Page 21 - living-ice

PHOTO: National Geographic
The oating grass of the
ocean
The Arctic Ocean is an enormous, three-dimensional system. Only the top 30–40 meters provide suf cient light for algae to grow. Beneath this depth are up to several thousand meters of what appears to be complete darkness. Outside the coastal zone microscopic unicellular plants alone produce most the energy for the entire Arctic ecosystem using sunlight and nutrients in the ocean. These microalgae are the grass of the ocean. Micro-algae have in common with land plants that they can convert sunlight into energy and, in many ways, ful l the role land plants have in terrestrial ecosystems.
There are very low concentrations of microalgae in the Arctic Ocean during winter. When the winter darkness of the polar night ends and the sunlight returns the ice still prevents part of the light from entering the ocean beneath. The light only reaches the microalgae when the sea ice cracks, melts or thins and the light can penetrate in between ice oes or through melt ponds. The microalgae then respond with immediate and explosive growth. Algae reproduce by cell division with one cell dividing into two or more. The population of algae in the Arctic Ocean can double within a couple of days. In just one week, a single alga reproduces into 64 algae. The water turns green as the algal bloom at the ice edge picks up pace. Algae grow if there are nutrients such as nitrogen and phosphorus in the seawater to sustain the growth.