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2018
Valencia, B, Decima M, Landry MR.  2018.  Environmental Effects on Mesozooplankton Size Structure and Export Flux at Station ALOHA, North Pacific Subtropical Gyre. Global Biogeochemical Cycles. 32:289-305.   10.1002/2017gb005785   AbstractWebsite

Using size-fractionated mesozooplankton biomass data collected over 23 years (1994-2016) of increasing primary production (PP) at station ALOHA (A Long-Term Oligotrophic Habitat Assessment), we evaluate how changing environmental conditions affect mesozooplankton size structure, trophic cycling, and export fluxes in the subtropical North Pacific. From generalized additive model analysis, size structure is significantly influenced by a nonlinear relationship with sea surface temperature that is mainly driven by the strong 1997-1998 El Nino and a positive and linear relationship with PP. Increasing PP has more strongly enhanced the biomass of smaller (0.2-0.5 mm) and larger (>5 mm) mesozooplankton, increasing evenness of the biomass spectra, while animals of 2-5 mm, the major size class for vertically migrating mesozooplankton, show no long-term trend. Measured PP is sufficient to meet feeding requirements that satisfy mesozooplankton respiration and growth rates, as determined by commonly used empirical relationships based on animal size and temperature, consistent with a tightly coupled food web with one intermediate level for protistan consumers. Estimated fecal pellet production suggests an enhanced contribution of mesozooplankton to passive particle export relative to the material collected in 150 m sediment traps. In contrast, the biomass of vertically migrants does not vary systematically with PP due to the varying responses of the different size classes. These results illustrate some complexities in understanding how varying environmental conditions can affect carbon cycling and export processes at the community level in open-ocean oligotrophic systems, which need to be confirmed and better understood by process-oriented mechanistic study.

2017
Steinberg, DK, Landry MR.  2017.  Zooplankton and the ocean carbon cycle. Annual Review of Marine Sciences, Vol 9. 9:413-444., Palo Alto: Annual Reviews   10.1146/annurev-marine-010814-015924   Abstract

Marine zooplankton comprise a phylogenetically and functionally diverse assemblage of protistan and metazoan consumers that occupy multiple trophic levels in pelagic food webs. Within this complex network, carbon flows via alternative zooplankton pathways drive temporal and spatial variability in production-grazing coupling, nutrient cycling, export, and transfer efficiency to higher trophic levels. We explore current knowledge of the processing of zooplankton food ingestion by absorption, egestion, respiration, excretion, and growth (production) processes. On a global scale, carbon fluxes are reasonably constrained by the grazing impact of microzooplankton and the respiratory requirements of mesozooplankton but are sensitive to uncertainties in trophic structure. The relative importance, combined magnitude, and efficiency of export mechanisms (mucous feeding webs, fecal pellets, molts, carcasses, and vertical migrations) likewise reflect regional variability in community structure. Climate change is expected to broadly alter carbon cycling by zooplankton and to have direct impacts on key species.

2016
Goes, JI, Gomes HD, Selph KE, Landry MR.  2016.  Biological response of Costa Rica Dome phytoplankton to Light, Silicic acid and Trace metals. Journal of Plankton Research. 38:290-304.   10.1093/plankt/fbv108   AbstractWebsite

The Costa Rica Dome (CRD) is a unique open-ocean upwelling system, with picophytoplankton dominance of phytoplankton biomass and suppressed diatoms, yet paradoxically high export of biogenic silica. As a part of Flux and Zinc Experiments cruise in summer (June-July 2010), we conducted shipboard incubation experiments in the CRD to examine the potential roles of Si, Zn, Fe and light as regulating factors of phytoplankton biomass and community structure. Estimates of photosynthetic quantum yields revealed an extremely stressed phytoplankton population that responded positively to additions of silicic acid, iron and zinc and higher light conditions. Size-fractioned Chl a yielded the surprising result that picophytoplankton, as well as larger phytoplankton, responded most to treatments with added silicic acid incubated at high incident light (HL + Si). The combination of Si and HL also led to increases in cell sizes of picoplankton, notably in Synechococcus. Such a response, coupled with the recent discovery of significant intracellular accumulation of Si in some picophytoplankton, suggests that small phytoplankton could play a potentially important role in Si cycling in the CRD, which may help to explain its peculiar export characteristics.

Freibott, A, Taylor AG, Selph KE, Liu HB, Zhang WC, Landry MR.  2016.  Biomass and composition of protistan grazers and heterotrophic bacteria in the Costa Rica Dome during summer 2010. Journal of Plankton Research. 38:230-243.   10.1093/plankt/fbv107   AbstractWebsite

We investigated biomass and composition of heterotrophic microbes in the Costa Rica Dome during June-July 2010 as part of a broader study of plankton trophic dynamics. Because picophytoplankton (, 2 mm) are known to dominate in this unique upwelling region, we hypothesized tight biomass relationships between size-determined predator-prey pairs (i.e. picoplankton-nano-grazers, nanoplankton-micro-grazers) within the microbial community. Integrated biomass of heterotrophic bacteria ranged from 180 to 487 mg C m(-2) and was significantly correlated with total autotrophic carbon. Heterotrophic protist (H-protist) biomass ranged more narrowly from 488 to 545 mg C m(-2), and was comprised of 60% dinoflagellates, 30% other flagellates and 11% ciliates. Nano-sized (<20 mu m) protists accounted for the majority (57%) of grazer biomass and were positively correlated with picoplankton, partially supporting our hypothesis, but nanoplankton and micro-grazers (>20 mu m) were not significantly correlated. The relative constancy of H-protist biomass among locations despite clear changes in integrated autotrophic biomass, Chl a, and primary production suggests that mesozooplankton may exert a tight top-down control on micro-grazers. Biomass-specific consumption rates of phytoplankton by protistan grazers suggest an instantaneous growth rate of 0.52 day(-1) for H-protists, similar to the growth rate of phytoplankton and consistent with a trophically balanced ecosystem dominated by piconanoplankton interactions.

2014
Taniguchi, DAA, Landry MR, Franks PJS, Selph KE.  2014.  Size-specific growth and grazing rates for picophytoplankton in coastal and oceanic regions of the eastern Pacific. Marine Ecology Progress Series. 509:87-101.   10.3354/meps10895   AbstractWebsite

Estimates of growth and grazing mortality rates for different size classes and taxa of natural picophytoplankton assemblages were measured in mixed-layer experiments conducted in 3 regions of the eastern Pacific: the California Current Ecosystem, Costa Rica Dome, and equatorial Pacific. Contrary to expectation, size-dependent rates for cells between 0.45 and 4.0 mu m in diameter showed no systematic trends with cell size both in and among regions. For all size classes, mean +/- SD growth rates ranged from -0.70 +/- 0.17 to 0.83 +/- 0.13 d(-1) and grazing rates between -0.07 +/- 0.13 and 1.17 +/- 0.10 d(-1). Taxon-specific growth rates for Prochlorococcus ranged from 0.17 +/- 0.12 to 0.59 +/- 0.01 d(-1), for Synechococcus from 0.68 +/- 0.03 to 0.97 +/- 0.04 d(-1), for picoeukaryotes from 0.46 +/- 0.13 to 1.03 +/- 0.06 d(-1), and for all cells combined between 0.45 +/- 0.03 and 0.65 +/- 0.02 d(-1). For grazing, Prochlorococcus rates ranged between 0.02 +/- 0.12 and 0.66 +/- 0.02 d(-1), Synechococcus rates between 0.24 +/- 0.08 and 0.92 +/- 0.05 d(-1), for picoeukaryotes between 0.19 +/- 0.10 and 0.78 +/- 0.09 d(-1), and for all cells between 0.16 +/- 0.05 and 0.75 +/- 0.02 d(-1). When comparing rates among taxa, only Prochlorococcus had consistently lower rates than Synechococcocus in all regions. No other trends were apparent. Temperature relationships based on the Metabolic Theory of Ecology were able to explain more of the variability among grazing rates than among growth rates for each taxon considered.

2013
Stukel, MR, Decima M, Selph KE, Taniguchi DAA, Landry MR.  2013.  The role of Synechococcus in vertical flux in the Costa Rica upwelling dome. Progress in Oceanography. 112:49-59.   10.1016/j.pocean.2013.04.003   AbstractWebsite

Despite evidence that picophytoplankton contribute to export from marine pelagic ecosystems to some extent, few field studies have experimentally evaluated the quantitative importance of that flux or specifically assessed the relative strengths of alternate ecological pathways in transporting picophytoplankton carbon to depth. In experimental studies in the Costa Rica Dome (CRD), we used a combination of methods - flow cytometry (FCM), microscopy, pigments, dilution assays, mesozooplanton gut contents and sediment traps - to follow production, grazing and export fates of the dominant picophytoplankter, Synechococcus spp. (Syn), relative to the total phytoplankton community. Syn accounted for an average of 25% (range 9-50%) of total phytoplankton production during four 4-day drifter experiments at CRD sites. During the same experiments, sediment trap deployments at the base of the euphotic zone measured total organic carbon export ranging from 50 to 72 mg C m(-2) d(-1). Flow cytometry measurements of the trap samples showed that only 0.11% of this carbon was recognizable as ungrazed sinking Syn. Phycoerythrin (PE) measurements on the same samples, which we attributed mostly to transport of intact cells in mesozooplankton fecal pellets, gave export contributions of unassimilated Syn eight-times higher than ungrazed sinking cells, though still <1% of total carbon. Grazing of mesozooplankton on Syn was confirmed by PE measurements of mesozooplankton guts and the visual presence of Syn cells in fecal pellets. Microzooplankton grazing estimates from dilution experiments, combined with degradation rates of mesozooplankton fecal material in the water column, allowed us to estimate indirectly the additional flux of carbon transferred through protozoan grazers before being exported as mesozooplankton fecal pellets. Assuming one to three protozoan trophic steps, this Syn pathway contributed on average an additional 0.5-5.7% of organic carbon flux. A similar budget for total phytoplankton, based on chlorophyll a and phaeopigments was consistent with fecal pellets as the dominant mechanism of sinking carbon. Therefore, while Syn sinking as ungrazed cells or aggregates were minor components of export, the indirect trophic pathway involving mesozooplankton predation on protozoan consumers of Syn comprised the major mode of bulk carbon export for Syn-generated primary production. (C) 2013 Elsevier Ltd. All rights reserved.

2012
Chekalyuk, AM, Landry MR, Goericke R, Taylor AG, Hafez MA.  2012.  Laser fluorescence analysis of phytoplankton across a frontal zone in the California Current ecosystem. Journal of Plankton Research. 34:761-777.   10.1093/plankt/fbs034   AbstractWebsite

Spatial variability of chlorophyll, phycobiliproteins, chromophoric dissolved organic matter and variable fluorescence (F-v/F-m) was analyzed across a deep-water density front in the Southern California Current Ecosystem using an Advanced Laser Fluorometer (ALF) calibrated to assess chlorophyll concentration (C-chl), total autotrophic carbon (AC) and Synechococcus carbon biomass (SYN). Three distinct autotrophic assemblages were identified. Fluorescence was found to be three to four times higher in cooler mesotrophic waters north of the front than in warm oligotrophic waters to the south. Northern waters were distinguished by a shallow pigment maximum dominated by a blue-water type of Synechococcus and by the presence of green-water Synechococcus and cryptophytes; only blue-water Synechococcus were detected at lower concentration south of the front. The highest C-chl and AC values, accompanied by elevated F-v/F-m and chlorophyll fluorescence per unit of C-chl, and minimal Synechococcus abundance, were found directly at the front in a 2040 m deep layer dominated by diatoms. The covariation of F-v/F-m with nitrate concentration in this layer, along with the structural changes in the phytoplankton community, suggest that it had been generated by in situ processes rather than advection. Strong structural responses to the local hydrography were also revealed by high-frequency underway ALF surface sampling, which detected an abrupt transition from low to high SYN on the northern side of a sharp salinity gradient at the front. Synechococcus-specific phycoerythrin fluorescence (F-PE12) and SYN were highly correlated in surface waters (R-2 0.95), while F-PE12:SYN gradually increased with depth. Strong relationships were found for chlorophyll fluorescence versus C-chl (R-2 0.95) and AC (R-2 0.79).

Samo, TJ, Pedler BE, Ball GI, Pasulka AL, Taylor AG, Aluwihare LI, Azam F, Goericke R, Landry MR.  2012.  Microbial distribution and activity across a water mass frontal zone in the California Current Ecosystem. Journal of Plankton Research. 34:802-814.   10.1093/plankt/fbs048   AbstractWebsite

Ocean fronts with accumulated biomass and organic matter may be significant sites of enhanced microbial activity. We sampled a frontal region (the A-Front) separating oligotrophic and mesotrophic water masses within the California Current Ecosystem (CCE) to assess the influence of frontal hydrography on several microbial parameters. Samples for heterotrophic bacterial, viral and flagellate abundance, dissolved and particulate carbon and nitrogen, transparent particles and bacterial carbon production were collected at 6 depths from the surface to 100 m with 59 conductivity/temperature/depth casts along a 26-km northerly transect across the front. Relative to adjacent oligotrophic and mesotrophic waters, the frontal transition displayed peaks in the mean estimates of cell-specific bacterial carbon and bulk bacterial production, particulate organic carbon and particulate organic nitrogen concentrations, and the abundance and size of transparent particles. Bacterial carbon production increased approximate to 5-fold northward from oligotrophic waters to the frontal zone, in agreement with an increase in the frequency of dividing cells, but bacterial abundance was lower than at adjacent stations. This may be partially explained by high chlorophyll, elevated virus:bacteria ratios and low nanoflagellate grazer abundance at the front. Our data suggest that CCE fronts can facilitate intense biological transformation and physical transport of organic matter, in sharp contrast to adjacent low productivity waters, and harbor dynamic microbial populations that influence nutrient cycling.

Stukel, MR, Landry MR, Ohman MD, Goericke R, Samo T, Benitez-Nelson CR.  2012.  Do inverse ecosystem models accurately reconstruct plankton trophic flows? Comparing two solution methods using field data from the California Current Journal of Marine Systems. 91:20-33.   10.1016/j.jmarsys.2011.09.004   AbstractWebsite

Despite the increasing use of linear inverse modeling techniques to elucidate fluxes in undersampled marine ecosystems, the accuracy with which they estimate food web flows has not been resolved. New Markov Chain Monte Carlo (MCMC) solution methods have also called into question the biases of the commonly used L(2) minimum norm (L(2)MN) solution technique. Here, we test the abilities of MCMC and L(2)MN methods to recover field-measured ecosystem rates that are sequentially excluded from the model input. For data, we use experimental measurements from process cruises of the California Current Ecosystem (CCE-LTER) Program that include rate estimates of phytoplankton and bacterial production, micro- and mesozooplankton grazing, and carbon export from eight study sites varying from rich coastal upwelling to offshore oligotrophic conditions. Both the MCMC and L(2)MN methods predicted well-constrained rates of protozoan and mesozooplankton grazing with reasonable accuracy, but the MCMC method overestimated primary production. The MCMC method more accurately predicted the poorly constrained rate of vertical carbon export than the L(2)MN method, which consistently overestimated export. Results involving DOC and bacterial production were equivocal. Overall, when primary production is provided as model input, the MCMC method gives a robust depiction of ecosystem processes. Uncertainty in inverse ecosystem models is large and arises primarily from solution under-determinacy. We thus suggest that experimental programs focusing on food web fluxes expand the range of experimental measurements to include the nature and fate of detrital pools, which play large roles in the model. (C) 2011 Elsevier B.V. All rights reserved.

Linacre, L, Landry MR, Cajal-Medrano R, Lara-Lara JR, Hernandez-Ayon JM, Mourino-Perez RR, Garcia-Mendoza E, Bazan-Guzman C.  2012.  Temporal dynamics of carbon flow through the microbial plankton community in a coastal upwelling system off northern Baja California, Mexico. Marine Ecology-Progress Series. 461:31-46.   10.3354/meps09782   AbstractWebsite

We investigated the temporal dynamics of carbon flow through the microbial food web of a coastal upwelling system (ENSENADA station) off northern Baja California during 6 cruises (September 2007 to November 2008). Carbon biomass assessments for major autotrophic size groups (pico- to micro-sized cells) and their microzooplankton grazers were based on analyses using flow cytometry, HPLC pigments and epifluorescence microscopy. Taxon-specific phytoplankton growth and microzooplankton grazing rates were determined from 24 h in situ incubations in the euphotic zone using an abbreviated 3-treatment dilution technique. Carbon biomass and instantaneous growth and grazing rate determinations were used to estimate daily rates of taxon-specific production and losses due to microzooplankton grazing. Overall, microbial biomass showed a close balance between autotrophic and heterotrophic components, except during a period of very strong upwelling (April 2008), which favored large phytoplankters and high primary production. Throughout a wide range of environmental conditions, the community primary production (PP) attributed both to small (mostly picophytoplankton and prasinophytes) and large (mostly diatoms and autotrophic dinoflagellates) autotrophs was significantly grazed (78 +/- 9% of PP) by small (< 20 mu m) and large (> 20 mu m) ciliates and flagellates (including mixotrophic dinoflagellates), respectively, showing complementary temporal shifts in protistan grazer types that matched the dominant phytoplankton. While large diatoms were strongly consumed by large ciliates during the 2 most productive periods (September 2007 and April 2008), pico- and nano-sized phytoplankton were grazed most by nanoflagellates and small ciliates from November 2007 to January 2008. Consequently, biogenic carbon production in this ecosystem is transferred through a multivorous food web.

2011
Taylor, AG, Landry MR, Selph KE, Yang EJ.  2011.  Biomass, size structure and depth distributions of the microbial community in the eastern equatorial Pacific. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:342-357.   10.1016/j.dsr2.2010.08.017   AbstractWebsite

We investigated the biomass, size structure and composition of microbial communities over a broad area of the eastern equatorial Pacific (4 degrees N-4 degrees S, 110-140 degrees W) during cruises in December 2004 (EB04) and September 2005 (EB05). Vertical-profile samples were collected at 30 stations at depths extending from the surface to the 0.1% light level, and each sample was analyzed quantitatively by flow cytometry and epifluorescence microscopy. Autotrophic biomass averaged 14.8 +/- 4.2 (1 s.d.) mu g CL(-1) for the euphotic zone, with dinoflagellates comprising 39%, Prochlorococcus 28%, other flagellates 18%, Synechococcus 7.5%, and diatoms 6.3%. Nanoplankton accounted for 46% of autotroph biomass, while pico- and microphytoplankton comprised 39 and 16%, respectively. C:Chl averaged 64 +/- 14 for the euphotic zone, with a mean mixed-layer value of 78 +/- 20 and a minimum of 36 +/- 15 at the 1% light level. Heterotrophic biomass averaged 7.0 +/- 1.2 mu g C L(-1) for prokaryotes, 1.6 +/- 0.9 mu g CL(-1) for dinoflagellates, 1.5 +/- 1.1 mu g C L(-1) for other flagellates, and 2.1 +/- 0.4 mu g C L(-1) for ciliates. Euphotic zone integrated biomass varied 2-fold, 1.2 to 2.5 g C m(-2), among stations, decreasing west to east with the gradient in euphotic zone concentrations of dissolved iron. Overall, community biomass and the contributions of functional groups displayed remarkable constancy over our study area, but some patterns were evident, such as the enhancement of picophytoplankton in the leading (upwelling) edges of tropical instability waves and larger diatoms in the trailing (downwelling) edges. Prochlorococcus, in particular, exhibited more variability than expected, given its generally assumed role as a stable background species in the tropical oceans, and was positively associated with the areas of enhanced autotrophic carbon and Chl a. With corrections for different methodological assumptions taken into account, our EB05 estimates of mixed-layer community biomass are 27-35% higher than values for JGOFS studies in 1992. (C) 2010 Elsevier Ltd. All rights reserved.

Decima, M, Landry MR, Rykaczewski RR.  2011.  Broad scale patterns in mesozooplankton biomass and grazing in the eastern equatorial Pacific. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:387-399.   10.1016/j.dsr2.2010.08.006   AbstractWebsite

We investigated biomass distributions and grazing rates of mesozooplankton in the eastern equatorial Pacific between 110 degrees-140 degrees W and 4 degrees S-4 degrees N during cruises in December 2004 (EB04) and September 2005 (EB05). Median (+/- SE) euphotic zone estimates of zooplankton biomass, collected with a 200-mu m mesh net, varied from 2.27 +/- 0.24 g dry weight m(-2) during EB04 to 3.13 +/- 0.22 g dry weight m(-2) for EB05 (however, when stations from overlapping regions were compared, no significant differences were found between years). Trends in gut fluorescence estimates of mesozooplankton grazing followed biomass, with significantly higher median rate estimates during EB05 (3.39 +/- 0.32 mg pigment m(-2) d(-1)) than during EB04 (2.31 +/- 0.34 mg pigment m(-2) d(-1)). Spatial gradients in mesozooplankton biomass and grazing on meridional transects sampled at 110 degrees W in 2004 and 140 degrees W in 2005 could be interpreted as either in situ growth/grazing responses or downstream advective flows relative to spatial patterns in phytoplankton. The present zooplankton biomass estimates for the equatorial Pacific are 80-90% higher than those from similar measurements made by the US Joint Global Ocean Flux Studies EqPac Program in 1992. Our grazing rates similarly exceed EqPac estimates by a factor of 2 or 3, in absolute terms and as percent of phytoplankton biomass consumed daily (11% - EB04; 14% - EB05). Although the equatorial region has not been regularly sampled between EqPac and the present study, both the magnitude and the direction of the observed changes are consistent with the documented decadal increase in mesozooplankton biomass in the adjacent North Pacific Subtropical Gyre based on monthly sampling at Stn. ALOHA, as well as an increase in the strength of the trade winds. These results may be indicative of a general shift up in productivity or community size structure and role of mesozooplankton in the open-ocean tropical/subtropical Pacific, and they provide important time points for validating the performance of ecosystem models of the region. (C) 2010 Elsevier Ltd. All rights reserved.

Stukel, MR, Landry MR, Selph KE.  2011.  Nanoplankton mixotrophy in the eastern equatorial Pacific. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:378-386.   10.1016/j.dsr2.2010.08.016   AbstractWebsite

Heterotrophic bacteria, cyanobacteria, and picoeukaryotic algae dominate the plankton community of high nutrient-low chlorophyll (HNLC) areas of the eastern equatorial Pacific (EEP). While grazing on these picoplankton is often attributed to aplastidic zooflagellates, mixotrophic nanoflagellates (phagotrophic phototrophs) may also exert a large grazing pressure. We assessed the relative contributions of mixotrophic nanoplankton and obligate heterotrophs to picoplankton phagotrophy in mixed-layer water of the EEP using 0.8-mu m Fluorescently-Labeled Bacteria (FLB). Obligate heterotrophs and phototrophs were distinguished from their ratios of microscopically measured red (chlorophyll a) to green (proflavin-stained protein) fluorescence. Sampling sites were located along a nutrient gradient formed by a tropical instability wave at 0.5 degrees N between 123.5 degrees W and 128 degrees W and at 1.75 degrees N, 125 degrees W. The majority of ingested particles were found within 3-5 mu m flagellates, with 54% of the demonstrated phagotrophs belonging to the high-pigment putatively phototrophic population and obligate heterotrophs responsible for 51% of the demonstrated phagotrophy due to their greater propensity to ingest multiple prey. The importance of mixotrophy as a means of alleviating nutrient stress is indicated by a strong inverse relationship between the proportion of community FLB uptake by mixotrophs and ambient nutrient concentration. Low ambient Fe concentration and a demonstrated community response to Fe-addition in shipboard grow-out experiments suggest that mixotrophs were primarily engaging in phagotrophy to offset Fe-deficiencies. (C) 2010 Elsevier Ltd. All rights reserved.

Selph, KE, Landry MR, Taylor AG, Yang EJ, Measures CI, Yang JJ, Stukel MR, Christensen S, Bidigare RR.  2011.  Spatially-resolved taxon-specific phytoplankton production and grazing dynamics in relation to iron distributions in the Equatorial Pacific between 110 and 140 degrees W. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:358-377.   10.1016/j.dsr2.2010.08.014   AbstractWebsite

Phytoplankton dynamics were investigated in the eastern equatorial Pacific at 32 stations sampled during two cruises (December 2004 and September 2005). Based on standing stock analyses from HPLC pigments, flow cytometry and microscopy, we used a modified 2-treatment approach of the seawater dilution method to estimate taxon-specific phytoplankton growth and mortality rates in 8-depth per station profiles. These data were complemented by contemporaneous measurements of dissolved iron (Fe). The stations encompassed an equatorial zonal gradient (110 to 140 degrees W) of diminishing eastward Fe availability in the euphotic zone from upwelling of the Equatorial Undercurrent (EUC). Latitudinal variation was assessed by meridional transects at 110 and 140 degrees W. Overall, euphotic zone averaged growth rates were 0.53 +/- 0.17 d(-1) (total chlorophyll a), 0.34 +/- 0.15 d(-1) (divinyl chlorophyll a) and 0.86 +/- 0.32 d(-1) (fucoxanthin). Microzooplankton grazing accounted for 50-60% of daily production of eukaryotic algae, whereas essentially all growth of phototrophic prokaryotes was consumed daily. Fucoxanthin, representing diatoms, was a minor component of the accessory pigments, but diatom growth rates were both significantly higher than other taxonomically defined groups and dropped off more sharply with depth (low light level). Strikingly, no spatial or temporal trends were seen in the 256 growth rate measurements for each measured pigment. However, the diminishing eastward equatorial Fe gradient was associated with deepening subsurface pigment maxima and decreasing surface-layer pigment stocks (down to the 8% light level). In addition, integrated standing stocks of total chlorophyll a and Prochlorococcus (divinyl chlorophyll a) were strongly correlated with integrated iron at equatorial upwelling stations, yet no correlation with Fe was seen for any of the eukaryotic groups, including diatoms. This latter result is contrary to expectations from previous Fe addition experiments (in situ or in bottles), where diatom biomass increased relative to other phytoplankton. We hypothesize that the natural supply of Fe to the base of the euphotic zone from the EUC is less favorable for diatoms because of light limitation. Rather, new Fe is rapidly incorporated into a small phytoplankton-dominated community in the deep euphotic zone, and tightly coupled grazing control results in a system regulated by return of recycled Fe. (C) 2010 Elsevier Ltd. All rights reserved.

Landry, MR, Selph KE, Yang EJ.  2011.  Decoupled phytoplankton growth and microzooplankton grazing in the deep euphotic zone of the eastern equatorial Pacific. Marine Ecology-Progress Series. 421:13-24.   10.3354/meps08792   AbstractWebsite

We conducted dilution depth-profile experiments in the eastern equatorial Pacific (EEP) to define regional characteristics of phytoplankton growth and microzooplankton grazing and to test the hypothesis that the process rates decouple in the deep euphotic zone where growth is negligible. We used an abbreviated 2-treatment dilution protocol to produce daily profiles at 8 depths of phytoplankton growth, microzooplankton grazing and cellular changes in chlorophyll a (chl a) content from surface waters to the 0.1% light depth. Experiments were conducted at 16 stations from 2 degrees N to 4 degrees S at 110 degrees W and from 110 degrees to 140 degrees W along the equator. Results were surprisingly robust and coherent over this broad spatial area and showed a euphotic zone essentially divided into 3 equal depth intervals. Mean (+/- SD) growth rates (0.83 +/- 0.16 d(-1)) exceeded grazing rates (0.42 +/- 0.15 d(-1)) in the light-saturated upper third of the water column. Growth, and to a lesser extent grazing, declined with light in the middle third. Effective cell growth was negligible (0.02 +/- 0.21 d(-1)) in the lower third (1 to 0.1% of surface irradiance), with grazing (0.18 +/- 0.17 d(-1)) exceeding growth in this layer. The deep euphotic zone accounted for 25.4 +/- 8.4% of the total euphotic zone chl a, 0.5 +/- 7.8% of depth-integrated phytoplankton growth and 12.7 +/- 7.2% of depth-integrated microzooplankton grazing on phytoplankton. The decoupling of growth and grazing processes under low light conditions at the base of the euphotic zone substantially affected our estimates of microzooplankton consumption of phytoplankton, which ranged from 51% of daily chlorophyll growth for experiments conducted in the upper euphotic zone to 69% for the depth-integrated euphotic zone. In addition, the excess of grazing over growth processes in the deepest stratum, which is typically overlooked in experimental studies, suggests that protistan grazers may have a much larger role in biogeochemical transformations of export fluxes than previously appreciated.

Li, QP, Franks PJS, Landry MR.  2011.  Microzooplankton grazing dynamics: parameterizing grazing models with dilution experiment data from the California Current Ecosystem. Marine Ecology-Progress Series. 438:59-69.   10.3354/meps09320   AbstractWebsite

Coupled physical-biological models are essential tools for enhancing our understanding of the potential effects of long-term climate change on planktonic ecosystems in the world's ocean. A major impediment to utilizing such models is obtaining accurate parameterizations of the modeled rate processes, such as growth and grazing. The California Current Ecosystem Long-Term Ecological Research (CCE-LTER) program has generated detailed data of phytoplankton growth and zooplankton grazing rates obtained in the field by the dilution technique. Here, we examine how data from dilution experiments can be used with nonlinear grazing models to study the dynamics of microzooplankton grazing. We use data from experiments conducted in 2007 to parameterize 3 different grazing functions and then test them against a more extensive data set from a CCE-LTER process cruise in 2006. We found that system-level parameterizations of the functional response relationships, representing the aggregate behaviors of predators and prey adapted to different environmental conditions, reasonably predict the shapes and magnitudes of vertical profiles of microzooplankton grazing in both coastal and open-ocean environments in the CCE. Predicting the magnitude of grazing rates-as opposed to just the concentrations of grazers-presents a much greater challenge for models in previous studies. Model-data comparisons are often difficult due to the lack of extensive data from different environments. Our study is a significant advance in the parameterization of zooplankton grazing models in the field and will serve as a solid base on which to pursue further studies of the planktonic ecosystems of the northeastern Pacific.

2010
Linacre, LP, Landry MR, Lara-Lara JR, Hernandez-Ayon JM, Bazan-Guzman C.  2010.  Picoplankton dynamics during contrasting seasonal oceanographic conditions at a coastal upwelling station off Northern Baja California, Mexico. Journal of Plankton Research. 32:539-557.   10.1093/plankt/fbp148   AbstractWebsite

The ecological dynamics of picoplankton were investigated at a coastal upwelling system of northern Baja California during six cruises (September 2007-November 2008). Populations of Prochlorococcus, Synechococcus, PicoEukaryotes and heterotrophic bacteria were assessed by flow cytometry (FCM). On each sampling date, we used an abbreviated three-treatment dilution technique and (14)C-uptake experiments to determine population (FCM) and community (TChl a) rates of growth, grazing and production from 24-h in situ incubations at three to four euphotic depths. Overall, picoplankton comprised an active and important component of the community, with biomass values (2.3-69.8 mu g C L(-1)) and production rates (0.8-68.4 mu g C L(-1) day(-1)) that varied positively with Chl a and community (14)C-production. The exception was an intense algal bloom (> 25 mu g Chl a L(-1)) during La Nina-intensified upwelling conditions in April 2008, during which biomass 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. Under extreme conditions of upwelling, however, the picophytoplankton declined abruptly, despite seemingly strong (average) growth rates. Future studies need to provide a better mechanistic understanding of the physical (advection), physiological (nutrient uptake and temperature) and ecological (food web) factors that result in this dramatic nonlinearity in picophytoplankton response to system forcing and richness.

2009
Hannides, CCS, Landry MR, Benitez-Nelson CR, Styles RM, Montoya JP, Karl DM.  2009.  Export stoichiometry and migrant-mediated flux of phosphorus in the North Pacific Subtropical Gyre. Deep-Sea Research Part I-Oceanographic Research Papers. 56:73-88.   10.1016/j.dsr.2008.08.003   AbstractWebsite

Export processes play a major role in regulating global marine primary production by reducing the efficiency of nutrient cycling and turnover in surface waters. Most studies of euphotic zone export focus on passive fluxes, that is, sinking particles. However, active transport, the vertical transfer of material by migrating zooplankton, can also be an important component of carbon (C) and nitrogen (N) removal from the surface ocean. Here we demonstrate that active transport is an especially important mechanism for phosphorus (P) removal from the euphotic zone at Station ALOHA (Hawaii Ocean Time-series program; 22 degrees 45'N, 158 degrees W), a P-stressed site in the North Pacific Subtropical Gyre. Migrant excretions in this region are P-rich (C(15):N(12):P(1)) relative to sinking particles (C(250):N(31):P(1)), and migrant-mediated P fluxes are almost equal in magnitude (82%) to P fluxes from sediment traps. Migrant zooplankton biomass and therefore the importance of this P removal pathway relative to sinking fluxes has increased significantly over the past 12 years, suggesting that active transport may be a major driving force for enhanced P-limitation of biological production in the NPSG. We further assess the C:N:P composition of zooplankton size fractions at Station ALOHA (C(88):N(18):P(1), on average) and discuss migrant-mediated P export in light of the balance between zooplankton and suspended particle stoichiometries. We conclude that, because active transport is such a large component of the total P flux and significantly impacts ecosystem stoichiometry, export processes involving migrant zooplankton must be included in large-scale efforts to understand biogeochemical cycles. (C) 2008 Elsevier Ltd. All rights reserved.

2008
Brown, SL, Landry MR, Selph KE, Yang EJ, Rii YM, Bidigare RR.  2008.  Diatoms in the desert: Plankton community response to a mesoscale eddy in the subtropical North Pacific. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 55:1321-1333.   10.1016/j.dsr2.2008.02.012   AbstractWebsite

As part of the E-Flux project, we documented spatial variability and temporal changes in plankton community structure in a cold-core cyclonic eddy in the lee of the Hawaiian Islands. Cyclone Opal spanned 200km in diameter, with sharply uplifted isopycnals (80-100m relative to surrounding waters) and a strongly expressed deep chlorophyll a maximum (DCM) in its central core region of 40 km diameter. Microscopic and flow cytometric analyses of samples from across the eddy revealed dramatic transitions in phytoplankton community structure, reflecting Opal's well-developed physical structure. Upper mixed-layer populations in the eddy resembled those outside the eddy and were dominated by picophytoplankton. In contrast, the DCM was composed of large chain-forming diatoms dominated by Chaetoceros and Rhizosolenia spp. Diatoms attained unprecedented levels of biomass (nearly 90 mu g Cl(-1)) in the center of the eddy, accounting for 85% of photosynthetic biomass. Protozoan grazers displayed two- to three-fold higher biomass levels in the eddy center as well. We also found a distinct and persistent layer of senescent diatom cells overlying healthy populations, often separated by less than 10 m, indicating that we were sampling a bloom in a state of decline. Time-series sampling over 8 days showed a successional shift in community structure within the central diatom bloom, from the unexpected large chain-forming species to smaller forms more typical of the subtropical North Pacific. The diatom bloom of Cyclone Opal was a unique, and possibly extreme, example of biological response to physical forcing in the North Pacific subtropical gyre, and its detailed study may therefore help to improve our predictive understanding of environmental controls on plankton community structure. (C) 2008 Elsevier Ltd. All rights reserved.

Landry, MR, Decima M, Simmons MP, Hannides CCS, Daniels E.  2008.  Mesozooplankton biomass and grazing responses to Cyclone Opal, a subtropical mesoscale eddy. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 55:1378-1388.   10.1016/j.dsr2.2008.01.005   AbstractWebsite

As part of E-Flux III cruise studies in March 2005, plankton net collections were made to assess the effects of a cyclonic cold-core eddy (Cyclone Opal) on the biomass and grazing of mesozooplankton. Mesozooplankton biomass in the central region of Cyclone Opal, an area of uplifted nutricline and a subsurface diatom bloom, averaged 0.80 +/- 0.24 and 1.51 +/- 0.59 g DW m(-2), for day and night tows, respectively. These biomass estimates were about 80% higher than control (OUT) stations, with increases more or less proportionately distributed among size classes from 0.2 to > 5 mm. Though elevated relative to surrounding waters south of the Hawaiian Islands (Hawai'i lee), total biomass and size distribution in Cyclone Opal were almost exactly the same as contemporary measurements made at Stn. ALOHA, 100 km north of the islands, by the HOT (Hawaii Ocean Time-series) Program. Mesozooplankton biomass and community composition at the OUT stations were also similar to ALOHA values from 1994 to 1996, preceding a recent decadal increase. These comparisons may therefore provide insight into production characteristics or biomass gradients associated with decadal changes at Stn. ALOHA. Gut fluorescence estimates were higher in Opal than in ambient waters, translating to grazing impacts of 0.11 +/- 0.02 d(-1) (IN) versus 0.03+0.01 d(-1) (OUT). Over the depth-integrated euphotic zone, mesozooplankton accounted for 30% of the combined grazing losses of phytoplankton to micro- and meso-herbivores in Opal, as compared to 13% at control stations. Estimates of active export flux by migrating zooplankton averaged 0.81 mmol Cm-2 d(-1) in Cyclone Opal and 0.37 mmol Cm-2 d(-1) at OUT stations, 53% and 24%, respectively, of the carbon export measured by passive sediment traps. Migrants also exported 0.18 mmol Nm(-2) d(-1) (117% of trap N flux) in Cyclone Opal compared to 0.08 mmol Nm(-2) d(-1) (51% of trap flux) at control stations. Overall, the food-web importance of mesozooplankton increased in Cyclone Opal both in absolute and relative terms. Diel migrants provided evidence for enhanced export flux in the eddy that was missed by sediment trap and Th-234 techniques, and migrant-mediated flux was the major export term in the observed bloom-perturbation response and N mass balance of the eddy. (C) 2008 Elsevier Ltd. All rights reserved.

2005
Selph, KE, Shacat J, Landry MR.  2005.  Microbial community composition and growth rates in the NW Pacific during spring 2002. Geochemistry Geophysics Geosystems. 6   10.1029/2005gc000983   AbstractWebsite

[1] The IOC North Pacific expedition in spring ( May 2002) sailed from Osaka, Japan, to Honolulu, Hawaii, and surveyed the region from roughly 22 to 50 degrees N and 147 degrees E to 158 degrees W. Nine stations were chosen to characterize three distinct water masses, as well as their boundary regions: the Kuroshio Current, the North Pacific subarctic gyre, and the North Pacific subtropical gyre. Grazing largely balanced growth at all stations, with an average net growth rate of 0.11 +/- 0.16 d(-1) (cell-based) and 0.06 +/- 0.15 d(-1) (chlorophyll-based). The stations could be distinguished, however, by phytoplankton community composition and growth response to nutrient additions ( ammonium, phosphate, manganese, and iron). Nutrients were undetectable in surface waters of the Kuroshio Current, where a centric diatom-dominated bloom showed a significant growth response to added nutrients. Iron limitation was observed for the cyanobacteria Synechococcus at the two subarctic gyre stations; however, the photosynthetic eukaryotes, which dominated the photosynthetic biomass at these stations, were not nutrient-limited. Four oligotrophic subtropical gyre stations were dominated by photosynthetic bacteria (Prochlorococcus and Synechococcus) and small (< 5 mu m) eukaryotic autotrophs, which exhibited a dramatic increase in growth rate with macronutrient additions but displayed little or no increased growth with iron additions alone. Overall, the ratio of heterotroph consumer biomass to autotroph biomass was higher in the Prochlorococcus-dominated subtropical gyre sites (0.5) than the subarctic or Kuroshio Current sites (0.2 - 0.3).

2004
Calbet, A, Landry MR.  2004.  Phytoplankton growth, microzooplankton grazing, and carbon cycling in marine systems. Limnology and Oceanography. 49:51-57.   10.4319/lo.2004.49.1.0051   AbstractWebsite

We present an analysis of the global impact of microplanktonic grazers on marine phytoplankton and its implications for remineralization processes in the microbial community. The data were obtained by an extensive literature search that yielded 788 paired rate estimates of autotrophic growth (mu) and microzooplankton grazing (m) from dilution experiments. From studies in which phytoplankton standing stock was measured in terms of carbon equivalents, we show that the production estimate from dilution experiments is a reasonable proxy (r = 0.89) for production determined by the standard C-14 method. The ratio m: mu, the proportion of primary production (PP) consumed by micrograzers, shows that microzooplankton consumption is the main source of phytoplankton mortality in the oceans, accounting for 67% of phytoplankton daily growth for the full data set. This ratio varies modestly among various marine habitats and regions, with data averages ranging from 60% for coastal and estuarine environments to 70% for the open oceans, and from similar to59% for temperate-subpolar and polar systems to 75% for tropical-subtropical regions. Given estimates for the metabolic requirements of micrograzers and assuming they consume most bacterial production, regionally averaged estimates of the protistan respiration are 35-43% of daily PP for the first level of consumer or 49-59% of PP for three trophic transfers. The estimated contributions of microbial grazers to total community respiration are of the same magnitude as bacterial respiration. Consequently, potential ecosystem differences in micrograzer activity or trophic structure are a large uncertainty for biogeochemical models that seek to predict the microbial community role in carbon cycling from bacterial parameters alone.

2003
Neveux, J, Dupouy C, Blanchot J, Le Bouteiller A, Landry MR, Brown SL.  2003.  Diel dynamics of chlorophylls in high-nutrient, low-chlorophyll waters of the equatorial Pacific (180 degrees): Interactions of growth, grazing, physiological responses, and mixing. Journal of Geophysical Research-Oceans. 108   10.1029/2000jc000747   AbstractWebsite

[1] In situ diel variations of extracted chlorophyllous pigments, beam attenuation by particles (c(p)), and in vivo chlorophyll fluorescence ( F-iv) were investigated during a 5-day time series in high-nutrient, low-chlorophyll waters of the equatorial Pacific ( date line = 180degrees). Samples were taken hourly at 10 depths in the upper 100 m during the first 48 hours, then sampling frequency decreased to 3 hours. In the 30 - 70 m layer the integrated chlorophyll concentrations, cp, and Fiv increased during the light period, but the minima and, especially, maxima were not fully synchronized. The lowest values of total chlorophyll a (Tchl a = chlorophyll a + divinyl-chlorophyll a) occurred around 5 - 6 hours, slightly ( 0 - 2 hours) before that of cp and Fiv. Tchl a reached a maximum around 1500 hours +/- 1 hour, clearly before c(p) (1700 hours) and F-iv (1900 hours). In the 0 - 30 m layer, diel variations of the integrated chlorophyll concentrations, cp, and Fiv were clearly out of phase. They showed a nocturnal increase in Tchl a, starting around midnight and peaking in early morning ( 0900 hours). In contrast, cp increased only during the light period in the upper 30 m, and variations of Fiv were largely opposite to those of extracted Tchl a. Specific phytoplankton growth (mu(0)) and grazing loss (g) rates were estimated from diel variations in the 30 - 70 m layer and compared to independent rate estimates from experimental incubations. These results are discussed in the context of physical processes and physiological responses of the cells to the daily photocycle.

Le Borgne, R, Landry MR.  2003.  EBENE: A JGOFS investigation of plankton variability and trophic interactions in the equatorial Pacific (180 degrees). Journal of Geophysical Research-Oceans. 108   10.1029/2001jc001252   AbstractWebsite

[1] The Etude du Broutage en Zone Equatoriale (EBENE) transect (8 degreesS - 8 degreesN) explored the equatorial high-nutrient, low-chlorophyll (HNLC) zone and adjacent oligotrophic areas during a La Nina period (October - November 1996). During this time the passage of a tropical instability wave also influenced the region north of the equator. We present a brief summary of EBENE findings, with an emphasis on phytoplankton utilization by the assemblage of protistan and animal consumers. Despite significant variability over the diel cycle, phytoplankton biomass at the equator was relatively constant on a 24-hour timescale, denoting a dynamic balance between growth and losses. The magnitude of the daily cycle in phytoplankton biomass was well constrained by in situ observations of the diel variability in pigments and suspended particulates, by (14)C uptake rates from in situ incubations, and from experimental determinations of specific growth and grazing rates. The general equilibrium of production and grazing processes is illustrated by applying biomass-specific grazing rates from the equatorial station to measured planktonic biomass along the EBENE transect and comparing them to measured (14)C uptake. Most of the grazing turnover is supported by the production of Prochloroccus (31%) and picoeukaryotic algae (34%). Among the consumers, microzooplankton (< 200 mu m) account for 59 - 98% of the grazing losses. The coherence of the results obtained by independent methods suggests that the essential features of the system have been adequately represented by rate and standing stock assessments from the EBENE study.

Selph, KE, Landry MR, Laws EA.  2003.  Heterotrophic nanoflagellate enhancement of bacterial growth through nutrient remineralization in chemostat culture. Aquatic Microbial Ecology. 32:23-37.   10.3354/ame032023   AbstractWebsite

Heterotrophic nanoflagellates are the principal consumers of picoplankton in the ocean. Their role as nutrient remineralizers is also well established. However, the coupled interactions between grazer consumption and prey growth are less well understood. In this work, we demonstrate a tight coupling among flagellate grazing, nitrogen remineralization, and prey growth, resulting in bacterial growth rates averaging 2- to 14-fold higher in the presence of flagellate grazers. These results were obtained using 2-stage, nitrogen-limited chemostats containing a mixed culture of heterotrophic bacteria enriched from seawater and Paraphysomonas bandaiensis, a chrysomonad flagellate. Abundance and biovolume of the flagellates were monitored on a daily basis, as was bacterial abundance. Grazing rates were measured using short-term tracer uptake experiments (fluorescently-labeled bacteria and beads), and these data were used to calculate gross bacterial growth rates in the presence of grazers. A mass balance approach was used to estimate reduced nitrogen regenerated by the protist and nitrogen demand of the heterotrophic bacteria. These independent methods of assessing grazer growth and feeding, coupled with estimates of flagellate gross growth efficiency, provided strong, internally consistent constraints on the estimates of bacterial growth rates in the presence of grazers. Under these culture conditions, P. bandaiensis had a carbon-based gross growth efficiency averaging 28%. This work shows that independently measured grazing rates are essential in protist culture work if system dynamics are to be understood. It also underscores the necessity of including protist remineralization pathways in models if realistic simulations are to be obtained.