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Coale, KH, Johnson KS, Fitzwater SE, Gordon RM, Tanner S, Chavez FP, Ferioli L, Sakamoto C, Rogers P, Millero F, Steinberg P, Nightingale P, Cooper D, Cochlan WP, Landry MR, Constantinou J, Rollwagen G, Trasvina A, Kudela R.  1996.  A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Nature. 383:495-501.   10.1038/383495a0   AbstractWebsite

The seeding of an expanse of surface waters in the equatorial Pacific Ocean with low concentrations of dissolved iron triggered a massive phytoplankton bloom which consumed large quantities of carbon dioxide and nitrate that these microscopic plants cannot fully utilize under natural conditions. These and other observations provide unequivocal support for the hypothesis that phytoplankton growth in this oceanic region is limited by iron bioavailability.

Benitez-Nelson, CR, Bidigare RR, Dickey TD, Landry MR, Leonard CL, Brown SL, Nencioli F, Rii YM, Maiti K, Becker JW, Bibby TS, Black W, Cai WJ, Carlson CA, Chen FZ, Kuwahara VS, Mahaffey C, McAndrew PM, Quay PD, Rappe MS, Selph KE, Simmons MP, Yang EJ.  2007.  Mesoscale eddies drive increased silica export in the subtropical Pacific Ocean. Science. 316:1017-1021.   10.1126/science.1136221   AbstractWebsite

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawaii. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.

Stukel, MR, Aluwihare LI, Barbeau KA, Chekalyuk AM, Goericke R, Miller AJ, Ohman MD, Ruacho A, Song H, Stephens BM, Landry MR.  2017.  Mesoscale ocean fronts enhance carbon export due to gravitational sinking and subduction. Proceedings of the National Academy of Sciences of the United States of America. 114:1252-1257.   10.1073/pnas.1609435114   AbstractWebsite

Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from U-238:Th-234 disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg C.m(-2).d(-1)) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front wasmechanistically linked to Fe-stressed diatoms and high-mesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional similar to 225 mg C.m(-2).d(-1) was exported as subduction of particle-rich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems.

Brown, SL, Landry MR.  2001.  Mesoscale variability in biological community structure and biomass in the Antarctic Polar Front region at 170 degrees W during austral spring 1997. Journal of Geophysical Research-Oceans. 106:13917-13930.   10.1029/1999jc000188   AbstractWebsite

The influence of the Antarctic Polar Front (PF) on microbial biomass and community structure was investigated during late spring, October - November 1997, as part of the U.S. Joint Global Ocean Flux Study Antarctic Environment and Southern Ocean Process Study. In conjunction with SeaSoar sampling, samples for flow cytometry and epifluorescence image analysis were collected across the PF region along a 170 degreesW transect and in two maps involving repeated crossings of the front. Phytoplankton abundance and size estimates clearly showed the influence of the front, with smaller, more numerous cells to the north and larger, less abundant cells to the south. Autotrophic biomass varied substantially across the region, ranging from 8 to 102 mug C L-1 Biomass accumulation, dominated by Phaeocystis spp. and Chaetoceros spp., was particularly apparent in discrete areas downstream of a frontal meander feature. Grazer biomass, ranging from 1 to 31 mug C L-1, was usually much less than 50% of phytoplankton biomass and did not show any spatial trends with regard to the PF. The distribution of heterotrophic bacteria was dearly influenced by the PF, with larger, less abundant cells south of the frontal zone. The developing assemblage of phytoplankton in the frontal meander was biologically distinct and spatially separated from the community sampled at the marginal ice zone. Analysis of phytoplankton biomass increases along PF current streamlines yielded net growth rates of similar to0.08 d(-1), pointing to in situ growth, rather than transport, as the primary mechanism for chlorophyll accumulation. The significance of the front on the development of the seasonal phytoplankton increase is evident, yet the spatial heterogeneity of the microbial assemblage indicates a complex physical environment with multiple mesoscale influences.

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.

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.

Landry, MR, Peterson WK, Fagerness VL.  1994.  Mesozooplankton grazing in the Southern California Bight .I. Population abundances and gut pigment contents. Marine Ecology-Progress Series. 115:55-71.   10.3354/meps115055   AbstractWebsite

Gut pigment content of the mesozooplankton community in Santa Monica Basin, California, USA was examined on 3 winter-spring and 3 autumn cruises between October 1985 and February 1988. Mean chlorophyll concentrations for the upper 70 m varied from 0.30 to 0.32 mg m(-3) for autumn cruises and from 0.35 to 1.7 mg m(-3) for winter-spring cruises. Larger crustacean zooplankton species, particularly the copepod Calanus pacificus, were more abundant in the winter-spring, and this trend was also evident in size structure of pelagic tunicates. Gut pigments of larger, migratory taxa (C. pacificus, Metridia lucens, Pleuromamma spp., large euphausiids, and ostracods) and often some of the smaller, non-migratory forms (Clausocalanus spp., Acartia spp., and appendicularians) indicated diel periodicity in feeding intensity with highest gut pigments generally at night or in the early morning. Feeding periodicity was weakest when chlorophyll concentration and mean gut pigment content (ng chl equ. ind.(-1)) was highest in the spring. The night:day ratio of community gut pigment (CGP) varied from 1.02 to 1.95 for the winter-spring period and from 1.77 to 3.39 for the autumn period. Migrating taxa explained most of the day-night difference in the winter-spring hut relatively little of the difference in the autumn. Small species and developmental stages (<1.5 mm body length) dominated daytime (95.1 to 99.8%) and nighttime (63.6 to 96.2%) CGP during the autumn and were important, though relatively less so, in the winter-spring cruises (day 58.2 to 76.9%, night 37.7 to 53.7%). Copepods accounted for the majority of CGP in May 1986 and April 1987. Thaliaceans dominated in February 1988 and were generally more important in winter-spring than autumn, as were euphausiids. Pteropods made a significant contribution to CGP in October 1985. Appendicularians (Oikopleura sp.) were major grazers during all cruises, but their contribution to CGP surpassed that of copepods on the 3 autumn cruises.

Landry, MR, Lorenzen CJ, Peterson WK.  1994.  Mesozooplankton grazing in the Southern California Bight .II. Grazing impact and particulate flux. Marine Ecology-Progress Series. 115:73-85.   10.3354/meps115073   AbstractWebsite

Mesozooplankton grazing on phytoplankton, as inferred from gut pigment contents and gut evacuation rates, was studied in relation to primary production and particulate export flux on 6 cruises in the Santa Monica Basin, California, USA. Gut evacuation rates did not vary significantly among different taxa or size classes examined and were consistent with extrapolations of published temperature relationships. Shipboard incubations with cultured phytoplankton and net-collected zooplankton indicated a seasonal difference in the extent to which gut passage converts chlorophyll to non-fluorescent by-products. In autumn experiments, only about 5% of ingested chlorophyll could not be recovered as phaeopigment. In winter-spring experiments, approximately 70% of ingested chlorophyll (chl) was destroyed. In contrast, other indices of pigment destruction, the ingestion rates of a dominant copepod species and the ratio of water-column phaeopigment:silica fluxes, did not reveal a significant gut passage effect during winter-spring cruises. Mesozooplankton community grazing impact varied from 1.7 to 7.3 mg chi m(-2) d(-1), with higher grazing during the winter-spring period (mean = 5.8 mg chi m(-2) d(-1)) as compared to the autumn (mean = 2.3 mg chi m(-2) d(-1)). On average, mesozooplankton grazing accounted for a loss of 11.7% of chlorophyll standing stock d(-1) with a 6 cruise range of 6 to 18% d(-1). Mesozooplankton grazing on phytoplankton accounted for 29 to 44% (mean = 39%) of measured primary production for the winter-spring cruises, but only 16 to 24% (mean = 19%) of production in the autumn. From measured phaeopigment fluxes into sediment traps below the euphotic zone, only 27.5% (range 23 to 32%) of this grazing on phytoplankton could be accounted for as export flux. Thus, in terms of contribution to particulate flux or remineralization, most mesozooplankton grazing in the Santa Monica Basin was functionally equivalent to that of microzooplankton. Direct grazing on phytoplankton contributed 15 to 38% of carbon flux into sediment traps during winter-spring and 8 to 13% during autumn. Nonetheless, if feeding on nonpigmented prey is considered from the available information on carbon:phaeopigment ratios of fresh fecal pellets, over 70% of the carbon flux to traps could have a mesozooplankton grazing origin.

Calbet, A, Landry MR.  1999.  Mesozooplankton influences on the microbial food web: Direct and indirect trophic interactions in the oligotrophic open ocean. Limnology and Oceanography. 44:1370-1380.   10.4319/lo.1999.44.6.1370   AbstractWebsite

The phytoplankton in warm oligotrophic regions of the open oceans is dominated by <2-mu m cells too small for efficient direct consumption by mesozooplankton. However, these primary producers are hypothetically linked to higher trophic levels via the cascading impacts of mesozooplankton grazing on intermediate consumers. To assess the magnitudes of these indirect trophic linkages, grazing experiments, involving different concentration treatments of the mixed mesozooplankton community, were performed during cruises in the subtropical North Pacific at station ALOHA. Mesozooplankton fed on a diverse assemblage of microzooplankton and nanoheterotrophs >5 mu m, and their predation indirectly enhanced net growth rates of phytoplankton and 2-5-mu m heterotrophs. Increasing the concentration of mesozooplankton also enhanced growth rates of heterotrophic bacteria, but this was more likely the result of organic enrichment than trophic transfer. Scaled to their natural abundance, the indirect grazing impacts of mesozooplankton on lower trophic levels are small, accounting for <0.005 d(-1) of the growth rates of each prey category examined. Thus, the larger consumers appear to exert little net influence on the dynamics at the base of the food web. Ln contrast, size-fraction manipulations of consumers between 2 and 20 mu m (i.e., the nanozooplankton) elicited strong responses among bacterial populations indicative of tightly coupled predatory chain of at least two steps. Given the present results, detailed studies of the interactions among pico- and nanoplankton appear to be the most profitable avenue for improving our understanding of community structure and function in this region and for acquiring useful data for developing and validating ecosystem models of the open oceans.

Landry, MR.  1994.  Methods and controls for measuring the grazing impact of planktonic protists. Mar. Microb. Food Webs. 8( Arndt H, Landry MR, Eds.).:37-57. AbstractWebsite

Although pelagic protists are mow recognized as important, if not dominant, consumers in many aquatic ecosystems, quantifying their grazing rates and trophic interactions continues to be problematic. Of the various approaches that have been used to assess protistan grazing impacts in natural communities, each has specific advantages as well as specific weakness and ambiguities which preclude application under all conditions. At least some of these methods should be viewed, not as alternatives, but as true complements--each to be used in a manner consistent with its strengths and each testing and supporting the others in the areas in which they overlap. "Hybrid" techniques are advanced as a way to test or control for critical methodological assumptions and to provide a common reference for comparing the results of different approaches.

Dupouy, C, Loisel H, Neveux J, Brown SL, Moulin C, Blanchot J, Le Bouteiller A, Landry MR.  2003.  Microbial absorption and backscattering coefficients from in situ and POLDER satellite data during an El Nino-Southern Oscillation cold phase in the equatorial Pacific (180 degrees). Journal of Geophysical Research-Oceans. 108   10.1029/2001jc001298   AbstractWebsite

Contributions of different microbial groups to absorption, backscattering, and marine reflectance (a(lambda), b(b)(lambda) and R(lambda), respectively) were quantified during an El Nino-Southern Oscillation cold phase in the equatorial Pacific during the Etude du Broutage en Zone Equatoriale cruise on board the R/V L'Atalante. In situ data were collected at every degree of latitude from 8degreesS to 8degreesN, 180degrees (26 October to 13 November 1996), and satellite reflectances were available from POLDER-ADEOS for the 1-10 November 1996 decade. Bulk absorption and backscattering coefficients were estimated at 440 nm for the major microbial groups enumerated in the upper surface layer (heterotrophic bacteria, Prochlorococcus, Synechococcus, and <20-μm eukaryotic algae). Total absorption and backscattering coefficients were retrieved from space by a new inverse method. The observed ecosystem was typical of a well-developed equatorial upwelling, with maximal values of 0.4 mg m(-3) for Tchl a, 0.026 m(-1) for a(p)(440), 0.023 m(-1) for a(phy)(440), and a low in situ a(det) (<14% of a(p)). Prochlorococcus and nanoeukaryotic algae (3.4-mum mean diameter) were the dominant absorbers (97%), contributing about equally to a(phy). The retrieved total absorption coefficient, a(sat)(440), from POLDER (maximum of 0.03 m(-1)) was higher than a(p)(440), as it included absorption by CDOM (estimated to be 15% of a(p)(440) + a(w)(440), where a(w) = absorption by pure water). Heterotrophic bacteria were the dominant contributors (73%) to total simulated microbial backscattering, b(bmic) (maximum = 3.7 x 10(-4) m(-1)), but b(bmic) was negligible compared to the inverted total backscattering by particles, b(bp) (2.7 x 10(-3) m(-1)), indicating that unidentified small nonliving particles contributed most to the satellite signal.

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.

Selph, KE, Landry MR, Allen CB, Calbet A, Christensen S, Bidigare RR.  2001.  Microbial community composition and growth dynamics in the Antarctic Polar Front and seasonal ice zone during late spring 1997. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 48:4059-4080.   10.1016/s0967-0645(01)00077-7   AbstractWebsite

The microbial community between the Antarctic Polar Front region (APFr) and the seasonal ice edge at 170 degreesW was examined as part of the US JGOFS AESOPS Process I Study during late spring (December) 1997. Water-mass identities of the sampled stations were determined based on physical properties, with three regions identified: the APFr. the South ACC region (SACCr), and the southern region of the ACC near the ice edge (SACCr-IE). The APFr (60.1-60.5 degreesS) was dominated by centric diatoms, with mixed-layer communities tending toward single genera (i.e., Chaetoceros and Corethron). Stations in the SACCr (61.4-63.0 degreesS) were also characterized by large centric diatoms, but no single genus dominated. In contrast, the SACCr-IE stations (64.1-64.4 degreesS), positioned near the retreating ice edge, were dominated by colony-forming Phaeocystis. A seasonal succession in phytoplankton is proposed, with the youngest communities near the ice edge and the most "mature" in the polar front region. We found an inverse relationship between chlorophyll a values and phytoplankton growth rates. Phytoplankton growth rates were the highest (up to 1 d(-1)) in the APFr, despite relatively low chlorophyll a values. Microzooplankton grazing accounted for similar to 50% of phytoplankton growth. The SACCr stations had intermediate chlorophyll a concentrations and phytoplankton growth rates (0.5-0.9 d(-1)), with grazing consuming similar to 50-70% of growth. The SACCr-IE stations had the highest chlorophyll a values and the lowest phytoplankton growth rates (0.2 d(-1)) of the transect, while grazing accounted for similar to 70% of growth in the one dilution experiment that gave a significant result. Given these growth and grazing estimates, a higher proportion of production in the northern diatom-dominated regions was available for export than in the Phaeocystis-dominated ice edge stations. However, since microzooplankton produce small fecal debris, the bulk of the consumed production was more likely remineralized in the upper water column. (C) 2001 Elsevier Science Ltd. All rights reserved.

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

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

Brown, SL, Landry MR, Christensen S, Garrison D, Gowing MM, Bidigare RR, Campbell L.  2002.  Microbial community dynamics and taxon-specific phytoplankton production in the Arabian Sea during the 1995 monsoon seasons. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 49:2345-2376.   10.1016/s0967-0645(02)00040-1   AbstractWebsite

As part of the US JGOFS Arabian Sea Process Study in 1995, we investigated temporal and spatial patterns in microbial dynamics and production during the late Southwest (SW) Monsoon (August-September 1995) and the early Northeast (NE) Monsoon (November-December 1995) seasons using the seawater-dilution technique. Experiments were coupled with population assessments from high-performance liquid chromatography, flow cytometry, and microscopy to estimate further taxon-specific phytoplankton growth, grazing and production. Dilution estimates of total primary production varied substantially, from 7 to 423 mug C l(-1) d(-1), and were generally in good agreement with rate estimates from 14 C-uptake incubations. Both primary production and secondary bacterial production were, on average, 2.5 x higher during the SW Monsoon than the NE Monsoon. Relative to the total community, photosynthetic prokaryotes contributed 23% and 53% of production during the SW and NE Monsoons, respectively. Prochlorococcus spp. production was well balanced by grazing losses, while > 50% of Synechococcus spp. production during the SW Monsoon appeared to escape grazing by protists. Diatoms comprised > 30% of primary production at a high biomass station during the SW Monsoon but < 30% at all stations during the NE Monsoon. Growth rates of Synechococcus spp. and diatoms appeared to be limited by inorganic nitrogen concentrations, while Prochlorococcus spp., dinoflagellates and Phaeocystis spp. were not. Losses to protistan grazing were strongly correlated with phytoplankton biomass and production. Despite sufficient prey levels, protistan biomass was modest and constant across the region during both seasons. Of the larger taxa, diatoms were grazed the least effectively with only 50% of daily production accounted for by protistan grazing. Combined estimates of protistan and mesozooplankton grazing at upwelling stations during the SW Monsoon leave similar to10% of primary production unaccounted for and available for sinking and/or lateral advection. Similarly high rates of net production at northern coastal stations during the NE Monsoon suggest that this area also may contribute to regional export flux. (C) 2002 Published by Elsevier Science Ltd.

Brown, SL, Landry MR.  2001.  Microbial community structure and biomass in surface waters during a Polar Front summer bloom along 170 degrees W. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 48:4039-4058.   10.1016/s0967-0645(01)00080-7   AbstractWebsite

As part of the US Joint Global Ocean Flux Study (US JGOFS) Southern Ocean Program, flow cytometry and epifluorescent microscopy were utilized to determine abundance, distribution and size structure of the microbial community in the Polar Front region during the summer biomass maximum. Surface samples were collected approximately every 10 km along 170 degreesW during two N-S transects, separated in time by two weeks. Phytoplankton abundance and size structure varied with distinct latitudinal trends. Autotrophic biomass was lowest north of the Polar Front reflecting the dominance of small cells. The highest biomass (170 mug Cl-1) occurred at 65 degreesS where the composition was strongly influenced by large centric diatoms. Farther south, the diatom community shifted to the dominance of smaller pennate diatoms. Total grazer biomass and size distributions followed similar patterns, ranging from 4 mug Cl-1 in the north to 52 mug Cl-1 in the south where larger (> 20 mum) grazers were more abundant, Heterotrophic bacteria varied over an order of magnitude in abundance across the study site, with size generally increasing from north to south. In the second transect. phytoplankton biomass at 65 degreesS was 50% lower, and grazer biomass and bacterial populations were slightly greater, indicating the decline of the bloom. The changes in biomass and community structure along 170 degreesW and the reduction of phytoplankton standing stock at 65 degreesS over time suggests adjacent, yet different, microbial systems in terms of carbon flux, spanning from primarily recycling to export-dominated. (C) 2001 Elsevier Science Ltd. All rights reserved.

Landry, MR, Kirchman DL.  2002.  Microbial community structure and variability in the tropical Pacific. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 49:2669-2693.   10.1016/s0967-0645(02)00053-x   AbstractWebsite

The spatially extensive tropical Pacific includes regions that are limited by macronutrients or iron, and is thus broadly representative of open-ocean systems in which microbial communities predominate. Despite strong physical forcing due to the El Nino-Southern Oscillation cycle and the local effects of tropical instability waves, microbial abundances from a variety of JGOFS and related studies show similar, modest levels of variability in the high-nutrient, low-chlorophyll (HNLC) equatorial upwelling region, the oligotrophic, western Pacific Warm Pool, and the North Pacific central gyre. Mean 0-50m abundances of some of the groups distinguished by flow cytometry are significantly enhanced in the HNLC region, including heterotrophic bacteria (HBACT; 720,000 versus 440,000 cells ml(-1)), Synechococcus spp. (SYN; 9800 versus 2000 cells ml(-1)) and pico-eukaryotic algae (PEUK; 6300 versus 800 cells ml(-1)). However, Prochlorococcus spp. (PRO) are slightly more abundant in the low-nitrate regions (180,000 versus 150,000 cells ml(-1)). The higher HNLC concentrations of SYN and PEUK are part of a broader expansion of the phytoplankton community over the relatively constant PRO base when the limiting nutrient (iron) pool is increased. Elevated biomass and production of phytoplankton and the greater availability of DOC presumably explain the higher HNLC abundances of HBACT. The mean biomass (+/-standard deviation) of bacterial populations for cross-equatorial transects (14.1+/-2.8 mug Cl-1) is similar to that in the subtropics (11.6+/-2.7 mug CI-1), with cruise variations falling generally within a 2-fold range. Heterotrophs comprise a significantly higher mean percentage of total prokaryote biomass (59+/-9%) in the HNLC region than in the low-nutrient subtropics (42+/-6%). The biomass production of photosynthetic bacteria (PRO and SYN) in the central equatorial Pacific is conservatively twice that of HBACT, but total carbon flux through bacteria (44-75% of phytoplankton C-14-production) is dominated by the high respiration, hence carbon demand, of heterotrophs. Given, the very different growth limiting factors (Fe, N, P, and organic carbon) among the various subregions and microbial groups in the tropical Pacific, it seems unlikely that direct controls on growth rate are sufficiently precise to account for the relatively low microbial variability observed. Among factors affecting loss rates, the regulatory role of viral lysis remains largely unexplored, as in most open-ocean systems. However, there is relatively good evidence, including the grazing response to the IronEx II perturbation and multi-level cascade influences, that protistan grazer are generally able to suppress large excursions in microbial abundance and biomass. The key elements of such a control mechanism are size or surface-chemistry characteristics that link the dynamics of different microbial populations to common (nanoflagellate) predators and the fact that such predators are held well below their maximum growth rate potential at ambient food concentrations. This latter point, in particular, ensures a rapid and approximately linear increase in protistan growth and grazing pressure up to prey concentrations many times ambient levels. (C) 2002 Elsevier Science Ltd. All rights reserved.

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

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

Pasulka, AL, Levin LA, Steele JA, Case DH, Landry MR, Orphan VJ.  2016.  Microbial eukaryotic distributions and diversity patterns in a deep-sea methane seep ecosystem. Environmental Microbiology. 18:3022-3043.   10.1111/1462-2920.13185   AbstractWebsite

Although chemosynthetic ecosystems are known to support diverse assemblages of microorganisms, the ecological and environmental factors that structure microbial eukaryotes (heterotrophic protists and fungi) are poorly characterized. In this study, we examined the geographic, geochemical and ecological factors that influence microbial eukaryotic composition and distribution patterns within Hydrate Ridge, a methane seep ecosystem off the coast of Oregon using a combination of high-throughput 18S rRNA tag sequencing, terminal restriction fragment length polymorphism fingerprinting, and cloning and sequencing of full-length 18S rRNA genes. Microbial eukaryotic composition and diversity varied as a function of substrate (carbonate versus sediment), activity (low activity versus active seep sites), sulfide concentration, and region (North versus South Hydrate Ridge). Sulfide concentration was correlated with changes in microbial eukaryotic composition and richness. This work also revealed the influence of oxygen content in the overlying water column and water depth on microbial eukaryotic composition and diversity, and identified distinct patterns from those previously observed for bacteria, archaea and macrofauna in methane seep ecosystems. Characterizing the structure of microbial eukaryotic communities in response to environmental variability is a key step towards understanding if and how microbial eukaryotes influence seep ecosystem structure and function.

Garrison, DL, Gowing MM, Hughes MP, Campbell L, Caron DA, Dennett MR, Shalapyonok A, Olson RJ, Landry MR, Brown SL, Liu HB, Azam F, Steward GF, Ducklow HW, Smith DC.  2000.  Microbial food web structure in the Arabian Sea: a US JGOFS study. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 47:1387-1422.   10.1016/s0967-0645(99)00148-4   AbstractWebsite

One of the main objectives of the Joint Global Ocean Flux Studies (JGOFS) program is to develop an understanding of the factors controlling organic carbon production in the ocean and the time-varying vertical flux of carbon from surface waters (US JGOFS (1990) US JGOFS Planning Report Number 11; Sarmiento and Armstrong (1997) US JGOFS Synthesis and Modeling Project Implementation Plan). A considerable amount of evidence suggests that carbon cycling and the potential for exporting carbon from ocean systems is a function of food web structure. As part of the US JGOFS Arabian Sea Studies, the biomass of planktonic organisms, ranging from heterotrophic bacteria through microplankton-sized organisms, was estimated using a variety of methods including flow cytometry and microscopy. This is a first attempt to combine biomass data from a number of sources, evaluate the structure of the food web, examine changes in food web structure in relation to seasonal or spatial features of the study area, and look for indications of how changing structure affects carbon-cycling processes. Biomass in the upper 100 m of the water column ranged from approximately 1.5 to > 5.2 gC m(-2). Heterotrophic bacteria (Hbac) made up from 16 and 44% of the biomass; autotrophs comprised 43-64%; and the remainder was made up of nano- and microheterotrophs. Autotrophs and nano- and microheterotrophs showed a general pattern of higher values at coastal stations, with the lowest values offshore. Heterotrophic bacteria (Hbac) showed no significant spatial variations. The Spring Intermonsoon and early NE Monsoon were dominated by autotrophic picoplankton, Prochlorococcus and Synechococcus. The late NE Monsoon and late SW Monsoon periods showed an increase in the larger size fractions of the primary producers. At several stations during the SW Monsoon, autotrophic microplankton, primarily diatoms and Phaeocystis colonies, predominated. Increases in the size of autotrophs were also reflected in increasing sizes of nano- and microheterotrophs. The biomass estimates based on cytometry and microscopy are consistent with measurement of pigments, POC and PON. Changes in community structure were assessed using the percent similarity index (PSI) in conjunction with multidimensional scaling (MDS) or single-linkage clustering analysis to show how assemblages differed among cruises and stations. Station clustering reflected environmental heterogeneity, and many of the conspicuous changes could be associated with changes in temperature, salinity and nutrient concentrations. Despite inherent problems in combining data from a variety of sources, the present community biomass estimates were well constrained by bulk measurements such as Chi a, POC and PON, and by comparisons with other quantitative and qualitative studies. The most striking correlation between Food web structure and carbon cycling was the dominance of large phytoplankton, primarily diatoms, and the seasonal maxima of mass flux during the SW Monsoon. High nutrient conditions associated with upwelling during the SW Monsoon would explain the predominance of diatoms during this season. The sinking of large, ungrazed diatom cells is one possible explanation for the flux observations, but may not be consistent with the observation of concurrent increases in larger microzooplankton consumers (heterotrophic dinoflagellates and ciliates) and mesozooplankton during this season. Food-web structure during the early NE Monsoon and Intermonsoons suggests carbon cycling by the microbial community predominated. (C) 2000 Elsevier Science Ltd. All rights reserved.

Landry, M.  2001.  Microbial Loops. Encyclopedia of Ocean Sciences. ( Editor-in-Chief:  John SH, Ed.).:1763-1770., Oxford: Academic Press   10.1006/rwos.2001.0293   Abstract
Baines, SB, Chen X, Vogt S, Fisher NS, Twining BS, Landry MR.  2016.  Microplankton trace element contents: implications for mineral limitation of mesozooplankton in an HNLC area. Journal of Plankton Research. 38:256-270.   10.1093/plankt/fbv109   AbstractWebsite

Mesozooplankton production in high-nutrient low-chlorophyll regions of the ocean may be reduced if the trace element concentrations in their food are insufficient to meet growth and metabolic demands. We used elemental microanalysis (SXRF) of single-celled plankton to determine their trace metal contents during a series of semi-Lagrangian drift studies in an HNLC upwelling region, the Costa Rica Dome (CRD). Cells from the surface mixed layer had lower Fe:S but higher Zn:S and Ni:S than those from the subsurface chlorophyll maximum at 22–30 m. Diatom Fe:S values were typically 3-fold higher than those in flagellated cells. The ratios of Zn:C in flagellates and diatoms were generally similar to each other, and to co-occurring mesozooplankton. Estimated Fe:C ratios in flagellates were lower than those in co-occurring mesozooplankton, sometimes by more than 3-fold. In contrast, Fe:C in diatoms was typically similar to that in zooplankton. RNA:DNA ratios in the CRD were low compared with other regions, and were related to total autotrophic biomass and weakly to the discrepancy between Zn:C in flagellated cells and mesozooplankton tissues. Mesozooplankton may have been affected by the trace element content of their food, even though trace metal limitation of phytoplankton was modest at best.

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, Constantinou J, Kirshtein J.  1995.  Microzooplankton grazing in the central equatorial Pacific during February and August, 1992. Deep-Sea Research Part II-Topical Studies in Oceanography. 42:657-671.   10.1016/0967-0645(95)00024-k   AbstractWebsite

Dilution studies were conducted on EqPac cruises in the central equatorial Pacific (2 degrees N to 2 degrees S, 140 degrees W) during February-March and August-September 1992 to determine phytoplankton growth rates and mortality rates attributable to microzooplankton grazing. Instantaneous growth rates (mu) based on bulk chlorophyll measurements were highly variable from day-to-day, but averaged 0.83 day(-1) for the upper (10-20 m), 0.34 day(-1) for the mid (40-50 m) and 0.22 day(-1) (70-80 m) for the lower euphotic zone on the first cruise. Corresponding rate estimates for microzooplankton grazing (m) were 0.72, 0.22 and 0.21 day(-1), respectively. During the second cruise, growth estimates strongly exceeded grazing estimates for the two upper (mu = 0.98 and 1.00 day(-1); m = 0.57 and 0.42 day(-1)), but not the lower depth strata (mu = 0.32 day(-1); m = 0.27 day(-1)). Grazing losses accounted for about 83% of depth-integrated phytoplankton growth in February/March experiments and only about 55% in August-September experiments. In addition, growth rates in the presence of added nutrients (including iron) showed evidence of limitation in February-March, which coincided with a major El Nino event, but not in August-September, which was more representative of climatological mean conditions. Differences in growth rates, implied nutrient limitation, and the balance of phytoplankton growth and microzooplankton grazing were consistent with a greater abundance of large diatoms in August-September. Despite the disparity between chlorophyll-based estimates of growth and grazing rates for this cruise, flowcytometric analyses of specific populations (Prochlorococcus, Synechococcus, and autotrophic nanoeukaryotes) in a subset of experiments conducted in August demonstrated that microzooplankton grazing was still sufficient to balance growth rates of the smaller components of the phytoplankton assemblage.

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.