title: Nutrient uplift in a cyclonic eddy increases diversity, primary productivity and iron demand of microbial communities relative to a western boundary current.
abstract: The intensification of western boundary currents in the global ocean will potentially influence meso-scale eddy generation, and redistribute microbes and their associated ecological and biogeochemical functions. To understand eddy-induced changes in microbial community composition as well as how they control growth, we targeted the East Australian Current (EAC) region to sample microbes in a cyclonic (cold-core) eddy (CCE) and the adjacent EAC. Phototrophic and diazotrophic microbes were more diverse (2-10 times greater Shannon index) in the CCE relative to the EAC, and the cell size distribution in the CCE was dominated (67%) by larger micro-plankton [Formula: see text], as opposed to pico- and nano-sized cells in the EAC. Nutrient addition experiments determined that nitrogen was the principal nutrient limiting growth in the EAC, while iron was a secondary limiting nutrient in the CCE. Among the diazotrophic community, heterotrophic NifH gene sequences dominated in the EAC and were attributable to members of the gamma-, beta-, and delta-proteobacteria, while the CCE contained both phototrophic and heterotrophic diazotrophs, including Trichodesmium, UCYN-A and gamma-proteobacteria. Daily sampling of incubation bottles following nutrient amendment captured a cascade of effects at the cellular, population and community level, indicating taxon-specific differences in the speed of response of microbes to nutrient supply. Nitrogen addition to the CCE community increased picoeukaryote chlorophyll a quotas within 24 h, suggesting that nutrient uplift by eddies causes a 'greening' effect as well as an increase in phytoplankton biomass. After three days in both the EAC and CCE, diatoms increased in abundance with macronutrient (N, P, Si) and iron amendment, whereas haptophytes and phototrophic dinoflagellates declined. Our results indicate that cyclonic eddies increase delivery of nitrogen to the upper ocean to potentially mitigate the negative consequences of increased stratification due to ocean warming, but also increase the biological demand for iron that is necessary to sustain the growth of large-celled phototrophs and potentially support the diversity of diazotrophs over longer time-scales.
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney , Ultimo NSW , Australia.
authors.forename: Martina A
authors.initials: MA
authors.lastname: Doblin
authors.affiliation: School of Life Sciences, University of Technology Sydney , Ultimo NSW , Australia.
authors.forename: Katherina
authors.initials: K
authors.lastname: Petrou
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Ultimo NSW, Australia; Faculty of Environmental Management, Prince of Songkla University, Kho Hong Songkhla, Thailand.
authors.forename: Sutinee
authors.initials: S
authors.lastname: Sinutok
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney , Ultimo NSW , Australia.
authors.forename: Justin R
authors.initials: JR
authors.lastname: Seymour
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney , Ultimo NSW , Australia.
authors.forename: Lauren F
authors.initials: LF
authors.lastname: Messer
authors.affiliation: School of Biotechnology and Biomolecular Sciences, University of New South Wales , Sydney NSW , Australia.
authors.forename: Mark V
authors.initials: MV
authors.lastname: Brown
authors.affiliation: Department of Plant Sciences, University of Cambridge , Cambridge , United Kingdom.
authors.forename: Louiza
authors.initials: L
authors.lastname: Norman
authors.affiliation: School of Biological, Earth and Environmental Sciences, University of New South Wales , Sydney NSW , Australia.
authors.forename: Jason D
authors.initials: JD
authors.lastname: Everett
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney , Ultimo NSW , Australia.
authors.forename: Allison S
authors.initials: AS
authors.lastname: McInnes
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology Sydney , Ultimo NSW , Australia.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Ralph
authors.affiliation: Oceans and Atmosphere Flagship, Commonwealth Scientific Industrial Research Organisation , Hobart Tas , Australia.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Thompson
authors.affiliation: Institute F.-A. Forel, Earth and Environmental Sciences, University of Geneva , Geneva , Switzerland.
authors.forename: Christel S
authors.initials: CS
authors.lastname: Hassler
keywords: Marine microbial diversity; Meso-scale processes; Nutrient limitation; Physiology; Upwelling