Aquatic primary producers constitute only 1% of the global biomass, but their productivity equals that of all terrestrial ecosystems taken together (cf. Chapter 3, this volume). They take up a large fraction of anthropogenically emitted CO2, thereby mitigating climate change. In addition, the oceans are a major CO2 sink, because part of the absorbed carbon sinks to the deep sea as oceanic snow consisting of dead organisms and excretion products. In the sediments its lifetime is millions of years so that this fraction of carbon is effectively removed from the atmosphere.