Envisat detects ’grass of the sea’ blooming early off Norway

Spring starts early this year in Norway’s fjords and coastal waters, with sunny weather awakening a colourful bloom of marine phytoplankton. ESA’s Envisat spacecraft is being used to monitor its development.

Microscopic phytoplankton have been called ’the grass of the sea’ – they are the basic food on which all other marine life depends. Provided with sufficient light planktonic algae multiply by absorbing mineral nutrients and converting solar energy into organic matter. They themselves are consumed by animal zooplankton that go on to provide food for larger animals and fish.

Although individually tiny, the collective chlorophyll pigments of algae species cause slight shifts in water hues, and dedicated ocean colour sensors such as Envisat’s Medium Resolution Imaging Spectrometer (MERIS) can identify phytoplankton concentrations.

The onset of this year’s bloom of phytoplankton in Norwegian waters was triggered by sunny weather in the last week of February. After a winter of heavy weather there were ample mineral nutrients in place for a bloom; sunlight was the only missing factor.

Earth Observation imagery by itself is not sufficient to identify the species that are blooming, but during such cloud free periods satellite data are essential to monitor the daily development of the bloom and to plan an optimal water-sampling strategy.

“The image shows a natural and annual phenomenon that happened at about the same time last year,” says Lasse Pettersson of Norway’s Nansen Environmental and Remote Sensing Center (NERSC). “Although it is occurring some weeks earlier than the long-term average time for the onset of early spring blooms in these waters.

“These spring blooms are generally governed by the availability of nutrients, solar light and ocean currents causing the advection of the water masses. This MERIS image shows the blooms in our major fjords and the extension of the Norwegian Coastal Current with its meanders and eddies.”

“The Nansen Center has been using Earth Observation data daily since 1998 to monitor Norwegian coastal waters in order to contribute to the early detection of possible harmful algae blooms in our waters. Analysis of water samples this week confirms that these algae species are normal for the season, and have no harmful impact on our environment.”

Some algae species are toxic or harmful. If they surge out of control during optimal blooming conditions they can kill fish and seabed life, or suffocate larger individuals as decaying phytoplankton exhaust all available oxygen across wide stretches of water.

Harmful algae blooms have in the past caused significant losses to the Norwegian aquaculture industry found along its coast. There are some 870 salmon and trout fish farms in Norway, with a total turnover of 9 486 million Norwegian Kroner (€1 143 million) in 2003.

“In addition to satellite monitoring an extensive network of in-situ measurements and numerical ocean and ecosystem models are used to predict the development of possible harmful algae blooms,” Pettersson added. “This type of integrated monitoring and forecasting provides the fishery authorities, the aquaculture industry, consumers and research community with significant information in order to initiate mitigation actions as well as better understand the causes and impact of possible harmful algae blooms.”

As part of its Global Monitoring for Environment and Security (GMES) Services Element, ESA is funding the development and enhancement of services to help manage and protect the entire European coastal environment. The two priorities are coastal water quality information – including detection of algae blooms likely to affect safety of fish farms and quality of coastal bathing water, and forecasting bloom evolution – and oil spill information – including the detection of illegal oil discharges and drift forecasting from large accidental spills such as the 2002 Prestige tanker disaster.

Although still under development, these information services have already demonstrated their worth. For instance, in 2004 Dutch shellfish farmers were spared an estimated bill of up to €200 million when an early warning of an algal bloom occurrence enabled the Royal Dutch Marine and Coastal Management Institute (RIKZ) to close estuary beds and prevent toxic algae from entering shellfish beds.