Dynamic Definition of Habitat Types in the Tropical Atlantic Ocean

Knowledge of the phytoplankton community structure is the one of the “holy grails” of microbial oceanography and is essential for both understanding how the current ocean system functions as well as how it will change in the future. Phytoplankton community structure is an emergent property resulting from the complex interactions between the bottom up physical and chemical properties of the upper water column and top down interaction within the biological community. While a variety of strategies such as the Longhurst Biomes have been employed to describe phytoplankton communities over large spatial and temporal scales, delineating habitats on spatial and temporal scales of days and 1000s of meters relevant to shipboard studies is challenging. We employ a method of dynamic habitat delineation based on environmental variables that are biologically relevant, that integrate over varying time scales, and that are derived from standard oceanographic measurements. We use sea surface temperature (SST), sea surface salinity (SSS), mixed layer depth (MLD), depth of the chlorophyll a maximum (ChlMD) as a proxy for light availability, and nitrate availability index, a measure of nitrate availability to surface waters to define the physical and chemical environment. We apply this method to measurements made in the Tropical Atlantic – along the 23W section stretching from Cape Verde to South of the Equator and in the Amazon River plume, to delineate habitat types and compare that to the phytoplankton community structure determined by cell counts from flow cytometry, HPLC diagnostic phytoplankton pigments, and spectrofluorometry.