4. Currents and Life in the Ocean


Ocean currents are a major influence on life in the ocean. Along with the availability of sunlight, the passing of the seasons, and the heating and cooling of surface waters, they control the physical conditions under which all marine life exists.

As on land, plants are at the bottom of the food chain, so the availability of food for marine animals depends on the productivity of marine plants.

Zoom Sign
Envisat MERIS true colour image of a phytoplankton bloom in the Barents Sea, 12th August 2008. During the Arctic summer, melting of sea ice helps to stabilise the water column, so phytoplankton are able to remain in the sunlit surface zone. The nutrients brought up from the deep where currents flow over sea-mounts or ridges, can give rise to bright blooms such as this.
Phytoplankton bloom in the Barents Sea

Plant life depends on light and nutrients

Unlike on land, life in the sea is very much a 3-D affair, and the vast majority of the ocean is cold, deep and dark. Plants can only photosynthesise in the sunlit zone near the surface. Plant growth on the sea floor is only possible in shallow water near land. The majority of plants in the ocean are tiny, single-celled plants that drift with the currents near the surface - the phytoplankton.

If turbulence brings the phytoplankton down into the dark zone, they die. But near the surface essential plant nutrients are soon used up. Unless new nutrient rich water flows up from the deep, growth will be slow. Thus concentrations of phytoplankton cells are low in areas with little vertical movement of water.

Why is the ocean teeming with life?

Sailing from Tromsø in Norway to Spitsbergen the Cape Farewell team suddenly found their ship surrounded by thousands of birds. Looking around they saw seals, dolphins and humpback whales. After over two days of seemingly empty seas, what was the reason for all this activity?

Can you explain their observations?

SeaWiFS image of the Gulf Stream
SeaWiFS. The Gulf Stream waters are warm and oligotrophic, low in nutrients. They are deep blue while the cooler, more-nutrient-rich waters to the north are milky blue due to the phytoplankton. (click on image for a larger version)
Source: NASA Visible Earth.


This chapter looks at how ocean currents affect the availability of plant nutrients, the global patterns of phytoplankton productivity, and the productivity of marine ecosystems around the world.