title: Water-column stratification governs the community structure of subtropical marine picophytoplankton.
abstract: The increase in the areal extent of the subtropical gyres over the past decade has been attributed to a global tendency towards increased water-column stratification. Here, we examine how vertical stratification governs the community structure of the picophytoplankton that dominate these vast marine ecosystems. We analysed phytoplankton community composition in the three Southern Subtropical basins of the Pacific, Indian and Atlantic Oceans using a variety of methods and show that the distributions of picocyanobacteria and photosynthetic picoeukaryotes (PPEs) are strongly correlated with depth and strength of vertical mixing: the changes in community structure occur at various taxonomic levels. In well-mixed waters, PPEs, in particular haptophytes, dominate, whereas in strongly stratified waters, picocyanobacteria of the genus Prochlorococcus are prevalent, regardless of whether the relative contributions to total biomass are assessed in terms of pigment or of carbon. This ecological diochotomy within the picophytoplankton supports the hypothesis that genomic streamlining provides a selective advantage for Prochlorococcus in highly stable, oligotrophic systems, but may restrict their ability to dominate in regions subject to dynamic mixing.
authors.affiliation: Departamento de Oceanografía and Centro de Investigación Oceanográfica COPAS, Universidad de Concepción, Casilla 160-C, Concepción, Chile. Marine and Coastal Management, Private Bag X2, Rogge Bay 8012 Cape Town, South Africa. Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada. Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK. Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: Heather A
authors.initials: HA
authors.lastname: Bouman
authors.forename: Osvaldo
authors.initials: O
authors.lastname: Ulloa
authors.forename: Ray
authors.initials: R
authors.lastname: Barlow
authors.forename: William K W
authors.initials: WK
authors.lastname: Li
authors.forename: Trevor
authors.initials: T
authors.lastname: Platt
authors.forename: Katrin
authors.initials: K
authors.lastname: Zwirglmaier
authors.forename: David J
authors.initials: DJ
authors.lastname: Scanlan
authors.forename: Shubha
authors.initials: S
authors.lastname: Sathyendranath