The grass of the sea

The satellite image below shows a large aquamarine-coloured phytoplankton bloom stretching across the west coast of Ireland. Plankton, the most abundant type of life found in the ocean, are microscopic marine plants that drift on or near the surface of the sea. Just like plants on land they employ green-pigmented chlorophyll for photosynthesis — the process of turning sunlight into chemical energy. While individually microscopic, plankton chlorophyll collectively tints the surrounding ocean waters, providing a means of detecting these tiny organisms from space with dedicated 'ocean colour' sensors onboard satellites.

Microscopic plankton are called ‘the grass of the sea' because they are the basic food on which all other marine life depends. They themselves are consumed by animal zooplankton that go on to provide food for larger animals and fish.

Found in abundance throughout the oceans, most algae and other plankton are not harmful to humans. As primary producers, fixing carbon dioxide from the atmosphere, they form the base of the marine food chain and help regulate the carbon cycle and through it the global climate system.

However, global warming in recent decades has resulted in a decline in plankton. A study based on 43 years of observations along the coast of southern California recorded an 80 per cent decrease in the biomass of large zooplankton. The decrease corresponded to a warming of oceanic surface layers by 1.5°C which changed the mechanics of water circulation. Convection currents in the ocean previously brought cold water to the surface from the nutrient-rich ocean depths. But because the surface temperature is now warmer, waters that upwell come from shallower depths, and are therefore less nutrient rich.

The decline in plankton represents a major disruption in the biotic web of the Californian region because large zooplankton are a significant part of the food web and are the main diet of some birds and many schooling, commercially mportant species of fish. More: earth.esa.int; gcrio.org