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Selph Et Al-2021.Pdf Journal of Plankton Research academic.oup.com/plankt Downloaded from https://academic.oup.com/plankt/advance-article/doi/10.1093/plankt/fbab006/6161505 by Florida State University Law Library user on 10 March 2021 J. Plankton Res. (2021) 1–20. doi:10.1093/plankt/fbab006 BLOOFINZ - Gulf of Mexico Phytoplankton community composition and biomass in the oligotrophic Gulf of Mexico KAREN E. SELPH1,*, RASMUS SWALETHORP2, MICHAEL R. STUKEL3,THOMASB.KELLY3, ANGELA N. KNAPP3, KELSEY FLEMING2, TABITHA HERNANDEZ2 AND MICHAEL R. LANDRY2 1department of oceanography, university of hawaii at manoa, 1000 pope road, honolulu, hi 96822, usa, 2scripps institution of oceanography, 9500 gilman drive, la jolla, ca 92093-0227, usa and 3department of earth, ocean and atmospheric science, florida state university, tallahassee, fl 32306, usa *corresponding author: [email protected] Received October 13, 2020; editorial decision January 10, 2021; accepted January 10, 2021 Corresponding editor: Lisa Campbell Biomass and composition of the phytoplankton community were investigated in the deep-water Gulf of Mexico (GoM) at the edges of Loop Current anticyclonic eddies during May 2017 and May 2018. Using flow cytometry, high-performance liquid chromatography pigments and microscopy, we found euphotic zone integrated chlorophyll aof ∼10 mg m−2 and autotrophic carbon ranging from 463 to 1268 mg m−2, dominated by picoplankton (<2μm cells). Phytoplankton assemblages were similar to the mean composition at the Bermuda Atlantic Time-series Study site, but differed from the Hawaii Ocean Times-series site. GoM phytoplankton biomass was ∼2-fold higher at the deep chlorophyll maximum (DCM) relative to the mixed layer (ML). Prochlorococcus and prymnesiophytes were the dominant taxa throughout the euphotic zone; however, other eukaryotic taxa had significant biomass in the DCM. Shallower DCMs were correlated with more prymnesiophytes and prasinophytes (Type 3) and reduced Prochlorococcus. These trends in ML and DCM taxonomic composition likely reflect relative nutrient supply—with ML populations relying on remineralized ammonium as a nitrogen source, and the taxonomically diverse DCM populations using more nitrate. These spatially separated phytoplankton communities represent different pathways for primary production, with a dominance of picoplankton in the ML and more nano- and microplankton at the DCM. KEYWORDS: phytoplankton; Gulf of Mexico; oligotrophic; Prochlorococcus; prymnesiophytes available online at academic.oup.com/plankt © The Author(s) 2021. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected]. JOURNAL OF PLANKTON RESEARCH VOLUME 00 NUMBER 00 PAGES 1–20 2021 INTRODUCTION used a combination of flow cytometry, microscopy and high performance liquid chromatography (HPLC) pig- Downloaded from https://academic.oup.com/plankt/advance-article/doi/10.1093/plankt/fbab006/6161505 by Florida State University Law Library user on 10 March 2021 Phytoplankton community structure directly impacts ment analyses on two expeditions conducted in May of upper trophic level organisms and export processes, and consecutive years, during the peak time of ABT spawning. hence ecosystem function (Finkel et al., 2010; Marañón, We assess biomass, composition and size structure of the 2019; Michaels and Silver, 1988; Mouw et al., 2016). phytoplankton community in profiles that covered the While phytoplankton communities are well studied depth range of the euphotic zone and thus also compare in coastal and shelf areas of the northern Gulf of assemblages, carbon (C) biomass and C:Chl ratios in the Mexico (GoM), especially in areas influenced by the mixed layer (ML) and the deep chlorophyll maximum Mississippi River Outflow and plume (Chakraborty and (DCM). While we found few obvious correlations between Lohrenz, 2015; Jochem, 2003; Liu et al., 2004; Qian et al., ABT larvae and phytoplankton community properties, we 2003; Wawrik and Paul, 2004), most studies beyond the compare our GoM sites to Hawaii Ocean Time-series shelf break have been based on satellite observations (HOT) and Bermuda Atlantic Times-series Study (BATS) (Hidalgo-González et al., 2005; Lindo-Atichati et al., 2012; sites as all are nitrogen-limited, oligotrophic, land-remote, Müller-Karger et al., 1991; Teo et al., 2007). The few warm, deep-water picoplankton-dominated systems. exceptions included data from stations on the continental shelf/slope edge (Chakraborty and Lohrenz, 2015; Qian et al., 2003) or used methods that gave an incomplete picture of composition and biomass of the community METHOD of the deep euphotic zone (Easson and Lopez, 2019; Williams et al., 2015). Since the oceanic GoM habitat is Sampling important for commercially important and endangered Two research expeditions were conducted aboard the fish species, filling this knowledge gap is essential for NOAA Ship Nancy Foster in May 2017 (NF1704) and informed ecosystem-based management. May 2018 (NF1802) during the peak month of ABT Oligotrophic regions like the northern and central larval abundance (Lindo-Atichati et al., 2012; Muhling GoM are typically dominated by cyanobacteria and et al., 2013; Teo et al., 2007). Station locations were chosen picoeukaryotes, i.e. cells ≤ 2 μm in diameter, (Agawin based on habitat characteristics known to be associated et al., 2000; Buitenhuis et al., 2012; Goericke, 1998; with ABT larvae (Domingues et al., 2016; Muhling Pasulka et al., 2013), implying inefficient food webs et al., 2013), detailed by Gerard et al. (2021). Once a (Buitenhuis et al., 2012; Flombaum et al., 2020). However, suitable location was found, we initiated a multi-day the substantial mesoscale variability within the GoM experiment, hereafter called cycle (Landry et al., 2009), could lead to other food-web characteristics. One consisting of daily intensive sampling with CTD profiles important impact on GoM productivity is the Loop and net tows, in situ incubation experiments, and a Current (Biggs and Ressler, 2001),whichisformedas sediment trap deployment, following a satellite-tracked the Caribbean Current and sheds westward-propagating drift array. Table I lists the abbreviations for variables mesoscale eddies as it moves northward from the Yucatan referred to in this study. Channel (Elliott, 1982; Leipper, 1970; Vukovich, 2007). Samples for most phytoplankton-related parameters Studies of spawning Atlantic bluefin tuna (ABT) show were collected pre-dawn from Niskin bottles mounted that their larvae are associated with the edges of on a 24-place rosette system equipped with a Seabird anticyclonic eddies that spin offfrom the Loop Current SBE911 CTD and a Seapoint fluorometer. Fluorescence (Lindo-Atichati et al., 2012; Muhling et al., 2013; Teo data (volts) from the fluorometer was compared with dis- et al., 2007). While anticyclonic eddies are downwelling crete bottle data analyzed with HPLC (Pigments section) features, their edges have been shown to be associated to convert the voltage data to total chlorophyll a (T CHLa) with higher production, depending upon feature age equivalents for the purpose of obtaining T CHLa inte- (Biggs, 1992; Salas-de-León et al., 2004; Wang et al., grals deeper than the bottle data allowed. 2018). They could therefore represent more favorable Same-day noon CTD casts were also conducted to local habitats for larval fish feeding and development, collect Trichodesmium (Microscopy section) and to deter- implying different phytoplankton assemblages and food- mine in situ light levels (%incident light or %I0) of pho- web characteristics than expected for oligotrophic oceanic tosynthetically active radiation (PAR) at the depths of waters. water collection on the pre-dawn cast. The PAR sensor, a The goal of the present study was to characterize Biospherical Instruments QSP-2300, was mounted on the the phytoplankton community associated with these eddy top of the CTD rosette frame. Five cycles were completed; edgesintheGoMatthetimeofABTspawning.We however, the PAR sensor was inoperable during Cycle 2 2 K. E. SELPH ET AL. PHYTOPLANKTON COMMUNITY COMPOSITION Table I: Abbreviations used for variables referred to in this study Downloaded from https://academic.oup.com/plankt/advance-article/doi/10.1093/plankt/fbab006/6161505 by Florida State University Law Library user on 10 March 2021 ABT: Atlanticnbluefin tuna ML: mixed layer AC: autotrophic carbon, includes cyanobacteria and eukaryotic MVCHLa: monovinyl chlorophyll a phytoplankton A-DINO: dinoflagellates MVCHLb: monvinyl chlorophyll b A-EUK: autotrophic eukaryotes NEO: neoxanthin ALLO: allophycocyanin NCF: normalized chlorophyll fluorescence BATS: Bermuda Atlantic Times-series Study NF1704: May 2017 expedition BUT: 19-but-fucoxanthin NF1802: May 2018 expedition C#: Cycle number, C1-C3 were during NF1704; C4-C5 were during NPP: Net Primary Production NF1802 C:CHL: carbon:chlorophyll (μg C L−1:μgCHLL−1) PAR: Photosynthetically Active Radiation CHL: total chlorophyll a, sum of MVCHLa and DVCHLa PELAG: pelagophytes CHLc3: chlorophyll c3 PER: peridinin CHLOR: chlorophytes PEUK: mainly ≤2 μm eukaryotic phytoplankton from flow cytometry CRYPT: cryptophytes PRAS3: prasinophytes—type 3 DCM: deep chlorophyll maximum PRAS: prasinoxanthin DIAT: diatoms PRO: Prochlorococcus DVCHLa: divinyl chlorophyll a PRYM: prymnesiophytes—type 6 FCM: flow cytometry SYN: Synechococcus FUCO: fucoxanthin T CHLa: total chlorophyll a (sum of MVCHLa and DVCHLa) HEX: 19-hex-fucoxanthin T CHLb: total chlorophyll b (sum of MVCHLb and DVCHLb) HOT: Hawaii Ocean
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