing growth but the mortality drivers, such as grazing, remaining poorly understood. This review investigates how grazing shapes pelagic ecology and biogeochemical cycling in the Southern Ocean, linking ecosystem structure to global carbon and silica fluxes. The authors synthesize field observations and propose hypotheses that can be tested through in‑situ iron fertilization experiments within an evolutionary‑ecology framework. The study finds that only a few species dominate the Southern Ocean’s trophic structure, with iron‑sufficient conditions promoting diatom–krill–whale blooms that drive the carbon pump, while iron limitation and grazing by small copepods and salps lead to large silicified diatoms that fuel the silicon pump.

Abstract

This review examines the links between pelagic ecology and ocean biogeochemistry with an 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. This paper addresses the role of grazing as a selective force operating on the structure and functioning of pelagic ecosystems within the larger conceptual framework of evolutionary ecology. That mortality due to grazing 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. Under iron-sufficient conditions, high growth rates of weakly silicified diatoms and Phaeocystis result in build-up of blooms that fuel “the food chain of the giants” (diatoms-krill-whales) and drive the carbon pump. In contrast, high grazing pressure of small copepods and salps on the regenerating microbial communities characteristic of the iron-limited Southern Ocean results in accumulation of large, heavily silicified diatoms that drive the silicon pump. The hypotheses we derive from field observations can be tested with in situ iron fertilization experiments.