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Kenitz, KM, Visser AW, Ohman MD, Landry MR, Andersen KH.  2019.  Community trait distribution across environmental gradients. Ecosystems. 22:968-980.   10.1007/s10021-018-0314-5   AbstractWebsite

Variability in community composition is often attributed to underlying differences in physical environments. However, predator-prey interactions can play an equally important role in structuring communities. Although environmental differences select for different species assemblages, less is known about their impacts on trait compositions. We develop a trait-based analysis of plankton communities of the southern California Current System across multiple trophic levels, from bacteria to mesozooplankton, and over a gradient of environmental conditions, from the oligotrophic open ocean to coastal upwelling. Across a factor of four differences in total community biomass, we observe similarities in the size structure along the environmental gradient, with the most pronounced departures from proportional variations in the biomasses found in the largest protists (> 40 mu m). Differences in the trait distributions emerge within a small range of size groups: greater biomass contribution of larger autotrophs (> 10 mu m) is observed only for the upwelling region.

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.

Valencia, B, Landry MR, Decima M, Hannides CCS.  2016.  Environmental drivers of mesozooplankton biomass variability in the North Pacific Subtropical Gyre. Journal of Geophysical Research-Biogeosciences. 121:3131-3143.   10.1002/2016jg003544   AbstractWebsite

The environmental drivers of zooplankton variability are poorly explored for the central subtropical Pacific, where a direct bottom-up food-web connection is suggested by increasing trends in primary production and mesozooplankton biomass at station ALOHA (A Long-term Oligotrophic Habitat Assessment) over the past 20 years (1994-2013). Here we use generalized additive models (GAMs) to investigate how these trends relate to the major modes of North Pacific climate variability. A GAM based on monthly mean data explains 43% of the temporal variability in mesozooplankton biomass with significant influences from primary productivity (PP), sea surface temperature (SST), North Pacific Gyre Oscillation (NPGO), and El Nino. This result mainly reflects the seasonal plankton cycle at station ALOHA, in which increasing light and SST lead to enhanced nitrogen fixation, productivity, and zooplankton biomass during summertime. Based on annual mean data, GAMs for two variables suggest that PP and 3-4 year lagged NPGO individually account for similar to 40% of zooplankton variability. The full annual mean GAM explains 70% of variability of zooplankton biomass with significant influences from PP, 4 year lagged NPGO, and 4 year lagged Pacific Decadal Oscillation (PDO). The NPGO affects wind stress, sea surface height, and subtropical gyre circulation and has been linked to mideuphotic zone anomalies in salinity and PP at station ALOHA. Our study broadens the known impact of this climate mode on plankton dynamics in the North Pacific. While lagged transport effects are also evident for subtropical waters, our study highlights a strong coupling between zooplankton fluctuations and PP, which differs from the transport-dominated climate influences that have been found for North Pacific boundary currents.

Baines, SB, Chen X, Twining BS, Fisher NS, Landry MR.  2016.  Factors affecting Fe and Zn contents of mesozooplankton from the Costa Rica Dome. Journal of Plankton Research. 38:331-347.   10.1093/plankt/fbv098   AbstractWebsite

Mineral limitation of mesozooplankton production is possible in waters with low trace metal availability. As a step toward estimating mesozooplankton Fe and Zn requirements under such conditions, we measured tissue concentrations of major and trace nutrient elements within size-fractioned zooplankton samples collected in and around the Costa Rica Upwelling Dome, a region where phytoplankton growth may be co-limited by Zn and Fe. The geometric mean C, N, P contents were 27, 5.6 and 0.21 mmol gdw(-1), respectively. The values for Fe and Zn were 1230 and 498 nmol gdw(-1), respectively, which are low compared with previous measurements. Migrant zooplankton caused C and P contents of the 2-5 mm fraction to increase at night relative to the day while the Fe and Zn contents decreased. Fe content increased with size while Zn content decreased with size. Fe content was strongly correlated to concentrations of two lithogenic tracers, Al and Ti. We estimate minimum Fe: C ratios in large migrant and resident mixed layer zooplankton to be 15 and 60 mu mol mol(-1), respectively. The ratio of Zn: C ranged from 11 mu mol mol(-1) for the 0.2-0.5 mm size fraction to 33 mu mol mol(-1) for the 2-5 mm size fraction.

Decima, M, Landry MR, Stukel MR, Lopez-Lopez L, Krause JW.  2016.  Mesozooplankton biomass and grazing in the Costa Rica Dome: amplifying variability through the plankton food web. Journal of Plankton Research. 38:317-330.   10.1093/plankt/fbv091   AbstractWebsite

We investigated standing stocks and grazing rates of mesozooplankton assemblages in the Costa Rica Dome (CRD), an open-ocean upwelling ecosystem in the eastern tropical Pacific. While phytoplankton biomass in the CRD is dominated by picophytoplankton (<2-mu m cells) with especially high concentrations of Synechococcus spp., we found high mesozooplankton biomass (similar to 5 g dry weight m(-2)) and grazing impact (12-50% integrated water column chlorophyll a), indicative of efficient food web transfer from primary producers to higher levels. In contrast to the relative uniformity in water-column chlorophyll a and mesozooplankton biomass, variability in herbivory was substantial, with lower rates in the central dome region and higher rates in areas offset from the dome center. While grazing rates were unrelated to total phytoplankton, correlations with cyanobacteria (negative) and biogenic SiO2 production (positive) suggest that partitioning of primary production among phytoplankton sizes contributes to the variability observed in mesozooplankton metrics. We propose that advection of upwelled waters away from the dome center is accompanied by changes in mesozooplankton composition and grazing rates, reflecting small changes within the primary producers. Small changes within the phytoplankton community resulting in large changes in the mesozooplankton suggest that the variability in lower trophic level dynamics was effectively amplified through the food web.

Stukel, MR, Kahru M, Benitez-Nelson CR, Decima M, Goericke R, Landry MR, Ohman MD.  2015.  Using Lagrangian-based process studies to test satellite algorithms of vertical carbon flux in the eastern North Pacific Ocean. Journal of Geophysical Research-Oceans. 120:7208-7222.   10.1002/2015jc011264   AbstractWebsite

The biological carbon pump is responsible for the transport of similar to 5-20 Pg C yr(-1) from the surface into the deep ocean but its variability is poorly understood due to an incomplete mechanistic understanding of the complex underlying planktonic processes. In fact, algorithms designed to estimate carbon export from satellite products incorporate fundamentally different assumptions about the relationships between plankton biomass, productivity, and export efficiency. To test the alternate formulations of export efficiency in remote-sensing algorithms formulated by Dunne et al. (2005), Laws et al. (2011), Henson et al. (2011), and Siegel et al. (2014), we have compiled in situ measurements (temperature, chlorophyll, primary production, phytoplankton biomass and size structure, grazing rates, net chlorophyll change, and carbon export) made during Lagrangian process studies on seven cruises in the California Current Ecosystem and Costa Rica Dome. A food-web based approach formulated by Siegel et al. (2014) performs as well or better than other empirical formulations, while simultaneously providing reasonable estimates of protozoan and mesozooplankton grazing rates. By tuning the Siegel et al. (2014) algorithm to match in situ grazing rates more accurately, we also obtain better in situ carbon export measurements. Adequate representations of food-web relationships and grazing dynamics are therefore crucial to improving the accuracy of export predictions made from satellite-derived products. Nevertheless, considerable unexplained variance in export remains and must be explored before we can reliably use remote sensing products to assess the impact of climate change on biologically mediated carbon sequestration.

Gutierrez-Rodriguez, A, Decima M, Popp BN, Landry MR.  2014.  Isotopic invisibility of protozoan trophic steps in marine food webs. Limnology and Oceanography. 59:1590-1598.   10.4319/lo.2014.59.5.1590   AbstractWebsite

According to modern oceanographic perspectives that emphasize microbial pathways, phagotrophic protists comprise one to several levels of intermediate consumers between phytoplankton and larger metazooplankton (copepods and krill). However, recent attempts to quantify pelagic trophic structure in the open ocean using nitrogen stable isotope techniques have brought into question whether such measurements adequately account for protistan trophic steps. Here, we use a two-stage chemostat system, with Dunaliella tertiolecta and Oxyrrhis marina as a predator-prey model, to address this question experimentally. To investigate N-15 trophic discrimination under different conditions of nitrogen availability and recycling, Oxyrrhis was fed in the light and in the dark on phytoplankton provided with high and low nutrient ratios of N : P. We used both bulk and amino acids-compound specific isotopic analysis (AA CSIA) to distinguish trophic fractionation from changes in the delta N-15 values of phytoplankton (isotopic baseline). Results demonstrate that protistan consumers are not, in fact, significantly enriched in N-15 relative to their prey, a marked departure from the general findings for metazoan consumers. In addition, we show that changes in the isotopic baseline propagate rapidly through the protistan food chain, highlighting the need to account for this variability at ecologically relevant time scales. If protistan trophic steps are largely invisible or significantly underestimated using nitrogen isotope measurements, research that utilize such measurements in ecological, fisheries, and climate change studies may miss a large part of the ocean's variability in food-web structure and ecosystem function.

Ohman, MD, Barbeau K, Franks PJS, Goericke R, Landry MR, Miller AJ.  2013.  Ecological transitions in a coastal upwelling ecosystem. Oceanography. 26:210-219. AbstractWebsite

The southern California Current Ecosystem (CCE) is a dynamic eastern boundary current ecosystem that is forced by ocean-atmosphere variability on interannual, multidecadal, and long-term secular time scales. Recent evidence suggests that apparent abrupt transitions in ecosystem conditions reflect linear tracking of the physical environment rather than oscillations between alternative preferred states. A space-for-time exchange is one approach that permits use of natural spatial variability in the CCE to develop a mechanistic understanding needed to project future temporal changes. The role of (sub)mesoscale frontal systems in altering rates of nutrient transport, primary and secondary production, export fluxes, and the rates of encounters between predators and prey is an issue central to this pelagic ecosystem and its future trajectory because the occurrence of such frontal features is increasing.

Landry, MR, Ohman MD, Goericke R, Stukel MR, Barbeau KA, Bundy R, Kahru M.  2012.  Pelagic community responses to a deep-water front in the California Current Ecosystem: overview of the A-Front Study. Journal of Plankton Research. 34:739-748.   10.1093/plankt/fbs025   AbstractWebsite

In October 2008, we investigated pelagic community composition and biomass, from bacteria to fish, across a sharp frontal gradient overlying deep waters south of Point Conception, California. This northsouth gradient, which we called A-Front, was formed by the eastward flow of the California Current and separated cooler mesotrophic waters of coastal upwelling origin to the north, from warm oligotrophic waters of likely mixed subarcticsubtropical origin to the south. Plankton biomass and phytoplankton growth rates were two to three times greater on the northern side, and primary production rates were elevated 5-fold to the north. Compared with either of the adjacent waters, the frontal interface was strongly enriched and uniquely defined by a subsurface bloom of large diatoms, elevated concentrations of suspension-feeding zooplankton, high bioacoustical estimates of pelagic fish and enhanced bacterial production and phytoplankton biomass and photosynthetic potential. Such habitats, though small in areal extent, may contribute disproportionately and importantly to regional productivity, nutrient cycling, carbon fluxes and trophic ecology. As a general introduction to the A-Front study, we provide an overview of its design and implementation, a brief summary of major findings and a discussion of potential mechanisms of plankton enrichment at the front.

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.

Chen, BZ, Landry MR, Huang BQ, Liu HB.  2012.  Does warming enhance the effect of microzooplankton grazing on marine phytoplankton in the ocean? Limnology and Oceanography. 57:519-526.   10.4319/lo.2012.57.2.0519   AbstractWebsite

We evaluated a hypothesis derived from the metabolic theory of ecology (MTE) that the ratio of microzooplankton herbivory (m) to phytoplankton growth (mu) will arise in a warming ocean because of the different temperature dependencies of autotrophic and heterotrophic organisms. Using community-level growth and grazing data from dilution experiments, generalized additive models (GAMs) were constructed to describe the effects of temperature and chlorophyll on m: mu. At low chlorophyll levels, m: mu decreases with increasing temperature, whereas at high chlorophyll levels, m: mu increases initially with temperature before reaching a peak and then declines. These complex responses of m: mu result from mixed effects of temperature and chlorophyll on microzooplankton biomass (B-z), biomass-specific microzooplankton grazing rate (m: B-z), and phytoplankton growth rate (mu). B-z decreases with rising temperature and increases with rising chlorophyll. m: B-z increases with temperature and decreases with chlorophyll. Nutrient-enriched growth rate of phytoplankton (mu(n)) and mu increase with increasing temperature and chlorophyll. Holding chlorophyll constant, the calculated activation energies of m: B-z and mu(n) are 0.67 +/- 0.05 and 0.36 +/- 0.05 eV, respectively, both consistent with previous MTE estimates for heterotrophs and autotrophs. Our study indicates that warming may enhance phytoplankton losses to microzooplankton herbivory in eutrophic but not in oligotrophic waters. The GAM analysis also provides important insights into underlying system relationships and reasons why community-level responses in natural systems may depart from theory based on laboratory data and individual species.

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.

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.

Landry, MR, Ohman MD, Goericke R, Stukel MR, Tsyrklevich K.  2009.  Lagrangian studies of phytoplankton growth and grazing relationships in a coastal upwelling ecosystem off Southern California. Progress in Oceanography. 83:208-216.   10.1016/j.pocean.2009.07.026   AbstractWebsite

Experimental studies of phytoplankton growth and grazing processes were conducted in the coastal upwelling system off Point Conception, California to test the hypothesis that phytoplankton growth and grazing losses determine, to first order, the local dynamics of phytoplankton in the upwelling circulation. Eight experiments of 3-5 days each were conducted over the course of two cruises in May-June 2006 and April 2007 following the trajectories of satellite-tracked drifters. Rates of phytoplankton growth and microzooplankton grazing were determined by daily in situ dilution incubations at 8 depths spanning the euphotic zone. Mesozooplankton grazing was assessed by gut fluorescence analysis of animals collected from net tows through the euphotic zone. We compared directly the net rates of change observed for the ambient phytoplankton community to the net growth rates predicted from experimental determinations of each process rate. The resulting relationship accounted for 91% of the variability observed, providing strong support for the growth-grazing hypothesis. In addition, grazing by mesozooplankton was unexpectedly high and variable, driving a substantial positive to negative shift in phytoplankton net rate of change between years despite comparable environmental conditions and similar high growth rates and suggesting strong top-down control potential. The demonstrated agreement between net ambient and experimental community changes is an important point of validation for using field data to parameterize models. Data sets of this type may provide an important source of new information and rate constraints for developing better coupled biological-physical models of upwelling system dynamics. (C) 2009 Elsevier Ltd. All rights reserved.

Landry, MR.  2009.  Grazing processes and secondary production in the Arabian Sea: A simple food web synthesis with measurement constraints. Indian ocean biogeochemical processes and ecological variability. ( Wiggert JD, Hood RR, Naqvi SWA, Brink KH, Smith SL, Eds.).:133-146.: American Geophysical Union, Washington, DC (USA)   10.1029/2008GM000781   Abstract

The Joint Global Ocean Flux Study in the Arabian Sea during the mid 1990s provides a rare opportunity to elucidate carbon flows in the lower food web of an open ocean ecosystem. Analysis of that data to date has, however, produced widely divergent perspectives on major flux pathways and roles of zooplankton: from zooplankton as controlling grazers tightly coupled to microbial processes to zooplankton as casual consumers who let a large fraction of production, mostly generated by picophytoplankton, flow directly to detritus and export. Synthesis of experimental grazing rates and production inferences for mesozooplankton and microzooplankton fit well in a conceptually simple food web, constrained by measured carbon flows through phytoplankton and bacteria. Microzooplankton dominate grazing processes, consuming over 70% of particulate primary production (PP), on average, and providing steady and significant supplemental nutrition to mesozooplankton. Direct grazing estimates of mesozooplankton, on the order of 25% of PP, are sufficient to balance the remaining particulate production, with additional transfer through a one- to two-step food chain of microzooplankton accounting for a total ingestion of 6 40% of PP required for mesozooplankton secondary production. Dissolved organic carbon fluxes to bacteria are provided mostly within the constraints of gross and net primary production. Contradictory results from inverse models are likely due to an assumption that exaggerates by approximately twofold the production contribution of picophytoplankton and to the failure to use measured rates of gross primary production as a system constraint. Grazing generally balances net particulate primary production in the Arabian Sea, but true grazer control of phytoplankton dynamics remains an open issue for further study.

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.

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.

Sheridan, CC, Landry MR.  2004.  A 9-year increasing trend in mesozooplankton biomass at the Hawaii Ocean Time-Series Station ALOHA. Ices Journal of Marine Science. 61:457-463.   10.1016/j.icesjm.2004.03.023   AbstractWebsite

Mesozooplankton biomass in the North Pacific Subtropical Gyre (NPSG), as measured by the Hawaii Ocean Time-series pro-ram at Station ALOHA (22.45degreesN 158degreesW), increased significantly from 1994 to 2002. Tide changes occurred at a rate of 60 mg DW m(-2) yr(-1) for night-time collections and 45 mg DW m(-2) yr-1 for daytime collections. Principal components analysis indicates that the 9-year trend was driven by an increase in small (0.5-2.0 mm) zooplankton that do not migrate on a diel cycle. This plankton class is known to increase during the summer at Station ALOHA when the water column is most stratified, and a strong summertime response is also apparent within the long-term trend from 1998 through 2002. Both long-term and seasonal changes in zooplankton biomass at Station ALOHA can be linked to an enhanced role of nitrogen fixation in ecosystem productivity. Climate forcing from El Nino-Southern Oscillation (ENSO) events may have influenced nitrogen fixation, general ecosystem productivity, and thus zooplankton biomass in the NPSG. However. it is difficult to evaluate the effect of climate cycles in this region without the benefit of a longer time-series at Station ALOHA. Because biomass trends in higher-level consumers like mesozooplankton can have cascading influences on lower levels, understanding the relative roles of bottom-up climate influences and top-down trophic processes will be important in resolving long-term trends in community composition and structure in the subtropical North Pacific Ocean. (C) 2004 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.

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.

Gaudy, R, Le Borgne R, Landry MR, Champalbert G.  2004.  Biomass, feeding and metabolism of mesozooplankton in the equatorial Pacific along 180 degrees. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 51:629-645.   10.1016/j.dsr2.2004.05.004   AbstractWebsite

Mesozooplankton biomasses, grazing rates, and metabolism (respiration, ammonium and phosphate excretion) were measured in the central Pacific (180; 8degreesS-8degreesN) during the EBENE cruise, in October-November 1996. In the HNLC zone, the highest mesozooplankton biomass occurred between 6degreesS and the equator. The decrease of biomass at the northern part of the HNLC zone was accounted to the passage of a tropical instability wave, carrying poorer zooplankton concentrations from the north. Within the HNLC zone, most of the biomass (60-96%) was found above 100 m in accordance with the upper distribution of phytoplankton and heterotrophic protists. No significant day-night differences were observed in mesozooplankton biomass. These results suggest a strong and permanent coupling between mesozooplankton and the first links of the food chain. Respiration and phosphate excretion rates were steady at the different latitudes, but at the equator, grazing rates were the lowest, and ammonium excretion the highest. These variations were accounted to latitudinal differences in microzooplankton density and in particles C/N ratios, suggesting that more nitrogen was available for food at the equator. Moreover, among the dominant copepod species, higher proportions of carnivorous were found at the equator. Higher particle grazing and ammonium excretion rates occurred at the equator during the day, while the chlorophyll gut content of copepods did not show any significant diel cycle. These results suggest a strong trophic link between mesozooplankton feeding activity, on the one hand, and cyclic variations of phytoplankton production and of of their main predators (microzooplankton protists), on the other hand. From biomass estimations of phytoplankton and protists in the euphotic layer and results of dilution experiments, the production and grazing mortality of the main autotrophic taxa (from picoplankton to microplankton) were computed at the equator. Mesozooplankton, which was mainly involved in the catch of the largest (>8 mum) food particles, such as diatoms or dinoflagellates, accounted for only a minor part (22%) in total phytoplankton grazing, compared to microzooplankton. As the chlorophyll ration was insufficient to balance its metabolism expenditures, mesozooplankton had to to complete its diet with other sources of food such as microzooplankton. The low level of zooplankton metabolic O:N ratios, suggesting the use of a food with a large protein content, supports of this hypothesis. (C) 2004 Elsevier Ltd. All rights reserved.

Piontkovski, SA, Landry MR.  2003.  Copepod species diversity and climate variability in the tropical Atlantic Ocean. Fisheries Oceanography. 12:352-359.   10.1046/j.1365-2419.2003.00250.x   AbstractWebsite

A database synthesized from 19 oceanographic expeditions conducted by the former Soviet Union was used to analyse interannual patterns in copepod species diversity in the tropical Atlantic. Mesozooplankton was collected predominately in vertical hauls through the upper 100 m with Juday nets. The samples from 744 oceanographic stations were identified and enumerated to the species level. To assess species diversity, the Shannon diversity index was used. On the interdecadal scale, no statistically confirmed trend was found in species diversity change over the years sampled ( 1963 - 89). Multiple regression analysis indicated that interannual fluctuations of the South Atlantic High ( pressure and latitude), the Azores High longitude and El Nino-Southern Oscillation (ENSO) index could explain 87% of species diversity fluctuations. Possible mechanisms that drive interannual fluctuations of species diversity are discussed.

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.

Piontkovski, SA, Landry MR, Finenko ZZ, Kovalev AV, Williams R, Gallienne CP, Mishonov AV, Skryabin VA, Tokarev YN, Nikolsy VN.  2003.  Plankton communities of the South Atlantic anticyclonic gyre. Oceanologica Acta. 26:255-268.   10.1016/s0399-1784(03)00014-8   AbstractWebsite

Data collected during cruises of the Former Soviet Union (in 1963-1989) and the British Atlantic meridional transect program (in 1995-1999) were used to analyse macroscale patterns in phyto- and zooplankton biomass, size structure, species diversity, chlorophyll a, and plankton bioluminescence in the macroscale anticyclonic gyre of the South Atlantic Ocean. The spatial pattern of bioluminescence intensity was in good agreement with that of remotely sensed (CZCS) chlorophyll a, phosphate, salinity, and copepod species diversity index distributions especially in terms of geographic inclinations of the isolines, both associated with the north-westward pattern off the South equatorial current. Among the 416 copepod species recorded in samples, 51 species were noted throughout the whole gyre. On the other hand, there were a number of species found only in one of the currents. The mesozooplankton biomass size spectra (calculated in carbon units), exhibited a fairly stable slope of the curve from the eastern periphery of the gyre to its centre. The British Atlantic meridional transect program meridional transect through the western part of the gyre showed mesozooplankton size spectra in greater detail between the equator and 50degrees S. Although the spectra change slowly along the transect as far as 36degrees S, there is a general trend toward increasing slopes from the equatorial region to the oligotrophic central gyre. The calculated phyto-to-zooplankton ratio indicated that for the tropical anticyclonic gyres, the mesozooplankton carbon biomass could be represented as the exponential function of the phytoplankton carbon. (C) 2003 Editions scientifiques et medicales Elsevier SAS and Ifremer/CNRS/IRD. All rights reserved.

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.