Ecology of marine phytoplankton

Abstract

Marine phytoplankton account for about 45 % of global net primary production (NPP). In addition, they perform other important biogeochemical functions including nitrogen fixation, calcium carbonate precipitation, and the production of climatically active gases such as dimethyl sulfide. Oceanographers employ a wide variety of platforms for studying marine phytoplankton ecology, including sampling from ships, sampling from autonomous remotely operated vehicles, and collecting observations from Earthorbiting satellites. Marine phytoplankton range in size from <1μm in diameter to about 1 mm in length and include representatives from at least five eukaryotic phyla together with the cyanobacteria. This wide size range and phylogenetic diversity presents challenges for quantifying and characterizing phytoplankton communities. Functional traits that quantify responses of growth rate, photosynthesis and nutrient uptake to temperature, irradiance, and nutrient availability provide a useful basis for understanding phytoplankton ecology. A variety of complementary approaches are used to measure gross and net primary production. These include measuring production of O2 and organic matter in bottle experiments and measuring diel and seasonal changes of O2 in open waters. Information obtained from satellite remote sensing of ocean color is used to calculate NPP on regional and global scales. The physical and chemical variables that drive NPP include temperature, nutrient availability, and solar radiation. These vary in time and space, and our understanding of this variability is largely encapsulated in the concepts of the seasonal production cycle and marine biogeochemical provinces. Nutrient limitation sets an upper limit to NPP over most of the ocean surface, with either inorganic iron or nitrogen being the proximate limiting element in different regions. The upper water column is stably stratified over much of the ocean, and pronounced vertical gradients of light and nutrients lead to depth separation of ecotypes with differing adaptations to nutrient availability and the light environment. Although the growth of individual phytoplankton cells is often limited by temperature and the availability of nutrients and light, biotic interactions including predation and disease often control the growth of phytoplankton populations and the species composition of phytoplankton communities. Anthropogenic impacts on the ocean, including nutrient loading to coastal waters, climatic forcing associated with global warming, and ocean uptake of anthropogenic CO2, are influencing the chemistry and physics of the upper ocean, with multiple potential impacts on the phytoplankton.