Despite its cold waters and harsh winds, the North Sea is a fertile basin for phytoplankton blooms. The drifting, plantlike organisms tend to be most abundant in late spring and early summer due to high levels of nutrients in the water and increasing sunlight. The intense winds blowing over the relatively shallow North Sea causes a lot of vertical and horizontal mixing that brings nutrients to the surface, as does runoff from European rivers.

The first image, acquired on June 6, 2015, by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, shows a mass of phytoplankton blooming between Denmark, the United Kingdom, and Germany. The milky, light-colored surface waters are likely filled with coccolithophores, whereas the greener areas are probably rich with diatoms or perhaps dinoflagellates. (It is impossible to know for sure without water samples.) Colors are more intense and vivid in places where concentrations of phytoplankton are highest, which should match where nutrients like nitrates, phosphates, iron, and sulfur are most abundant and water temperatures are ideal. The patterns traced by the phytoplankton also give some insight to the currents and eddies in the area.

On June 11, 2015, the MODIS instrument on NASA’s Aqua satellite caught a glimpse (middle image) of the same area in what was likely a different stage of the bloom cycle. Areas of concentrated phytoplankton are smaller, and most have the milky color characteristic of coccolithophores; there are few to no green areas. The change could be due to the short life span of phytoplankton—two to six days—and differences between the species. Some outlast others because of their ability to survive at lower nutrient levels.

The third image, taken on July 1, 2015, by MODIS on Terra, shows an area slightly north of the earlier bloom. It could be a new burst of coccolithophores, or it could be a continuation and migration of the same bloom from earlier in June.

Some researchers have found that numbers of plankton can actually begin to increase in the middle of winter, when growth conditions would seem to be at their worst. Studies suggest that winter storms churn the ocean and cause deep water mixing. This water mixing allows for phytoplankton to grow and live at depth without being spotted by their predators. When spring arrives, phytoplankton can fully bloom because not only are the nutrients available, but there is a longer period of sunlight.

NASA Earth Observatory images by Jesse Allen, using data from the Level 1 and Atmospheres Active Distribution System (LAADS). Caption by Rachel Carlowicz with Mike Carlowicz. Interpretation insight provided by Mike Behrenfeld, Oregon State University, and Jochen Wollschläger, Helmholtz-Zentrum Geesthacht.