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

Abstract

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 ( 14 C-incorporation) and group-specific growth and net growth rates (two-treatment seawater dilution method) were estimated from samples incubated in situ at eight depths. Our cruise coincided with a mild El Niño 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 ( 0.5 -0.9 day 21 ), but higher on the edge of the dome ( 0.9 -1.0 day 21 ) and in adjacent coastal waters (0.9 -1.3 day 21 ). 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 ( 0.5 -0.6 day 21 ), 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 22 day 21 on average, even during a period of reduced upwelling.

The unique ecological role of pyrosomes in the Eastern Tropical Pacific

“…The only other study to document the habitat for Pyrosoma spinosum was done in the Arabian Sea (Gauns et al ). In this area, Pyrosoma spinosum was found under conditions of high macronutrient concentrations, modest Chl a , dominance of Synechococcus in the upper 40 m, and overlying a well‐developed shallow oxygen minimum zone, all of which are very similar to the characteristics of our study area (Buchwald et al ; Selph et al ), and suggest these as typical habitat characteristics for Pyrosoma spinosum .…”

“…We encountered large colonies of a pyrosome in the Eastern Tropical Pacific (ETP), later identified as Pyrostremma spinosum , a much lesser studied pyrosome with a more delicate consistency that P. atlanticum , as part of the Costa Rica Dome (CRD) FLUx and Zinc Experiments (FLUZiE) voyage (Landry et al ). The CRD is a unique open‐ocean upwelling system within the ETP, characterized by very high abundances of Synechococcus spp., often exceeding 10 6 cells mL −1 (Li et al ; Fiedler ; Selph et al ), large zooplankton stocks (Décima et al ), and a food web that supports many charismatic megafauna (including sea turtles, which feed on gelatinous zooplankton such as salps and pyrosomes) and top predators (Ballance et al ). Pyrostremma spinosum has not, to our knowledge, been reported previously in the area (Van Soest , ), but is known in other regions of the tropical Pacific (Griffin and Yaldwyn ; Van Soest ) as well as in the Arabian Sea (Gauns et al ).…”

Decoupling Between Phytoplankton Growth and Microzooplankton Grazing Enhances Productivity in Subantarctic Waters on Campbell Plateau, Southeast of New Zealand

“…Photoacclimation related artefacts during incubations can significantly affect pigment‐based growth rates due to uncoupling between pigment and cell growth (Brown et al, ; Selph et al, ; Worden & Binder, ), and specific measures should be adopted to quantify and correct for them (Gutierrez‐Rodriguez & Latasa, ; Landry, ). The photoacclimation response estimated in our study (mean ± sd (range), Phi nut = −0.050 ± 0.092, (−0.220–0.100) and Phi nonut = −0.057 ± 0.092, (−0.200–0.100)) was modest (~5% change in cell pigment content during the incubations) compared to other studies (Brown et al, ; Gutiérrez‐Rodríguez et al, ; Selph et al, ), suggesting that the mean daily light exposure in the deck experiments was consistent with the in situ mixed‐layer averaged irradiance (Latasa et al, ). Further support for the applied photoacclimation correction comes from the fact that the Phi factor obtained across experiments varied with MLD in a way that was consistent with our current understanding of photoacclimation dynamics (e.g., cell pigment decrease, and higher Phi, in experiments conducted with communities collected from deeper mixed layers and vice versa) (Figure S3).…”

Nitrogen and Isotope Flows Through the Costa Rica Dome Upwelling Ecosystem: The Crucial Mesozooplankton Role in Export Flux

“…Cruise measurements included systematic sampling of the planktonic community from primary producers to mesozooplankton consumers (Table ). Net primary production was measured daily by H 14 CO 3 − uptake in triplicate samples incubated in situ at eight depths spanning the euphotic zone (Selph et al, ). Taxon‐specific rates of phytoplankton growth and protozoan grazing were also quantified at eight depths in situ by two‐point dilution experiments and subsequent sample analyses by high‐pressure liquid chromatography (HPLC) for nanophytoplankton and microphytoplankton taxa and flow cytometry for picophytoplankton (Selph et al, ).…”

“…Net primary production was measured daily by H 14 CO 3 − uptake in triplicate samples incubated in situ at eight depths spanning the euphotic zone (Selph et al, ). Taxon‐specific rates of phytoplankton growth and protozoan grazing were also quantified at eight depths in situ by two‐point dilution experiments and subsequent sample analyses by high‐pressure liquid chromatography (HPLC) for nanophytoplankton and microphytoplankton taxa and flow cytometry for picophytoplankton (Selph et al, ). Instantaneous estimates of growth and grazing rates were converted to carbon‐based production and loss rates using standard cell‐biomass conversion rates for picophytoplankton and epifluorescence microscopy‐derived biomass estimates for the larger taxa (Landry, Selph, et al, ; Taylor et al, ).…”