Marine Chemistry and Biological Productivity

Marine biogeochemistry and productivity

Marine biogeochemistry studies how elements such as carbon, nitrogen, phosphorus, silicon and oxygen cycle among seawater, sediments and the atmosphere, with living organisms acting as the main agents of transformation. Marine productivity is the rate at which phytoplankton convert dissolved inorganic carbon into organic matter by photosynthesis in the sunlit euphotic zone. Productivity is controlled mainly by light and by nutrients such as nitrate and phosphate, which reach the surface through upwelling, vertical mixing and river inputs.

Reef-building corals show how light, temperature, salinity and carbonate chemistry together set the limits of a marine ecosystem. Corals depend on symbiotic zooxanthellae, which photosynthesise and need strong sunlight, so reefs are confined mainly to shallow tropical and subtropical waters. Normal oceanic salinity of about 35 psu and temperatures of roughly 23 to 29 °C favour optimal growth, whereas fresh or low-salinity water is harmful. Because corals build calcium carbonate skeletons, they require seawater that is supersaturated, not undersaturated, with respect to carbonate minerals.

The biological pump is the set of processes by which photosynthesis moves carbon from the surface ocean to the deep ocean. Phytoplankton take up dissolved CO_2 and fix it into organic matter, which sinks as particles and is remineralised back to CO_2 at depth. Most of this organic matter is respired in the upper water column, and only a small fraction reaches the sea floor. The net vertical gradient keeps surface dissolved inorganic carbon low and maintains a large air-sea CO_2 difference. Without the biological pump, atmospheric CO_2 would be higher by roughly 150 to 200 ppm.

Phytoplankton are the organisms that start the biological pump, because they fix dissolved inorganic carbon drawn from surface seawater, which is replenished from the atmosphere through air-sea exchange. Zooplankton, fish and radiolarians transfer, consume or repackage organic matter and carbonate, but they do not themselves initiate the uptake of atmospheric carbon.

Calcification has the opposite effect on surface CO_2 from photosynthesis. Calcifying organisms build shells from calcium and bicarbonate through \mathrm{Ca^{2+} + 2HCO_3^{-} \rightarrow CaCO_3 + CO_2 + H_2O}, a reaction that releases CO_2 and raises surface p\mathrm{CO_2}. This is why the carbonate pump acts as a source of CO_2 to the surface ocean, in contrast with the carbon removal achieved by soft-tissue production.

Vertical profiles reveal these processes directly. Nitrate increases steadily with depth as sinking organic matter is remineralised, so it is lowest in the surface layer and rises through the thermocline. Chlorophyll shows a subsurface maximum, the deep chlorophyll maximum, where phytoplankton receive enough light and still draw nutrients from below the mixed layer. Dissolved oxygen is highest in the surface layer, supplied by air-sea exchange and photosynthesis, and falls with depth as respiration consumes it.

In the northern Indian Ocean, intense respiration of sinking organic matter combined with sluggish ventilation produces an oxygen minimum zone at intermediate depths of several hundred metres. Oxygen therefore shows a surface maximum, a steep decline into the minimum layer, and a slow recovery in the deep water.

• euphotic zone — the sunlit upper layer in which photosynthesis can occur. 예) Phytoplankton growth is confined to the euphotic zone.
• remineralisation — the breakdown of organic matter by bacteria back into inorganic nutrients and CO_2. 예) Sinking detritus is remineralised as it descends.
• zooxanthellae — symbiotic dinoflagellates living inside coral tissue that supply the coral with photosynthetic products. 예) Reef corals rarely thrive where light is weak because their zooxanthellae cannot photosynthesise.
• deep chlorophyll maximum — a subsurface peak in chlorophyll concentration below the mixed layer. 예) Nutrient supply from below sustains a deep chlorophyll maximum.