On land, green plants form the center of the food web, and nearly all other life radiates out from there—consuming those plants or the creatures who eat the plants. In the ocean, phytoplankton are the equivalent of grasses, trees, and shrubs. Floating near the ocean surface, phytoplankton use chlorophyll to harness sunlight, turning carbon dioxide from the air and dissolved nutrients in the water into sugars and oxygen. Nearly all life in the ocean traces its food supply back to these primary producers.
Off the coast of Hokkaido, Japan, there was a lot of primary production going on in late May and early June 2019. On May 26 and June 2, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite caught glimpses of vast blooms of phytoplankton. Their green and light blue tones traced the edges of swirling water masses, currents, and eddies.
Blooms are common in this region, especially in spring, as it is dominated by the Oyashio current. The “parent stream” (oya shio in Japanese) nurtures so much life because it carries cool, lower-salinity water from the Bering Sea and sub-Arctic North Pacific. It bears iron and other nutrients from Arctic waters and from the coasts of Kamchatka and Siberia. More nutrients are stirred up from the depths through upwelling. This combination of ocean conditions provides an incredibly fertile environment for bursts of phytoplankton growth, often led by diatoms.
Blooms tend to be largest here in the early spring because surface waters have been “resting” all winter. That is, the diminished sunlight and turbulent storms of winter keep phytoplankton productivity at a minimum. This allows the iron- and silica-rich dust and ash from Asian deserts and Kamchatkan volcanoes to accumulate in surface waters. The spring blooms then deplete most of these nutrients. Later blooms can be spurred by upwelling, by the collision and mixing of water masses between the Oyashio and the Kuroshio currents, or by sporadic natural events like dust storms that can seed the ocean.
The blooms on the Oyashio current in turn support some of the most productive fisheries in the world. The phytoplankton feed abundant populations of copepods, euphausiids, and other zooplankton. Walleye pollock, Pacific cod, chum salmon, and pink salmon feed on the plankton buffet, and other migrants—such as sardines, anchovies, Pacific saury, chub mackerel, and squid—pass through seasonally. Whales and seabirds feast on the bounty, and humans reap a strong commercial harvest here.
NASA Earth Observatory images by Joshua Stevens, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview. Caption by Michael Carlowicz.