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Decima, M, Landry MR, Bradley CJ, Fogel ML.  2017.  Alanine delta(15) N trophic fractionation in heterotrophic protists. Limnology and Oceanography. 62:2308-2322.   10.1002/lno.10567   AbstractWebsite

We evaluated differences in the N-15 isotopic enrichment factors of trophic amino acids (AA) for protistan (microzooplankton) and metazoan (mesozooplankton) consumers, testing the hypothesis that delta N-15 of alanine (ala) increases in both consumer types, while glutamic acid (glu) enriches mainly in mesozooplankton. AA delta N-15 values were measured for dinoflagellate and ciliate grazers and their respective algal prey (Oxyrrhis marina/Dunaliella tertiolecta and Favella sp./Heterocapsa triquetra) in four two-stage chemostat experiments, including treatments with different nitrogen : phosphorous nutrient ratios and light/dark recycling conditions. Propagation of AA N-15 enrichment to a metazoan consumer was also assessed in two-and three-stage chemostat experiments simulating simple "classical" (Calanus pacificus and the diatom Thalassiosira weissflogii) and "multivorous" (C. pacificus, O. marina, and D. tertiolecta) food chains. We found small or negligible 15 N-enrichment of glu for both protistan grazers, while ala enrichment was consistently greater and similar to that in metazoan consumers. Ala and glu delta N-15 values were both highly elevated in C. pacificus relative to prey, and enrichment was higher with autotrophic diets. These laboratory results suggest that ala may be used as an alternate, accurate isotopic proxy for quantifying protistan contributions to trophic structure in aquatic systems.

Li, QP, Franks PJS, Landry MR.  2017.  Recovering growth and grazing rates from nonlinear dilution experiments. Limnology and Oceanography. 62:1825-1835.   10.1002/lno.10536   AbstractWebsite

Biological rate measurements provide critical information for understanding key processes and modeling future states of marine ecosystems. Experimentally derived rates can be challenging to interpret when methodological assumptions are untested or potentially violated under variable natural conditions, such as the assumed linear grazing response of the dilution technique for estimating rates of phytoplankton growth and microzooplankton grazing impact. Here, we show that grazing nonlinearity can be related to the ratio of initial phytoplankton biomass to the half-saturation parameter in the Holling II model, while not being affected by varying grazer biomass during dilution experiments. From this, we present theory to recover growth and grazing rates from multi-treatment dilution experiments with nonlinear grazing results. We test our analyses with data collected during the California Current Ecosystem-Long-Term Ecological Research (CCE-LTER) program. We show that using a linear regression in 2-treatment dilution experiments may lead to underestimates of microzooplankton grazing rates, particularly in high-phytoplankton-biomass coastal regions where grazing can be saturated. Using the Holling II grazing model and a correction factor, growth and grazing rates from 2-treatment experiments can also be estimated, as illustrated by application to Lagrangian watertracking studies of growth and grazing dynamics in the CCE.

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.

Chappell, PD, Vedmati J, Selph KE, Cyr HA, Jenkins BD, Landry MR, Moffett JW.  2016.  Preferential depletion of zinc within Costa Rica upwelling dome creates conditions for zinc co-limitation of primary production. Journal of Plankton Research. 38:244-255.   10.1093/plankt/fbw018   AbstractWebsite

The Costa Rica Dome (CRD) is a wind-driven feature characterized by high primary production and an unusual cyanobacterial bloom in surface waters. It is not clear whether this bloom arises from top-down or bottom-up processes. Several studies have argued that trace metal geochemistry within the CRD contributes to the composition of the phytoplankton assemblages, since cyanobacteria and eukaryotic phytoplankton have different transition metal requirements. Here, we report that total dissolved zinc (Zn) is significantly depleted relative to phosphate (P) and silicate (Si) within the upper water column of the CRD compared with other oceanic systems, and this may create conditions favorable for cyanobacteria, which have lower Zn requirements than their eukaryotic competitors. Shipboard grow-out experiments revealed that while Si was a limiting factor under our experimental conditions, additions of Si and either iron (Fe) or Zn led to higher biomass than Si additions alone. The addition of Fe and Zn alone did not lead to significant enhancements. Our results suggest that the depletion of Zn relative to P in upwelled waters may create conditions in the near-surface waters that favor phytoplankton with low Zn requirements, including cyanobacteria.

Pasulka, AL, Samo TJ, Landry MR.  2015.  Grazer and viral impacts on microbial growth and mortality in the southern California Current Ecosystem. Journal of Plankton Research. 37:320-336.   10.1093/plankt/fbv011   AbstractWebsite

Protistan grazers and viruses are major agents of mortality in marine microbial communities with substantially different implications for food-web dynamics, carbon cycling and diversity maintenance. While grazers and viruses are typically studied independently, their impacts on microbial communities may be complicated by direct and indirect interactions of their mortality effects. Using a modification of the seawater dilution approach, we quantified growth and mortality rates for total phytoplankton and picophytoplankton populations (Prochlorococcus, Synechococcus, picoeukaryotes) at four contrasting sites in the California Current Ecosystem. Grazing mortality was significant in 10 of 15 cases, while viral effects were significant for 2 cases. Nonetheless, mortality estimates for the entire phytoplankton community based on chlorophyll a were 38 +/- 13% higher when viral effects were included, relative to grazing alone. Mortality estimates for picophytoplankton varied in space and among groups. We also explored a potential methodological constraint of this method and hypothesize that heterotrophic bacteria may be affected by the dilution of their growth-sustaining substrates. For all picophytoplankton, estimates of grazing and viral mortality were inversely related within and across experiments. Indirect interactions among grazers and viruses may be important to consider if there are tradeoffs in the grazing and virus resistance strategies of prey/host cells.

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.

Taniguchi, DAA, Franks PJS, Landry MR.  2012.  Estimating size-dependent growth and grazing rates and their associated errors using the dilution method. Limnology and Oceanography-Methods. 10:868-881.   10.4319/lom.2012.10.868   AbstractWebsite

Size-dependent properties are pervasive in nature but difficult to measure for natural communities. Here, we develop a technique to estimate size-specific phytoplankton growth and grazing rates based on the two-point dilution method, enhanced by the acquisition of the size spectra of the phytoplankton in the samples. We describe a way to estimate standard deviations associated with the rate estimates, which can be applied either to the size-dependent or total community rates. We tested the accuracy of rates estimated using the size-dependent dilution method by applying it to dilution experiments simulated using a complex size-structured ecosystem model. The strong agreement between model and size-dependent dilution method rates (two-sample Kolmogorov-Smirnov test, P = 1) supports the accuracy of this new technique. Because size-dependent rates vary with the size interval over which they are calculated, we display the size-dependent growth and grazing rates and their standard deviations as a function of the size interval. This technique easily allows the assessment of rates for any size class of interest. Finally, we apply the size-dependent dilution method to data collected in the equatorial Pacific. There is a general agreement between size-based and previously published taxonomic-based rates, with differences reflecting the extent to which size classes are mixtures of taxa. The use of the size-dependent dilution method will provide new insights into the structure and dynamics of planktonic communities. Future applications of this method to other natural communities will help in assessing the size-dependencies of phytoplankton growth and grazing rates in their environments.

Macias, D, Franks PJS, Ohman MD, Landry MR.  2012.  Modeling the effects of coastal wind- and wind-stress curl-driven upwellings on plankton dynamics in the Southern California current system. Journal of Marine Systems. 94:107-119.   10.1016/j.jmarsys.2011.11.011   AbstractWebsite

We use a Nitrogen-Phytoplankton-Zooplankton-Detritus (NPZD) biogeochemical model implemented in a time-dependent box model scheme to simulate the temporal dynamics of the pelagic ecosystem in the Southern California Current System (SCCS). The model was forced by winds, sea surface temperature and light. Nutrient inputs to the modeled box were driven by coastal upwelling or upwelling due to wind-stress curl in order to assess the importance of each process in the temporal dynamics of the SCCS ecosystem. Model results were compared to the CalCOR dataset, both in terms of climatological annual cycles and actual values. This comparison led to modifications of the basic model structure to better represent the coastal ecosystem, particularly phytoplankton growth and zooplankton mortality terms. Wind-stress curl-induced upwelling was found to be significant only in the offshore regions while coastal upwelling better represented the dynamics of the inshore areas. The two upwelling mechanisms work in synchrony, however, to bring nutrients to surface waters during the same time periods. Finally, the effect of low-frequency perturbations, such as those associated with the ENSO and NPGO, were assessed by comparing model results and data. Since the NPGO cycle largely impacts the SCCS through modifications of upwelling-favorable winds, its effects were well represented in the model results. In contrast, ENSO responses were poorly captured in the simulations because such perturbations alter the system by changing surface water mass distributions via mechanisms that were not included in the model forcing. (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.

Stukel, MR, Landry MR, Benitez-Nelson CR, Goericke R.  2011.  Trophic cycling and carbon export relationships in the California Current Ecosystem. Limnology and Oceanography. 56:1866-1878.   10.4319/lo.2011.56.5.1866   AbstractWebsite

We constructed a simple non-steady-state model of trophic cycling relationships in the California Current Ecosystem and tested its predictions of mesozooplankton fecal-pellet export against vertical carbon-flux measurements by the (234)Th method taken during Lagrangian experiments. To assess trophic relationships, we simultaneously measured (14)C-primary production and chlorophyll-based rate estimates of phytoplankton growth, microzooplankton grazing, mesozooplankton grazing, and net phytoplankton growth. Study locations ranged from coastal upwelling to offshore oligotrophic conditions. E-ratios (carbon export : (14)C-primary production) predicted by the model ranged from 0.08 to 0.14, in good agreement with both the magnitude and the variability found in contemporaneous measurements of (234)Th export and C: (234)Th-ratios of sinking particles. E-ratios were strongly decoupled from new production estimates. The lowest measured and predicted e-ratios were associated with higher nutrient chlorophyll parcels with net accumulating phytoplankton in the inshore region. For our study sites, variability in export efficiency was determined by the local net balance of growth and grazing and the relative strengths of grazing pathways to microzooplankton and mesozooplankton. Despite very different plankton assemblages studied, the consistently good agreement between independently measured production-grazing processes and biogeochemical rates suggest that zooplankton are the major drivers of vertical carbon-flux in this system during springtime.

Brzezinski, MA, Baines SB, Balch WM, Beucher CP, Chai F, Dugdale RC, Krause JW, Landry MR, Marchi A, Measures CI, Nelson DM, Parker AE, Poulton AJ, Selph KE, Strutton PG, Taylor AG, Twining BS.  2011.  Co-limitation of diatoms by iron and silicic acid in the equatorial Pacific. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:493-511.   10.1016/j.dsr2.2010.08.005   AbstractWebsite

The relative roles of silicon (Si) and iron (Fe) as limiting nutrients in the eastern equatorial Pacific (EEP) were examined in a series of nine microcosm experiments conducted over two years between 110 degrees W and 140 degrees W longitude. Si and Fe additions had consistently different but synergistic effects on macronutrient use, phytoplankton biomass and phytoplankton community structure. Silicon addition increased silicic acid use and biogenic silica production, but had no significant effect on the use of inorganic nitrogen or orthophosphate, chlorophyll accumulation, particulate inorganic (PIC) carbon accumulation, or plankton community composition relative to controls. That result, together with observations that Si addition increased the cellular Si content of the numerically dominant diatom by similar to 50%, indicates that the main effect of Si was to regulate diatom silicification. Like the effect of Si, Fe addition increased the rate of silicic acid use and biogenic silica production and had no effect on PIC production. Unlike the effect of Si, Fe addition also enhanced rates of organic matter production, had no effect on cellular Si content of diatoms, and resulted in the growth of initially rare, large (> 40 mu m) diatoms relative to controls, indicating that Fe limitation acts mainly through its effects on growth rate and phytoplankton community composition. A pennate diatom of the genus Pseudo-nitzschia dominated the diatom assemblage in situ, grew readily in the controls and did not show a strong growth response to either Fe or Si addition suggesting that its growth was regulated by other factors such as grazing or light. Addition of germanium, an inhibitor of diatom cell division, eliminated the effects of Fe on macronutrient use, biogenic silica production and chlorophyll accumulation and phytoplankton community composition, consistent with a predominantly diatom response to Fe addition. The lack of a response of PIC production to Fe suggests that coccolithophores were not Fe limited. Addition of Fe and Si together resulted in the greatest levels of nutrient drawdown and biomass accumulation through the effect of Fe in promoting the growth of large diatoms. The results suggest a form of co-limitation with Si regulating diatom silicification and the rate of biogenic silica production while Fe regulates the production of organic matter through limitation of phytoplankton growth rates, in particular those of large diatoms. The results argue against Si regulation of new production in the EEP under average upwelling conditions. Iron addition was necessary and sufficient to stimulate complete removal of nitrate within the equatorial upwelling zone suggesting that new production was restricted by low ambient dissolved Fe consistent with results from in situ Fe fertilization experiments conducted to the south of the equator outside of the equatorial upwelling zone. (C) 2010 Elsevier Ltd. All rights reserved.

Landry, MR, Selph KE, Taylor AG, Decima M, Balch WM, Bidigare RR.  2011.  Phytoplankton growth, grazing and production balances in the HNLC equatorial Pacific. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:524-535.   10.1016/j.dsr2.2010.08.011   AbstractWebsite

We investigate the hypothesis that phytoplankton growth and grazing processes are strongly balanced in high-nutrient low-chlorophyll (HNLC) waters of the equatorial Pacific using euphotic-zone estimates of rates and biomass determined for 30 stations during EB04 (December 2004) and EB05 (September 2005). As predicted by the balance hypothesis, depth-averaged instantaneous rates of phytoplankton growth and grazing losses to micro- and mesozooplankton show a net growth difference of zero. Contemporaneous estimates of phytoplankton biomass and specific rates from flow cytometry, microscopy and taxon-specific accessory pigments allow determination of constrained production-consumption trophic balances for the phytoplankton community as a whole and for major component populations. The magnitude of growth-based production (867 mg C m(-2) d(-1)) is consistent with measured (14)C primary production, given methodological differences. 70% of production is utilized by protistan herbivores within the microbial community; 30% is consumed by mesozooplankton. Among picophytoplankton (Prochlorococcus, Synechococcus and small eukaryotes), representing 40% of community biomass and 27% of daily biomass growth, microzooplankton consume almost all production. Among groups of larger eukaryote taxa, including diatoms but dominated by dinoflagellate biomass, micro-grazers consume 51-62% of production, with the remainder available to mesozooplankton. Some leakage from the balance is expected as export of sinking phytoplankton cells and aggregates, but is constrained to no more than a few percent of daily production from alternate determinations of mesozooplankton grazing. The demonstrated balance of growth and grazing processes in the equatorial Pacific is inconsistent with recent claims from inverse models that a large flux associated with ungrazed picophytoplankton production dominates euphotic zone carbon export in the region. (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.

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.

Stukel, MR, Landry MR.  2010.  Contribution of picophytoplankton to carbon export in the equatorial Pacific: A reassessment of food web flux inferences from inverse models. Limnology and Oceanography. 55:2669-2685.   10.4319/lo.2010.55.6.2669   AbstractWebsite

The paradigm that carbon export is derived almost exclusively from the primary production of large phytoplankton has been challenged by inverse ecosystem modeling studies that suggest that most carbon export in the open ocean is fueled by picophytoplankton. To readdress this hypothesis, we use an inverse model to synthesize the planktonic rate measurements from a pair of recent cruises in the equatorial Pacific. The analysis based on this new experimental data, which crucially include vertically integrated taxon-specific production and grazing estimates, largely resolve the unexpected results of the previous inverse studies, including unbalanced growth and grazing processes and the dominance of production by picophytoplankton. While this very small size class does not produce the majority of phytoplankton carbon that is eventually exported to depth (only 23%, vs. 73% from a previous analysis of Joint Global Ocean Flux Study Equatorial Pacific data), our base model supports the conclusion that the role of picophytoplankton in vertical carbon flux is largely proportional to their contribution to net primary productivity (though neither is proportional to biomass). We show, however, that export-production proportionality is sensitive to the model representation of the detrital pool such that the relative export role of picophytoplankton declines substantially for an alternate model with size-structured detritus. A definitive assessment of the role of picoplankton in vertical carbon flux will thus require detailed experimental examination of the origin, composition, and fate of euphotic zone detrital material.

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.

Huntley, ME, Lopez MDG, Zhou M, Landry MR.  2006.  Seasonal dynamics and ecosystem impact of mesozooplankton at station ALOHA based on optical plankton counter measurements. Journal of Geophysical Research-Oceans. 111   10.1029/2005jc002892   AbstractWebsite

[1] Abundances of mesozooplankton-sized particles were measured at 45-m depth at station ALOHA (22.75 degrees N, 158 degrees W) during 18 cruises from February 1995 through December 1996 with an optical plankton counter (OPC). Mesozooplankton were also sampled with oblique net tows to 155 m depth. Vertical OPC profiles showed uniform total abundance in the upper mixed layer, usually > 45 m. OPC and net data agreed with respect to total abundance, size composition, abundance of individual size classes, and seasonal cycle of abundance. We found no evidence for significant contributions to OPC particle counts by diatom aggregates, Trichodesmium spp., or detritus. Variations in OPC estimates of abundance are well explained by diel behavior and seasonal cycles of species that dominate mesozooplankton abundance, of which 80% are copepods. The summer maximum in mesozooplankton abundance is due primarily to the increase of the six smallest OPC size classes (< 1.15 mm equivalent spherical diameter), dominated by 14 nonmigrating copepod species that account for more than 95% of average copepod abundance. Seasonal cycles of zooplankton egestion estimated from OPC measurements were highly correlated, and comparable in magnitude, with observed sinking flux measurements of both C and N. Sinking flux at the base of the euphotic zone was 0.67 and 0.77 mol C m(-2) yr(-1) and 81 and 87 mmol N m(-2) yr(-1), for 1995 and 1995, respectively. The potential contribution of mesozooplankton egestion in the mixed layer, based on OPC measurements, accounts for 95% and 90% of C and 86% and 81% of N, respectively.

Hoover, RS, Hoover D, Miller M, Landry MR, DeCarlo EH, Mackenzie FT.  2006.  Zooplankton response to storm runoff in a tropical estuary: bottom-up and top-down controls. Marine Ecology-Progress Series. 318:187-201.   10.3354/meps318187   AbstractWebsite

Zooplankton successional patterns and response times were characterized in a tropical estuary following a major storm-runoff event to evaluate the effects of a nutrient perturbation on community composition and dynamics. Intensive water-column monitoring in southern Kaneohe Bay, Hawaii, showed that dissolved macronutrients - NO3- + NO2-, SRP (soluble reactive phosphorus) and Si(OH)(4) - increased significantly immediately following the initial runoff event. Bottom-up effects were evident in both phytoplankton and zooplankton communities. An initial phytoplankton bloom was dominated by small cells and lasted only a few days, while post-bloom pigment concentrations showed a more gradual increase in total chlorophyll a and a shift to a diatom-dominated community. The initial bloom had an unexpectedly large influence on zooplankton growth and reproduction on extremely short time scales. Appendicularians exhibited the most dramatic response, with biomass increasing 6-fold in 1 d, and abundances reaching values only rarely observed in these waters. Response covaried with organism size, with larger components of the community, especially calanoid copepods and gelatinous zooplankton, increasing as new resources became available. Post-bloom changes in zooplankton and phytoplankton community structure also suggest significant top-down controls on phytoplankton and zooplankton community biomass and structure, with increased predation on appendicularians and copepods resulting in partial release of grazing pressure on small and large cells, respectively. Nutrient-rich runoff can have significant and surprisingly rapid impacts on zooplankton population dynamics in tropical coastal waters via direct, pulsed, food influences on the growth and reproduction of omnivorous organisms and the indirect stimulation of secondary consumers.

de Baar, HJW, Boyd PW, Coale KH, Landry MR, Tsuda A, Assmy P, Bakker DCE, Bozec Y, Barber RT, Brzezinski MA, Buesseler KO, Boye M, Croot PL, Gervais F, Gorbunov MY, Harrison PJ, Hiscock WT, Laan P, Lancelot C, Law CS, Levasseur M, Marchetti A, Millero FJ, Nishioka J, Nojiri Y, van Oijen T, Riebesell U, Rijkenberg MJA, Saito H, Takeda S, Timmermans KR, Veldhuis MJW, Waite AM, Wong CS.  2005.  Synthesis of iron fertilization experiments: From the iron age in the age of enlightenment. Journal of Geophysical Research-Oceans. 110   10.1029/2004jc002601   AbstractWebsite

[1] Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/ Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate ( 10 - 30 mu m), medium ( 30 - 60 mu m), and large (> 60 mu m) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of "dissolved'' Fe ( filtrate < 0.2 mu m) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the "dissolved'' pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe similar to 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO(2) fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Landry, MR, Calbet A.  2004.  Microzooplankton production in the oceans. ICES Journal of Marine Science. 61:501-507.   10.1016/j.icesjms.2004.03.011   AbstractWebsite

A literature synthesis of phytoplankton growth (It) and grazing (m) rate estimates front dilution experiments reveals that microzooplankton account for most phytoplankton mortality in the oceans, averaging 60-75% of daily phytoplankton production (PP) across a spectrum of open-ocean and coastal systems. For reasonable estimates of gross growth efficiency (GGE = 30-40%), such impacts imply that secondary production rates of microzooplankton (MP2degrees) are typically in the range 21-34% of PP. However, multiple trophic transfers within the microbial community can further enhance total microzooplankton production by an additional third to a half (MPtot = 28-55% of PP). These estimates are 2-5 times typical values for bacterial production (10-15% of PP). Thus, in aggregate and on average, microzooplankton consume substantially more (6-7 times) production from phytoplankton than from heterotrophic bacteria. High grazing impacts and relatively high GGEs are consistent with population growth rates for microzooplankton and phytoplankton that are roughly equivalent under ambient conditions, which may be requisite for grazing regulation. Transfer efficiencies of microzooplankton production to mesozooplankton depend critically on the number of predatory interactions among micro-consumers. and may be one way in which systems differ substantially. Overall, the ability to quantify microzooplankton production in terms of more broadly measured rates of PP provides a potential avenue for broadening our understanding of ocean community dynamics through,h remote sensing and modelling. (C) 2004 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.

Coale, KH, Johnson KS, Chavez FP, Buesseler KO, Barber RT, Brzezinski MA, Cochlan WP, Millero FJ, Falkowski PG, Bauer JE, Wanninkhof RH, Kudela RM, Altabet MA, Hales BE, Takahashi T, Landry MR, Bidigare RR, Wang XJ, Chase Z, Strutton PG, Friederich GE, Gorbunov MY, Lance VP, Hilting AK, Hiscock MR, Demarest M, Hiscock WT, Sullivan KF, Tanner SJ, Gordon RM, Hunter CN, Elrod VA, Fitzwater SE, Jones JL, Tozzi S, Koblizek M, Roberts AE, Herndon J, Brewster J, Ladizinsky N, Smith G, Cooper D, Timothy D, Brown SL, Selph KE, Sheridan CC, Twining BS, Johnson ZI.  2004.  Southern ocean iron enrichment experiment: Carbon cycling in high- and low-Si waters. Science. 304:408-414.   10.1126/science.1089778   AbstractWebsite

The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Twomesoscaleexperiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic acid, were performed in the Southern Ocean. These experiments demonstrate iron's pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide.

Brown, SL, Landry MR, Neveux J, Dupouy C.  2003.  Microbial community abundance and biomass along a 180 degrees transect in the equatorial Pacific during an El Nino-Southern Oscillation cold phase. Journal of Geophysical Research-Oceans. 108   10.1029/2001jc000817   AbstractWebsite

As part of the French Joint Global Ocean Flux Study Etude du Broutage en Zone Equatoriale program, we investigated the distributions of microorganisms (bacteria and protists <200 mm) in the upper 120 m of the equatorial Pacific from 8&DEG;S to 8&DEG;N, along 180&DEG;. Population distributions, determined by a combination of flow cytometry, microscopy and spectrofluorometry, were closely related to physical features across the study site. Phytoplankton biomass, ranging from 1.2 to 34.2 mg C L-1 and averaging 15.5 mg C L-1, was most enhanced in the divergence zone. Carbon to chlorophyll ratios were also enhanced in the divergence zone and showed distinct latitudinal variations. Heterotrophic biomass, excluding ciliates, was patchy across the area, ranging from 5 to 36 mg C L-1 and averaging 13 mg C L-1. Prokaryotic species (Prochlorococcus spp., Synechococcus spp., and heterotrophic bacteria) showed similar patterns of abundance, with the main feature being their distributional asymmetry to the south of the equator. Both autotrophic and heterotrophic biomass were enriched in the convergent zone at 4&DEG;-5&DEG;N between the South Equatorial Current and the North Equatorial Counter Current. Heterotrophic biomass exceeded phytoplankton biomass in the more nutrient-impoverished waters to the north and in the branch of a tropical instability wave eddy. Microplankton represented only a small portion of the total autotrophic carbon and was comprised mostly of dinoflagellates. Large species dominated the relatively modest diatom biomass. Food web interactions and biogeochemical fluxes in the central equatorial Pacific may be significantly influenced by temporal and spatial variability of the microbial community associated with physical features of the region.

Landry, MR.  2002.  Integrating classical and microbial food web concepts: evolving views from the open-ocean tropical Pacific. Hydrobiologia. 480:29-39.   10.1023/a:1021272731737   AbstractWebsite

Over the past half-century, and particularly the last two decades, new paradigms, perspectives and technological capabilities have greatly advanced our understanding of open-ocean pelagic ecosystems. Major new insights have come from the microbial loop concept and related discoveries, the iron limitation hypothesis and ocean time series. Focusing mainly on the tropical and subtropical Pacific Ocean, I review the influences of these new perspectives on classical views of food web complexity, phytoplankton regulation and diversity, and temporal dynamics.

Liu, H, Campbell L, Landry MR, Nolla HA, Brown SL, Constantinou J.  1998.  Prochlorococcus and Synechococcus growth rates and contributions to production in the Arabian Sea during the 1995 Southwest and Northeast Monsoons. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 45:2327-2352.   10.1016/s0967-0645(98)00073-3   AbstractWebsite

We investigated the growth rates of Prochlorococcus and Synechococcus spp. and their relative contributions to carbon production at five stations in the Arabian Sea during the late Southwest and early Northeast Monsoon seasons in 1995. Estimates of Prochlorococcus growth rates were based on diel cell cycle analysis. Accurate determination of the duration of the cell cycle terminal event, e.g., t(S +G2) was not possible for Synechococcus because of its highly variable cell cycle patterns (e.g., imperfect phasing, multiple DNA-replication peaks, and dark-arrested division). Consequently, growth and mortality rates of Synechococcus were estimated from diel variations in population abundance. The assumptions of this approach were validated by observations that Synechococcus cell division occurred only during the daytime as well as good agreement between growth rate estimates for Prochlorococcus compared to cell cycle analyses. Prochlorococcus growth rates were typically less than 1 doubling per day, although higher rates occurred in the surface waters at an offshore site (Stn. N7) during the SW Monsoon and at a coastal station (S2) during the NE Monsoon. For Synechococcus, maximum growth rates >2 d(-1) were observed at mesotrophic (nitrate concentration 0.1-3 mu m) onshore stations during both seasons. Synechococcus spp. grew much faster than Prochlorococcus in the upper water column at almost every station during both seasons, but the depth range of its maximum growth rate was shallower and its growth and abundance decreased sharply in deeper waters. In addition, growth rates of Synechococcus increased with nutrient availability whereas Prochlorococcus growth rates did not vary dramatically with nutrients. Although there was no significant difference in Synechococcus growth rates between the late SW and early NE Monsoon seasons, the estimated carbon production and relative contribution to primary production were greater during the early NE Monsoon due to the larger biomass of Synechococcus and lower total primary production. Maximum Prochlorococcus production was found only in the most oligotrophic regions, and Prochlorococcus was not a major contributor of primary production for the most part of the Arabian Sea during the SW and NE Monsoons. Overall, Prochlorococcus and Synechococcus were inversely related in terms of their relative contributions to primary production. (C) 1998 Elsevier Science Ltd. All rights reserved.

Landry, MR, Barber RT, Bidigare RR, Chai F, Coale KH, Dam HG, Lewis MR, Lindley ST, McCarthy JJ, Roman MR, Stoecker DK, Verity PG, White JR.  1997.  Iron and grazing constraints on primary production in the central equatorial Pacific: An EqPac synthesis. Limnology and Oceanography. 42:405-418.   10.4319/lo.1997.42.3.0405   AbstractWebsite

Recent studies in the central equatorial Pacific allow a comprehensive assessment of phytoplankton regulation in a high-nutrient, low-chlorophyll (HNLC) ecosystem. Elemental iron enters the euphotic zone principally via upwelling and is present at concentrations (less than or equal to 30 pM) wen below the estimated half-saturation constant (120 pM) for the large cells that bloom with iron enrichment. In addition, the meridional trend in quantum yield of photosynthesis suggests that even the dominant small phytoplankton are held below their physiological potential by iron deficiency. Grazing by microzooplankton dominates phytoplankton losses, accounting for virtually all of the measured phytoplankton production during El Nino conditions and similar to 66% during normal upwelling conditions, with mesozooplankton grazing and lateral advection closing the balance. Nitrate uptake is strongly correlated with the pigment biomass of diatoms, which increase in relative abundance during normal upwelling conditions. Nonetheless, the f-ratio remains low (0.07-0.12) under all conditions. Iron budgets are consistent with the notions that new production is determined by the rate of new iron input to the system while total production depends on efficient iron recycling by grazers. Although the limiting substrates differ, the interactions of resource limitation and grazing in HNLC regions are conceptually similar to the generally accepted view for oligotrophic subtropical regions. In both systems, small dominant phytoplankton grow at rapid, but usually less than physiologically maximal, rates; they are cropped to low stable abundances by microzooplankton; and their sustained high rates of growth depend on the remineralized by-products of grazing.