Half of the photosynthesis that occurs in the world happens in the oceans, fixing carbon dioxide and giving us oxygen in return. Despite this, surprisingly little is known about the cycle and the environmental dynamics that may influence the process.
Much of that is due to the difficulty of studying ocean processes that play out over time scales many orders of magnitude shorter than terrestrial systems. On land, vegetation persists for decades, but the photosynthesizers in the ocean live for only one week before they are eaten.
When zooplankton and fish graze on phytoplankton and algae, the carbon ingested comes out in the fecal material that sinks down into the water column. Moving the carbon away from the surface of the water keeps it from returning to the atmosphere.
Some plankton and mollusks also lock carbon in the form of calcium carbonate shells, fragments of which help the organic matter sink into the deep ocean.
The ocean’s biological pump is thus considered a natural form of carbon sequestration.
Scientists are trying to understand the effect of human activities on this natural system in the ocean. This includes the effect of fisheries, pollution and increased levels of carbon dioxide in the atmosphere. What is known is that dissolved carbon dioxide has increased the acidification of the oceans.