Depth-stratified phytoplankton dynamics in Cyclone Opal, a subtropical mesoscale eddy Michael R. Landry a,Ã , Susan L. Brown b , Yoshimi M. Rii b , Karen E. Selph b , Robert R. Bidigare b,c , Eun Jin Yang d , Melinda P. Simmons e a Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0227, USA b Department of Oceanography, University of Hawaii at Manoa, Honolulu, HI 96822, USA c Hawaii Institute of Marine Biology, University of Hawaii at Manoa, Honolulu, HI 96822, USA d Marine Environment Research Department, Korea Ocean Research & Development Institute, Ansan P.O. Box 29, Seoul 425-600, South Korea e Gordon and Betty Moore Foundation, San Francisco, CA 94129-0910, USA article info Article history: Accepted 26 February 2008 Available online 5 May 2008 Keywords: Physical–biological coupling Prochlorococcus Diatom bloom Microzooplankton grazing Growth rate abstract As part of E-Flux III cruise studies in March 2005, we investigated phytoplankton community dynamics in a cyclonic cold-core eddy (Cyclone Opal) in the lee of the Hawaiian Islands. Experimental incubations were conducted under in situ temperature and light conditions on a drift array using a two-treatment dilution technique. Taxon-specific estimates of growth, grazing and production rates were obtained from analyses of incubation results based on phytoplankton pigments, flow cytometry and microscopy. Cyclone Opal was sampled at a biologically and physically mature state, with an 80–100 m doming of isopycnal surfaces in its central region and a deep biomass maximum of large diatoms. Depth-profile experimentation defined three main zones. The upper (mixed) zone (0–40 m), showed little compositional or biomass response to eddy nutrient enrichment, but growth, grazing and production rates were significantly enhanced in this layer relative to the ambient community outside of the eddy. Prochlorococcus spp. dominated the upper mixed layer, accounting for 50–60% of its estimated primary production both inside and outside of Opal. In contrast, the deep zone of 70–90 m showed little evidence of growth rate enhancement and was principally defined by a 100-fold increase of large (420-mm) diatoms and a shift from Prochlorococcus to diatom dominance (80%) of production. The intermediate layer of 50–60 m marked the transition between the upper and lower extremes but also contained an elevated biomass of physiologically unhealthy diatoms with significantly depressed growth rates and proportionately greater grazing losses relative to diatoms above or below. Microzooplankton grazers consumed 58%, 65% and 55%, respectively, of the production of diatoms, Prochlorococcus and the total phytoplankton community in Cyclone Opal. The substantial grazing impact on diatoms suggests that efficient recycling was the major primary fate of diatom organic production, consistent with the low export fluxes and selective export of biogenic silica, as empty diatom frustules, in Cyclone Opal. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction Although general understanding of open-ocean biogeochem- istry and ecology has been greatly advanced by systematic study over the past two decades, there are still major issues—e.g., discrepancies in net system auto- or heterotrophy and nutrient sources for new production—that seem to defy explanation from mean system measurements (e.g., Michaels et al., 1996; McGilli- cuddy et al., 1998; Karl et al., 2003). Mesoscale eddies, as well as waves and fronts of various kinds, have therefore emerged as mechanisms of interest for explaining pulsed nutrient delivery, cryptic production and flux anomalies at scales that are difficult to extract from classic sampling designs (e.g., McGillicuddy et al., 1999; Siegel et al., 1999; Sakamoto et al., 2004). Eddies, in particular, are common and often long-lived features of ocean circulation with demonstrable, though highly variable, impacts on phytoplankton production, biomass and community structure (Allen et al., 1996; Froneman and Perissinotto, 1996; Tarran et al., 2001; Bidigare et al., 2003; Vaillancourt et al., 2003). Despite these general findings, however, we know very little about the ecology of eddies as perturbed ecosystems, or the comparative dynamics of component populations within their water columns. The present study was designed to investigate the depth- stratified responses of a subtropical phytoplankton community to strong nutrient perturbation in a cold-core, cyclonic eddy. In effect, the eddy was viewed as a large, relatively well-contained ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2008.02.001 Ã Corresponding author. Tel.: +1858 534 4702; fax: +1858 534 6500. E-mail address: mlandry@ucsd.edu (M.R. Landry). Deep-Sea Research II 55 (2008) 1348– 1359