Phytoplankton production and taxon-specific growth rates in the Costa Rica Dome

During summer 2010, we investigated phytoplankton production and growth rates at 19 stations in the eastern tropical Pacific, where winds and strong opposing currents generate the Costa Rica Dome (CRD), an open-ocean upwelling feature. Primary production (C-14-incorporation) and group-specific growth and net growth rates (two-treatment sea-water dilution method) were estimated from samples incubated in situ at eight depths. Our cruise coincided with a mild El Nino event, and only weak upwelling was observed in the CRD. Nevertheless, the highest phytoplankton abundances were found near the dome center. However, mixed-layer growth rates were lowest in the dome center (similar to 0.5-0.9 day(-1)), but higher on the edge of the dome (similar to 0.9-1.0 day(-1)) and in adjacent coastal waters (0.9-1.3 day(-1)). We found good agreement between independent methods to estimate growth rates. Mixed-layer growth rates of Prochlorococcus and Synechococcus were largely balanced by mortality, whereas eukaryotic phytoplankton showed positive net growth (similar to 0.5-0.6 day(-1)), that is, growth available to support larger (mesozooplankton) consumer biomass. These are the first group-specific phytoplankton rate estimates in this region, and they demonstrate that integrated primary production is high, exceeding 1 g C m(-2) day(-1) on average, even during a period of reduced upwelling.