Abandoning Sverdrup’s Critical Depth Hypothesis on phytoplankton blooms. Climate-driven trends in contemporary ocean productivity. Bulletin of the American Meteorological Society. Baringer (Eds.), State of the Climate in 2008. Accurate global mapping of phytoplankton taxonomic groups is one of the primary goals of proposed future NASA missions like the Aerosol, Cloud, Ecology (ACE) mission. These shifts in species composition may be benign, or they may result in a cascade of negative consequences throughout the marine food web. Shifts in the relative abundance of larger versus smaller species of phytoplankton have been observed already in places around the world, but whether it will change overall productivity remains uncertain. As the ocean has warmed since the 1950s, it has become increasingly stratified, which cuts off nutrient recycling.Ĭontinued warming due to the build up of carbon dioxide is predicted to reduce the amounts of larger phytoplankton such as diatoms), compared to smaller types, like cyanobacteria. Changes in water clarity, nutrient content, and salinity change the species that live in a given place.īecause larger plankton require more nutrients, they have a greater need for the vertical mixing of the water column that restocks depleted nutrients. Hundreds of thousands of species of phytoplankton live in Earth's oceans, each adapted to particular water conditions. These low-nutrient “marine deserts” appear to be expanding due to rising ocean surface temperatures. For example, ocean scientists documented an increase in the area of subtropical ocean gyres-the least productive ocean areas-over the past decade. Over the past decade, scientists have begun looking for this trend in satellite observations, and early studies suggest there has been a small decrease in global phytoplankton productivity. In the equatorial upwelling zone, there is very little seasonal change in phytoplankton productivity. As the winds reverse direction (offshore versus onshore), they alternately enhance or suppress upwelling, which changes nutrient concentrations. In lower-latitude areas, including the Arabian Sea and the waters around Indonesia, seasonal blooms are often linked to monsoon-related changes in winds. Phytoplankton use up the nutrients available, and growth falls off until winter storms kick-start mixing. With warm, buoyant water on top and cold, dense water below, the water column doesn't mix easily. As surface waters warm up through the summer, they become very buoyant. In the subtropical oceans, by contrast, phytoplankton populations drop off in summer. Recent research suggests the vigorous winter mixing sets the stage for explosive spring growth by bringing nutrients up from deeper waters into the sunlit layers at the surface and separating phytoplankton from their zooplankton predators. In high latitudes, blooms peak in the spring and summer, when sunlight increases and the relentless mixing of the water by winter storms subsides. Like plants on land, phytoplankton growth varies seasonally.
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