Springtime in the northern hemisphere brings green buds and sprouts to plants on land. It also brings blooms in the seas, where increasing sunlight and nutrient-rich waters promote the growth of phytoplankton. These microscopic, plant-like organisms use chlorophyll to harness energy from the Sun and fuel their own growth. They are the center of the ocean food web, as everything from zooplankton to fish to whales consume phytoplankton.
The image above shows chlorophyll concentrations in milligrams per cubic meter of water. Blue indicates areas with low concentrations of chlorophyll, and shades of yellow indicate high concentrations. Land is dark gray, while light gray areas depict where MODIS could not collect data due to clouds blocking the sensor’s view.
Blooms off the coast of Hokkaido are a yearly springtime occurrence, supporting one of the most productive fisheries in the world. The cool, nutrient-rich Oyashio Current arises in Arctic waters, flows south and west along the coast of Siberia and Kamchatka, and then turns east around Hokkaido. In springtime, the coastal waters off of the island as well as the incoming Oyashio Current are both enriched with iron, a critical nutrient for phytoplankton growth. Later in the spring and summer, the interaction of the Oyashio with the northeast-flowing Kuroshio Current can provoke eddies that stir up nutrients from deeper waters, fueling more blooms.
NASA Earth Observatory image by Jesse Allen, using data from the Ocean Color Web. Caption by Michael Carlowicz