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Picoplankton dynamics duringcontrasting seasonal oceanographicconditions at a coastal upwelling stationoff Northern Baja California, Mexico

LORENA P. LINACRE1*, MICHAEL R. LANDRY2, J. RUBEN LARA-LARA3, J. MARTIN HERNANDEZ-AYON4

AND CARMEN BAZAN-GUZMAN31

PROGRAMA DE DOCTORADO EN OCEANOGRAFIA COSTERA, FACULTAD DE CIENCIAS MARINAS/INSTITUTO DE INVESTIGACIONES OCEANOLOGICAS, UNIVERSIDADAUTONOMA DE BAJA CALIFORNIA (UABC), ENSENADA, BAJA CALIFORNIA, MEXICO, 2INTEGRATIVE OCEANOGRAPHY DIVISION, SCRIPPS INSTITUTION OFOCEANOGRAPHY, LA JOLLA, CA, USA, 3DEPARTAMENTO DE OCEANOGRAFIA BIOLOGICA, CENTRO DE INVESTIGACION CIENTIFICA Y DE EDUCACION SUPERIOR DEENSENADA, ENSENADA, BAJA CALIFORNIA, MEXICO AND 4INSTITUTO DE INVESTIGACIONES OCEANOLOGICAS, UABC, ENSENADA, BAJA CALIFORNIA, MEXICO

*CORRESPONDING AUTHOR: llinacre@uabc.mx; lorenalinacre@gmail.com

Received October 15, 2009; accepted in principle December 18, 2009; accepted for publication December 23, 2009

Corresponding editor: William K.W. Li

The ecological dynamics of picoplankton were investigated at a coastal upwellingsystem of northern Baja California during six cruises (September 2007November2008). Populations of Prochlorococcus, Synechococcus, PicoEukaryotes and heterotrophicbacteria were assessed by flow cytometry (FCM). On each sampling date, we usedan abbreviated three-treatment dilution technique and 14C-uptake experiments todetermine population (FCM) and community (TChl a) rates of growth, grazingand production from 24-h in situ incubations at three to four euphotic depths.Overall, picoplankton comprised an active and important component of thecommunity, with biomass values (2.369.8 mg C L21) and production rates(0.868.4mg C L21 day21) that varied positively with Chl a and community14C-production. The exception was an intense algal bloom (.25 mg Chl a L21)during La Nina-intensified upwelling conditions in April 2008, during whichbiomass and production estimates of picophytoplankton were at their lowest levels,suggesting that the smallest primary producers were being replaced by larger cells.Thus, for most of the environmental circumstances encountered during our study,our results supported the recent rising tide hypothesis that improved growth(nutrient) conditions benefit all size classes, including picophytoplankton. Underextreme conditions of upwelling, however, the picophytoplankton declinedabruptly, despite seemingly strong (average) growth rates. Future studies needto provide a better mechanistic understanding of the physical (advection),physiological (nutrient uptake and temperature) and ecological (food web) factorsthat result in this dramatic nonlinearity in picophytoplankton response to systemforcing and richness.

KEYWORDS: picoplankton dynamics; dilution method; growth rate; grazingrate; coastal upwelling system

doi:10.1093/plankt/fbp148, available online at www.plankt.oxfordjournals.org. Advance Access publication January 27, 2010

# The Author 2010. Published by Oxford University Press. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

JOURNAL OF PLANKTON RESEARCH j VOLUME 32 j NUMBER 4 j PAGES 539557 j 2010

I N T RO D U C T I O N

The coastal region off western Baja California (WBC) isthe southern limit of the California Current System(CCS). The hydrography of WBC is characterized byan alongshore near-surface equatorward flow carryingrelatively cool and fresh water from the subarctic, a sub-surface poleward current flowing along the edge of thecontinental slope, and coastal upwelling episodes drivenby northerly winds during most of the year. At the sea-sonal scale, subarctic waters dominate during the peakof the upwelling season in spring and summer, whiletropical and subtropical influences are commonlyobserved during later summer and fall (Lynn andSimpson, 1987; Durazo et al., in press).

Physical and chemical characteristics of the coastalwaters from a monitoring observatory off Ensenada,Mexico (ENSENADA station), display the seasonal andinterannual characteristics derived from 11 years ofrecent oceanographic measurements in the northernregion off WBC (Linacre et al., in preparation), rangingfrom strong seasonal upwelling that produces densealgal blooms in the euphotic layer to periods of strongstratification and oligotrophy. Seasonal climatologicalmeans from ENSENADA station demonstrate that thiscoastal site is representative of a broader area (Linacreet al., in preparation). Consequently, it is a convenientsite for the study of plankton community and pro-duction responses to strongly variable conditionsthroughout this dynamic coastal ecosystem.

Classical and microbial pathways are a useful dichot-omy for distinguishing among the alternate fates ofprimary production carbon in marine ecosystems(Calbet and Landry, 2004). In highly productive coastalupwelling areas, it has long been assumed that carbonproduction via chain-forming diatoms is either effi-ciently transferred to higher trophic levels through theclassical food web (Ryther, 1969) or exported from theeuphotic zone as fecal pellets, detritus or by sedimen-tation of marine snow aggregates (Turner, 2002).However, there is increasing evidence that the microbialcomponents of food webs are an ubiquitous and impor-tant feature of not only oligotrophic but also eutrophicsystems, including seasonally variable coastal upwellingareas (Cuevas et al., 2004; Worden et al., 2004; Vargaset al., 2007). Most productive and seasonal systemsinvolve multivorous food webs, where both classical andmicrobial trophic components play significant roles(Legendre and Rassoulzadegan, 1995). It has also beenrecently noted that the onset of favorable growth con-ditions for diatom-dominated blooms does not necess-arily lead, as often assumed, to the successionalreplacement of the smaller cells that dominate during

less productive times (Barber and Hiscock, 2006). Thepicophytoplankton assemblage can respond positively ingrowth rates and biomass to improved growth con-ditions, although its biomass increase is often modestcompared with larger phytoplankton, which escapecontrol by protistan grazers via the loophole bloom-ing mechanism (Irigoien et al., 2005; Barber andHiscock, 2006).

Heterotrophic protists (here defined as nano- tomicro-sized grazers ,200 mm) play an important rolein pelagic food webs, where they are a major source ofmortality for small and large primary producers, as wellas heterotrophic bacteria (Sherr and Sherr, 2002).Globally, grazing impact by protists has been estimatedto consume two-thirds of phytoplankton daily growth(production), with moderate variations among marinehabitats and regions (Calbet and Landry, 2004). Incoastal environments, protistan grazing accounts for60% of daily primary production on average,although this value appears to vary dynamically withseasonality and the state of the phytoplankton bloomand bust cycles (Neuer and Cowles, 1994; Bottjer andMorales, 2005; McManus et al., 2007). Similar to proti-stan consumption of autotrophs, strong grazing pressureis exerted on heterotrophic bacteria mainly by nanofla-gellates. Removal of .100% of daily bacterial pro-duction has been reported in the coastal upwellingareas of the Humboldt Current System, mostly duringnon-upwelling seasons (Cuevas et al., 2004; Vargas et al.,2007). Since bacterial growth is ultimately supported byautotrophic sources of DOC, even when the seasons ofproduction and utilization are temporally separate, asignificant fraction of annual primary production ischanneled through bacteria to heterotrophic nanoflagel-lates, reflecting the importance of microbial food websin carbon cycling in coastal upwelling systems (Cuevaset al., 2004; Vargas et al., 2007).

The aim of the present study was to assess the tem-poral dynamics of phytoplankton and bacteria in thecoastal upwelling system off northern WBC, focusingon abundance, biomass, growth, grazing mortality andproduction of autotrophic and heterotrophic picoplank-ton populations under contrasting seasonal oceano-graphic conditions.

M E T H O D

In situ dilution experiments

Experimental studies of phytoplankton growth and pro-tistan grazing were conducted at the ENSENADAstation (31840.1050N, 116 841.5960W) at the northern

JOURNAL OF PLANKTON RESEARCH j VOLUME 32 j NUMBER 4 j PAGES 539557 j 2010

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region off WBC (Fig. 1), as part of the FLUCAR (CarbonSources and Sinks in the Continental Margins of the MexicanPacific Waters) project. During the period from 24September 2007 to 11 November 2008, six sets ofexperiments were incubated in situ for 24 h at a fixedcoastal station, following the experimental approach ofLandry et al. (Landry et al., 2008). Routine station activi-ties included CTD/rosette casts to 100 m depth withcontinuous measurements of pressure, temperature,conductivity, dissolved oxygen, chlorophyll fluorescenceand PAR light, as well as seawater collection with 5-LNiskin bottles for flow cytometric (FCM) and nutrientanalyses (NO3

2NO22, Si(OH)4 and PO432) at 10depths which were variable among cruises. Nutrientanalyses were performed with a Skalar SANplus

segmented-flow nutrient analyzer; NO32NO22 deter-

mination was based on a modification of the Armstronget al. (Armstrong et al., 1967) procedure. Seawater forthe experiments was also collected with 5-L Niskinbottles at variable depths among cruises within theeuphotic zone (from 56 to 0.3% of average PAR lightfrom the first meter, %Io). Based on the in situ fluor-escence profiles, the collection depths were at the deepchlorophyll maximum (DCM) and one level above andone level below the DCM. For each set of experiments,the treatments were prepared in clear polycarbonatebottles (2.0 L) with 100 (undiluted), 30 and 10% ofwhole seawater (diluted with 0.1-mm filtered

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