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2016
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.

2013
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.

2012
Li, QP, Franks PJS, Ohman MD, Landry MR.  2012.  Enhanced nitrate fluxes and biological processes at a frontal zone in the southern California current system. Journal of Plankton Research. 34:790-801.   10.1093/plankt/fbs006   AbstractWebsite

Processes that occur at mesoscale and submesoscale features such as eddies and fronts are important for marine ecosystem dynamics and biogeochemical fluxes. However, their impacts on the fate of biogenic organic carbon in coastal oceans are not well quantified because physical and biological interactions at such features are very complex with short time-and small spatial scales variability. As part of the California Current Ecosystem Long-Term Ecological Research (CCE-LTER) Process studies in the southern California Current in October 2008, we sampled across a strong temperature and chlorophyll front ('A-Front') separating water masses with distinct hydrographic and biogeochemical characteristics and a modified biological assemblage at the frontal interface. Thorpe-scale analyses of the hydrographic data from a free-fall moving vessel profiler suggested an increased diapycnal diffusive nitrate flux at the front zone. Based on these field data, we use data-driven diagnostic biogeochemical models to quantify how the front-induced physical mixing influenced the production, grazing and transport of phytoplankton carbon in the southern California Current. Our results suggest that enhanced diffusive diapycnal fluxes of nutrients stimulated phytoplankton primary production at the front; this effect, together with reduced microzooplankton grazing, increased net growth of the phytoplankton community leading to locally enhanced biomass of large phytoplankton, such as diatoms, in the frontal zone.

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

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

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.

2009
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.

2003
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.

2001
Landry, MR, Brown SL, Selph KE, Abbott MR, Letelier RM, Christensen S, Bidigare RR, Casciotti K.  2001.  Initiation of the spring phytoplankton increase in the Antarctic Polar Front Zone at 170 degrees W. Journal of Geophysical Research-Oceans. 106:13903-13915.   10.1029/1999jc000187   AbstractWebsite

During austral summer 1997, satellite imagery revealed enhanced chlorophyll associated with the Antarctic Polar Front at 170 degreesW. Phytoplankton growth conditions during the early stages of the spring increase were investigated on the Antarctic Environment and Southern Ocean Process Study Survey I cruise using flow cytometry (FCM) and microscopy to characterize community biomass, composition and biological stratification and dilution experiments to estimate growth and grazing rates, Physical and biological measures showed a general shoaling of mixed layer depth from similar to 200 to < 100 m from late October to early November. Plankton assemblages on the southern side of the frontal jet (similar to0 degreesC waters) differed from those on the northern side (similar to2 degreesC) in enhanced relative importance of larger (> 20 mum) cells, greater contributions of diatoms and ciliates, and a twofold higher ratio of protistan grazers to photoautotrophs. Phytoplankton community growth rates from incubations at 10 and 23% of surface incident light showed good agreement between high performance liquid chromatography estimates of chlorophyll a (Chl a) (0.20 d(-1)) and FCM cell-based (0.21 d(-1))results. Fucoxanthin-based estimates for diatoms were 0.2-1 d(-1). Mean estimates of microzooplankton grazing from the three phytoplankton measures were 0.16, 0.12, and 0.11 d(-1), respectively. Heterotrophs typically consumed 40-100% of their body carbon per day and thus presumably grew at rates similar to phytoplankton. The low net rates of Chl a increase in shipboard bottle incubations (0.04 d(-1)) were consistent with the slow downstream accumulation of phytoplankton biomass (0.03 d(-1)) as measured with instrumented Lagrangian drifters through the month of November. Both were slightly less than the net rate estimates from SeaSoar surveys (0.05 d(-1)) because of the effects of pigment photoadaptation (bleaching) during this time of increasing light level and water column stratification.

Landry, MR, Al-Mutairi H, Selph KE, Christensen S, Nunnery S.  2001.  Seasonal patterns of mesozooplankton abundance and biomass at Station ALOHA. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 48:2037-2061.   10.1016/s0967-0645(00)00172-7   AbstractWebsite

The zooplankton community of the central North Pacific has been sampled at Stn. ALOHA since 1994 as part of routine data collections for the Hawaii Ocean Time-series (HOT) Program. Typically, three day and three night tows are collected in the upper 150 m on each cruise with a l-m, 200-mum mesh net. Size-fractioned carbon and dry weight biomass have been determined for each sample, and one day and one night sample per cruise have been enumerated microscopically through 1996. The accumulated data show statistically significant seasonal signals, with peak biomass and abundance during the summer months for the total community and for smaller size fractions (0.2-0.5, 0.5-1, 1-2 mm). Interannual variability is seen as significantly higher nighttime biomass during summer 1996, which is also related to the enhancement of small size fractions. Comparisons of the present composition of the community to the results of classic studies at the CLIMAX site show remarkable similarities in the species sampled and their abundance ranges. Nonetheless, biomass estimates, corrected for relative capture efficiencies of the different net systems, suggest that there may have been an increase of about a factor of two in zooplankton standing stocks over the past two decades. This hypothesis raises issues about the comparability of the two sites, but the increase, if real, would be consistent with the observed decadal-scale increase in phytoplankton chlorophyll a. The small harpacticoid copepod, Macrosetella gracilis, demonstrates a strong summer maximum consistent with the increased abundance of nitrogen-fixing Trichodesmium spp. during mid-summer months, as described in recent studies at Stn. ALOHA. Accordingly, nitrogen fixation could provide the source of new nutrients to support higher productivity, larger phytoplankton, and enhanced zooplankton standing stocks in the summer, when the upper water column is most stratified and isolated from nutrient influxes from below. (C) 2001 Elsevier Science Ltd. All rights reserved.

2000
Bollens, GCR, Landry MR.  2000.  Biological response to iron fertilization in the eastern equatorial Pacific (IronEx II). II. Mesozooplankton abundance, biomass, depth distribution and grazing. Marine Ecology-Progress Series. 201:43-56.   10.3354/meps201043   AbstractWebsite

Mesozooplankton (202 to 2000 mu m) biomass, abundance, taxonomic composition, depth distributions and gut pigment contents were measured inside and outside of an iron-enriched patch during the IronEx II study in the eastern equatorial Pacific. Mean carbon biomass remained nearly constant in the ambient community, but increased 2- to 3-fold during early stages of the phytoplankton bloom. The increases were due primarily to small calanoid and cyclopoid copepods and copepod nauplii in the mixed layer and appeared to be the result of 2 processes. First, significantly higher abundances of nauplii in the patch indicated that adult copepods responded reproductively, at least initially, to the increased food. Second, changes in copepod vertical migratory behaviors in response to reduced light penetration and increased food abundance in the patch apparently resulted in an upward displacement of copepods from the lower euphotic zone into the mixed-layer. Mesozooplankton gut pigment content also increased significantly inside the patch, largely in proportion to the increased concentration of phytoplankton chlorophyll a, and estimates of carbon consumed suggest that mesozooplankton standing stock was growing at maximal, or near maximal, temperature-dependent rates (1.0 d(-1)) at the peak of the patch bloom. Nonetheless, zooplankton abundance and biomass declined, rather than increased, during this period. The premature decline of mesozooplankton in the patch suggests that they might have been cropped by their predators in a tightly coupled trophic network or that their reproductive output may have failed to produce viable young when the food resources were dominated by diatoms.

1996
Chung, SP, Gardner WD, Richardson MJ, Walsh ID, Landry MR.  1996.  Beam attenuation and micro-organisms: Spatial and temporal variations in small particles along 140 degrees W during the 1992 JGOFS EqPac transects. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 43:1205-&.   10.1016/0967-0645(96)00030-6   AbstractWebsite

As part of the U.S. JGOFS EqPac program, transmissometer profiles were made from 12 degrees N to 12 degrees S at 140 degrees W during February-March (cruise TT007) and August-September (cruise TT011) 1992. Meridional sections of c(p) (beam attenuation due to particles) were prepared by selecting profiles made at a specific time during the day (similar to 18:00 h) to reduce the influence of diel variability and to facilitate point-to-point comparisons with other variables (e.g. T, nitrate, bioabundance, etc.). A tight correlation between beam c(p) and total scattering cross-section of micro-organisms was observed, suggesting that heterotrophic bacteria, prochlorophytes, cyanobacteria and small autotrophic eukaryotes (all < 3 mu m) were dominant contributors to beam c(p). Size-filtration experiments also confirmed that small particles (< 8 mu m) accounted for 41-89% of the c(p) signal in the equatorial Pacific. Contributions of the bacterial fraction and detrital material were assessed. Three biohydrographic regimes [northern (7 degrees-12 degrees N), equatorial (5 degrees N-5 degrees S) and southern (7 degrees-12 degrees S)] could be distinguished from characteristic profiles of c(p) and other variables. While the northern and southern regimes remained relatively constant in c(p) between El Nino (TT007) and cold surface water (TT001) conditions, the equatorial regime showed > 30% increase in surface beam c(p) and IBAC (integrated beam attenuation coefficient) during TT011 compared to TT007. This suggests that only the equatorial regime responds sensitively to the hydrodynamic factors (e.g. upwelling, currents, El Nino. tropical instability waves, etc.) regulating particle distributions. The c(p):chlorophyll a ratio, a proxy for the C:chlorophyll a ratio, also was calculated to obtain insight into biogeochemical cycles in the upper waters of the equatorial Pacific. Copyright (C) 1996 Elsevier Science Ltd.

1994
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.