BIOCOMPLEXITY: Collaborative Research: Factors Affecting, and Impact of, Diazotrophic Microorganisms in the Western Equatorial Atlantic Ocean

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BIOCOMPLEXITY: Collaborative Research: Factors affecting, and impact of,diazotrophic microorganisms in the western Equatorial Atlantic Ocean This biocomplexity research focuses on plankton dynamics in the western Equatorial Atlantic Ocean (WEQAT). This is a complex and understudied ecosystem that has significant impacts on marine resourcesin the region as well as in downstream areas such as the Caribbean Sea. The study centers ondiazotrophic (nitrogen fixing) microorganisms as keystone species. Geological, physical, biological,chemical and even social factors all have a major influence on population biology and activity ofdiazotrophs in the WEQAT. Diazotrophs in turn have a major impact on other phytoplankton andtrophic levels through input of fixed nitrogen (N). The Amazon River affects the regionphysically by changing salinity and thereby water column stratification, and geochemically byintroducing iron and silicate which can then biologically stimulate the growth of diatoms thatcontain the N2 fixing endosymbiont Richelia intracellularis. Furthermore, the area receivessignificant seasonal atmospheric inputs of iron in dust from the Sahel region of Africa, which canpromote the growth of the important N2 fixing cyanobacterium Trichodesmium. Thisatmospheric iron source is directly deposited on the surface waters where biological activity isgreatest. For Trichodesmium, the physical environment (e.g. high wind speed) can also inhibitactivity and the formation of blooms. Diazotrophs may be affected by land use practices in theAmazon Basin and the African Sahel, and N2 fixed by marine plankton can affect humans bystimulating primary productivity and fishery yields. Using both remote sensing and shipboard measurements, scientists will examine the complex processes which structure these planktonic diazotroph populations, influence their importance in CO2 andN2 fixation, which, in turn, affect other planktonic processes. The seasonal and spatialrelationships of Trichodesmium and Hemiaulus / Richelia associations will be examined withdirect reference to the major routes of inputs of Fe and Si, and with regard to the physicalenvironment. The group of collaborating scientists will examine the trophic structures associated with each diazotrophic community, including the vertical distribution of processes and associated autotrophic andheterotrophic plankton populations. These data will be used to develop and verifybiogeochemical and trophodynamic models that incorporate the complex physical, chemical andbiological interactions that characterize the WEQAT region. The models will, in turn, be used toexamine the hypothesis that physical forcing, through its effect on the diazotrophic populationsand the structure of the food web, influences N2 fixation and, in part, determines the highproductivity of the WEQAT. The work uses a combination of both observations and models to address threefundamental issues in biocomplexity: 1) the relationship between ecosystem structure andfunction in a system that is both nonlinear and high-dimensional; 2) the response of a nonlinearecosystem to environmental forcing; and 3) the relevant level of detail, including the resolutionof physical space, that must be incorporated in nonlinear systems to capture the dynamics of aglobal ecosystem property (here, high productivity). The research will significantly advance our understanding of the interaction between physical and biogeochemical processes in an important area the world's oceans, and identify how these interactions regulate variability in marine ecosystem productivity.