Author's personal copy The transient oasis: Nutrient-phytoplankton dynamics and particle export in Hawaiian lee cyclones Yoshimi M. Rii a,Ã , Susan L. Brown a , Francesco Nencioli b , Victor Kuwahara b , Tommy Dickey b , David M. Karl a , Robert R. Bidigare c a Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawai’i, Honolulu, HI 96822, USA b Ocean Physics Laboratory, University of California Santa Barbara, Santa Barbara, CA 93117, USA c Hawai’i Institute of Marine Biology, Kaneohe, HI 96744, USA article info Article history: Accepted 25 January 2008 Available online 15 May 2008 Keywords: Mesoscale eddies Phytoplankton Macronutrients Photosynthetic pigments Particle export North Pacific Ocean abstract Macronutrients, photosynthetic pigments, and particle export were assessed in two eddies during the E-Flux I and III cruises to investigate linkages between biogeochemical properties and export flux in Hawaiian lee cyclonic eddies. Cyclone Noah (E-Flux I), speculated to be in the ‘decay’ stage, exhibited modest increases in macronutrients and photosynthetic pigments at the eddy center compared to ambient waters. Cyclone Opal (E-Flux III) also exhibited modest increases in macronutrient concentrations, but a 2-fold enhancement in total chlorophyll a (TChl a) concentration within the eddy center. As indicated by fucoxanthin concentrations, the phytoplankton community in the deep chlorophyll maximum (DCM) of Opal was comprised mainly of diatoms. During an 8-day time series in the center of Opal, TChl a concentration and fucoxanthin in the DCM decreased by 50%, which was potentially triggered by silicic acid limitation. Despite the presence of a substantial diatom bloom, Opal did not deliver the expected export of particulate carbon and nitrogen, but rather a large biogenic silica export (4-fold increase relative to export in surrounding waters). Results suggest that controls on the life cycle of a Hawaiian lee cyclone are likely a combination of physical (eddy dynamics), chemical (nutrient limitation), and biological (growth and grazing imbalance) processes. Comparisons between Noah and Opal and previously studied cyclones in the region point to a relationship between the spin-up duration of a cyclone and the resulting biological response. Nonetheless, Hawaiian lee cyclones, which strongly influence the biogeochemistry of areas 100’s of km in scale in the subtropical North Pacific Ocean, still remain an enigma. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction Phytoplankton community structure in the Subtropical North Pacific Ocean (SNPO) is primarily influenced by the lack of inorganic macronutrient availability (nitrogen, N; phosphorus, P; and silicon, Si) in the upper euphotic zone. Minute organisms with high surface area-to-volume ratios, such as photosynthetic bacteria (i.e. Prochlorococcus spp.) and picophytoplankton (0.2–2 mm), are efficient at nutrient uptake and light harvesting and comprise the ‘climax community’ in the SNPO (Clements, 1916; Campbell and Vaulot, 1993; Anderson et al., 1996; Karl et al., 2001a). In a two-layer distribution typical of the SNPO, cyano- bacteria (i.e. Synechococcus and Prochlorococcus spp.) dominate the mixed layer (p50 m) and Prochlorococcus spp. and photo- synthetic pico- and nanoeukaryotes (i.e. prymnesiophytes and pelagophytes) comprise the deep chlorophyll maximum (DCM, 90–110 m) (Bidigare et al., 1990, 2008; Ondrusek et al., 1991; Letelier et al., 1993). A standing stock of smaller phytoplankton suggest minimal particulate organic matter flux in the SNPO, since large eukaryotic phytoplankton are often reported to have a critical role in export production (Eppley, 1969; Goldman, 1993; Legendre and Le Fevre, 1995). Additionally, since organic matter production is controlled by the nutrient available in the lowest concentration relative to the needs of phytoplankton growth (Law of the Minimum, Justus von Liebig), lack of organic matter export in the SNPO has been attributed to N or P deficiencies in the euphotic zone (Karl et al., 2001a, b; Karl, 2002) and/or high rates of respiration (Laws et al., 2000). The input of fixed N into the nutrient-limited surface waters of the SNPO via the processes of nitrogen fixation and nitrification plays an important role in the N:P stoichiometry of available nutrient pools in the system. Due to varying supply rates, the SNPO community is reported to alternate between states of N and P limitation via El Nin ˜ o-Southern Oscillation 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.dsr2.2008.01.013 Ã Corresponding author. Tel.: +1808 956 7632; fax: +1808 956 5308. E-mail address: shimi@hawaii.edu (Y.M. Rii). Deep-Sea Research II 55 (2008) 1275– 1290