Seasonal Phytoplankton Blooms at High Latitudes

Primary production in the ocean is driven by phytoplankton. Oceanographers often examine primary production by measuring chlorophyll-a concentration (a proxy for phytoplankton biomass) via fluorescence. In the ocean two main factors limit primary production – light and nutrients. In this nugget, the annual cycle of primary production (chlorophyll) from two OOI Global Array sites are examined – Irminger Sea Array and Argentine Basin Array.

Primary production in the Irminger Sea shows a strong signal of seasonal light limitation. During winter months, day length is shorter and there is deep mixing in the water column, so primary production is limited even with the plentiful nutrients in the area. As spring unfolds off the coast of Greenland, melting ice and solar heating produce fresh, warm surface water that stratifies the water column. The combination of a stratified water column and longer day length lead to a large bloom in May. Productivity continues through September, though much lower than the initial bloom. As fall arrives, the days shorten and the water column mixes again reducing primary production.

Similar to the Irminger Sea, productivity is focused in the summer and fall time periods in the Argentine Basin as mixed layer depths shoal (get shallower). As the Argentine Basin is in the Southern Hemisphere, however, its seasons are reversed with austral summer/fall occurring from January to April. The Argentine Basin Array is located in a convergence zone where nutrient-poor subtropical water flowing south in the Brazil Current meets nutrient rich water from the subarctic traveling north in the Malvinas Current. This area is collectively referred to as the Brazil-Malvinas Confluence Area.