A phytoplankton vortex in the Baltic Sea

Every summer, phytoplankton spread across the northern basins of the North Atlantic and Arctic Oceans, with blooms spanning hundreds and sometimes thousands of kilometres. Nutrient-rich, cooler waters tend to promote more growth among marine plants and phytoplankton than is found in tropical waters. Blooms this year off Scandinavia seem to be particularly intense.

On July 18, 2018, the Operational Land Imager (OLI) on Landsat 8 acquired a natural-colour image of a swirling green phytoplankton bloom in the Baltic Sea. Note how the phytoplankton trace the edges of a vortex.

Three decades of satellite observations suggest that these green blooms are likely to be cyanobacteria (blue-green algae), an ancient type of marine bacteria that capture and store solar energy through photosynthesis (like plants). Some of the greens also could come from diatoms, which are also rich in chlorophyll. According to news outlets, the Finnish Environment Institute (SYKE) has observed the recent bloom from the water and found it to be mostly cyanobacteria.

In recent years, the proliferation of algae blooms in the Baltic Sea has led to the regular appearance of “dead zones” in the basin. Phytoplankton and cyanobacteria consume the abundant nutrients in the Baltic—fueled in part by past and current runoff from sewage and agriculture—and reproduce in such vast numbers that their growth and decay deplete the oxygen content of the water. According to researchers from Finland’s University of Turku, the dead zone this year is estimated to span about 70,000 square kilometres.

A research team from Finland and Germany reported this month that oxygen levels in recent years in the Baltic Sea are at their lowest levels in the past 1500 years. More frequent and massive blooms, combined with warming seas due to climate change, are making it harder for fish and other marine life to thrive in this basin.