Effect of environmental forcing on the biomass, production and growth rate of size-fractionated phytoplankton in the central Atlantic Ocean

In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t a r t i c l e i n f o To ascertain the response of phytoplankton size classes to changes in environmental forcing, we determined size-fractionated biomass, carbon fixation and growth (production/biomass) rates in surface waters along the central Atlantic Ocean (26°N–5°S). As a result of the enhanced input of nutrients into the euphotic layer and the higher water column stability found at the equatorial upwelling, we observed increases not only in phytoplankton biomass and primary production, but also in turnover rates, suggesting nutrient limitation of phytoplankton physiology in the oligotrophic central Atlantic. The phytoplankton groups analysed (pico-, small nano-, large nano-and micro-phytoplankton) showed different responses to the equatorial environmental forcing, in terms of carbon biomass, primary production and growth rate. Large nano-and micro-phytoplankton consistently showed higher growth rates and carbon fixation to chl a ratios than smaller phytoplankton. We observed a higher stimulating effect of increased nitrate supply on the small phytoplankton growth rates. This observation can be explained by the dynamics of the equatorial upwelling, where the continuous but small nutrient input into the euphotic layer provide a competitive advantage for smaller cells adapted to oligotrophic conditions. The size-fractionated approach shown here reveals important group-specific differences in the response to environmental forcing, which cannot be appreciated in bulk measurements of the whole community. Phytoplankton size structure is a critical property of pelagic ecosystems as it largely determines their food-web organisation and, dominated by small cells (b5 μm in Equivalent Espherical Diameter, ESD), typical of oligotrophic waters, are characterised by the recycling of matter within the microbial food web and, in consequence, by a small downward carbon export (Azam et al., 1983; Goldman, 1988). By contrast, the dominance by larger cells (N5 μm in ESD), commonly observed in turbulent and nutrient-rich areas, favours the transfer of organic matter through short-food chains towards higher trophic levels as well as its sedimentation towards the deep ocean, thus increasing the potential of these ecosystems for CO 2 sequestration (Chisholm, 1992; Cushing, 1989). In addition, both chemical and physical characteristics of ecosystems are key factors in controlling the size distribution of phytoplankton community. Among …