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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.

2014
Freibott, A, Linacre L, Landry MR.  2014.  A slide preparation technique for light microscopy analysis of ciliates preserved in acid Lugol's fixative. Limnology and Oceanography-Methods. 12:54-62.   10.4319/lom.2014.12.54   AbstractWebsite

A slide preparation method for seawater samples preserved in acid Lugol's is presented here as an alternative to the traditional Utermohl settling chamber method for microplankton analysis. This preparation maintains the integrity of fragile cells, such as ciliates, resolves issues associated with the transience of samples prepared in settling chambers, and allows the use of automated image acquisition methods. Samples are filtered onto polycarbonate membranes and analyzed with transmitted light microscopy. The visibility of pore outlines is minimized by using mounting oil (Cargille Series A immersion oil, Certified Refractive Index, n(D)(25 degrees C) 1.5840 +/- 0.0002) with a refractive index matching that of the membrane material. We assessed the efficacy of this new method by comparing abundance and biomass estimates for ciliates in settled and filtered samples. Acceptable results were found for the most delicate of samples stored long-term in acid Lugol's. Some cell shrinkage occurred during the filtration and brief drying steps. Therefore, corrections for ciliate length and width measurements in filtered samples were determined to counteract this effect on total cell biovolume. Overall, the method provides a simple and stable alternative to settling chamber analysis for ciliates preserved in acid Lugol's.

2013
Pasulka, AL, Landry MR, Taniguchi DAA, Taylor AG, Church MJ.  2013.  Temporal dynamics of phytoplankton and heterotrophic protists at station ALOHA. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 93:44-57.   10.1016/j.dsr2.2013.01.007   AbstractWebsite

Pico- and nano-sized autotrophic and heterotrophic unicellular eukaryotes (protists) are an important component of open-ocean food webs. To date, however, no direct measurements of cell abundance and biomass of these organisms have been incorporated into our understanding of temporal variability in the North Pacific Subtropical Gyre (NPSG). Based primarily on epifluoresence microscopy augmented with flow cytometry, we assessed the abundance and biomass of autotrophs and heterotrophic protists at Station ALOHA between June 2004 and January 2009. Autotrophic eukaryotes (A-EUKS) were more abundant in both the upper euphotic zone and deep chlorophyll maximum layer (DCML) during winter months, driven mostly by small flagellates. A higher ratio of A-EUKS to heterotrophic protists (A:H ratio) and a structural shift in A-EUKS to smaller cells during the winter suggests a seasonal minimum in grazing pressure. Although Prochlococcus spp. comprised between 30% and 50% of autotrophic biomass in both the upper and lower euphotic zone for most of the year, the community structure and seasonality of nano-and micro-phytoplanlcon differed between the two layers. In the upper layer, Trichodesmium spp. was an important contributor to total biomass (20-50%) in the late summer and early fall. Among A-EUKS, prymnesiophytes and other small flagellates were the dominant contributors to total biomass in both layers regardless of season (10-20% and 13-39%, respectively). Based on our biomass estimates, community composition was less seasonally variable in the DCML relative to the upper euphotic zone. In surface waters, mean estimates of C:Chl a varied with season highest in the summer and lowest in the winter (means=156 +/- 157 and 89 +/- 32, respectively); however, there was little seasonal variability of C:Chl a in the DCML (100 m mean=29.9 +/- 9.8). Biomass of heterotrophic protists peaked in the summer and generally declined monotonically with depth without a deep maximum. Anomalous patterns of A:H variability during summer 2006 (low mesozooplankton, high A-EUKS and H-dinoflagellates) suggest that top-down forcing is strong enough to impact lower trophic levels in the NPSG. Continued studies of community abundance and biomass relationships are needed for adequate representations of plankton dynamics in ecosystem models and for developing a predictive understanding of both intra-and inter-lower trophic level responses to climate variability in the NPSG. (c) 2013 Elsevier Ltd. All rights reserved.

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

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

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

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

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

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

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

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

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

We investigate the hypothesis that phytoplankton growth and grazing processes are strongly balanced in high-nutrient low-chlorophyll (HNLC) waters of the equatorial Pacific using euphotic-zone estimates of rates and biomass determined for 30 stations during EB04 (December 2004) and EB05 (September 2005). As predicted by the balance hypothesis, depth-averaged instantaneous rates of phytoplankton growth and grazing losses to micro- and mesozooplankton show a net growth difference of zero. Contemporaneous estimates of phytoplankton biomass and specific rates from flow cytometry, microscopy and taxon-specific accessory pigments allow determination of constrained production-consumption trophic balances for the phytoplankton community as a whole and for major component populations. The magnitude of growth-based production (867 mg C m(-2) d(-1)) is consistent with measured (14)C primary production, given methodological differences. 70% of production is utilized by protistan herbivores within the microbial community; 30% is consumed by mesozooplankton. Among picophytoplankton (Prochlorococcus, Synechococcus and small eukaryotes), representing 40% of community biomass and 27% of daily biomass growth, microzooplankton consume almost all production. Among groups of larger eukaryote taxa, including diatoms but dominated by dinoflagellate biomass, micro-grazers consume 51-62% of production, with the remainder available to mesozooplankton. Some leakage from the balance is expected as export of sinking phytoplankton cells and aggregates, but is constrained to no more than a few percent of daily production from alternate determinations of mesozooplankton grazing. The demonstrated balance of growth and grazing processes in the equatorial Pacific is inconsistent with recent claims from inverse models that a large flux associated with ungrazed picophytoplankton production dominates euphotic zone carbon export in the region. (C) 2010 Elsevier Ltd. All rights reserved.

Parker, AE, Wilkerson FP, Dugdale RC, Marchi AM, Hogue VE, Landry MR, Taylor AG.  2011.  Spatial patterns of nitrogen uptake and phytoplankton in the equatorial upwelling zone (110 degrees W-140 degrees W) during 2004 and 2005. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:417-433.   10.1016/j.dsr2.2010.08.013   AbstractWebsite

The equatorial Pacific plays a central role in the global carbon cycle as the largest oceanic source of CO(2) to the atmosphere. The region has been described as a HNLC (High Nutrient, Low Chlorophyll) region with low chlorophyll being explained by either Fe limitation, Si(OH)(4) limitation, or a combination of the two. The autotrophic community in the equatorial upwelling zone (EUZ) is dominated numerically by small cell-sized picoplankton but also contains larger cells, including diatoms that require Si(OH)(4) and have been hypothesized to drive NO(3) uptake. The 1-D CoSiNE (Carbon, Silicon, Nitrogen Ecosystem) model was developed for the EUZ to evaluate the potential drivers of the HNLC condition. The model assumes two phytoplankton groups (small non Si(OH)(4) users and larger Si(OH)(4) requiring cells). Many of its predicted variables have not been measured in the field - specifically, uptake of NO(3), NH(4) and Si(OH)(4) by the larger cells versus the smaller picoplankton and biomass values (as particulate nitrogen, PN) of the two size classes. Two cruises to the EUZ were undertaken in December 2004 (EB04) and September 2005 (EB05) to collect these data, in part to test the conceptual basis and functioning of the CoSiNE model. These EUZ cruises were unique in having size-fractionated uptake rates and phytoplankton community composition at the same depths throughout the euphotic zone and at many stations representing a range of environmental conditions and community composition. Here, we report results pertaining to NO(3) and NH(4) uptake, PN, and f-ratio by different size fractions of the phytoplankton community collected during zonal (0 degrees Eq and 0.5 degrees N) and meridional surveys at 110 degrees W and 140 degrees W. Larger sized (> 5 mu m) phytoplankton contributed more to NO(3) uptake (up to 85%) than cells < 5 mu m and so had higher f-ratios. Several autotrophic functional groups were likely responsible for NO(3) uptake in the > 5-mu m size class, including both diatoms and dinoflagellates. The importance of diatoms in NO3 uptake appears to be variable, with biological "hotspots" around regions of locally enhanced upwelling associated with tropical instability waves (TIWs). In some cases. TIW activity favored diatoms, and Si(OH)(4) and NO(3) uptake were elevated strongly during TIW activity. Largely as a result of increased diatom productivity, integrated NO(3) uptake rates during the passage of these waves reached 6.74 mmol m(-2) d(-1), exceeding the mean values previously reported for the equator by > 2-fold. NH(4) concentrations and uptake were also dynamic, with lower values south of the equator and higher values to the north. The major contributors to NH(4) uptake were the smaller (< 5 mu m) phytoplankton that resulted in low total community f-ratios of 0.26 over both years. A > 2-fold increase in NH(4) uptake rates in 2005 was consistent with the observed roughly 2-fold increase in Prochlorococcus biomass. Our results highlight a potential area for improvement to the CoSiNE model, to include the role of autotrophic dinoflagellates. (C) 2010 Elsevier Ltd. All rights reserved.

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

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

Chen, BZ, Liu HB, Landry MR, Chen M, Sun J, Shek L, Chen XH, Harrison PJ.  2009.  Estuarine nutrient loading affects phytoplankton growth and microzooplankton grazing at two contrasting sites in Hong Kong coastal waters. Marine Ecology-Progress Series. 379:77-90.   10.3354/meps07888   AbstractWebsite

To investigate the effects of enhanced nutrient loading in estuarine waters on phytoplankton growth and microzooplankton grazing, we conducted monthly dilution experiments at 2 stations in Hong Kong coastal waters with contrasting trophic conditions. The western estuarine station (WE) near the Pearl River estuary is strongly influenced by freshwater discharge, while the eastern oceanic station (EO) is mostly affected by the South China Sea. Growth rates of phytoplankton were often limited by nutrients at EO, while nutrient limitation of phytoplankton growth seldom Occurred at WE due to the high level of nutrients delivered by the Pearl River, especially in the summer rainy season. Higher chlorophyll a, microzooplankton biomass, phytoplankton growth and microzooplankton grazing rates were found at WE than at EO. However, the increase in chlorophyll greatly exceeded the increase in phytoplankton growth rate, reflecting different response relationships to nutrient availability. Strong seasonality was observed at both stations, with temperature being an important factor affecting both phytoplankton growth and microzooplankton grazing rates. Picophytoplankton, especially Synechococcus, also exhibited great seasonality at EO, with summer abundances being 2 or 3 orders of magnitude higher than those during winter, Our results confirm that in eutrophic coastal environments, microzooplankton grazing is a dominant loss pathway for phytoplankton, accounting for the utilization of >50%, of primary production on average.

2008
Landry, MR, Brown SL, Rii YM, Selph KE, Bidigare RR, Yang EJ, Simmons MP.  2008.  Depth-stratified phytoplankton dynamics in Cyclone Opal, a subtropical mesoscale eddy. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 55:1348-1359.   10.1016/j.dsr2.2008.02.001   AbstractWebsite

As part of E-Flux III cruise studies in March 2005, we investigated phytoplankton community dynamics in a cyclonic cold-core eddy (Cyclone Opal) in the lee of the Hawaiian Islands. Experimental incubations were conducted under in situ temperature and light conditions on a drift array using a two-treatment dilution technique. Taxon-specific estimates of growth, grazing and production rates were obtained from analyses of incubation results based on phytoplankton pigments, flow cytometry and microscopy. Cyclone Opal was sampled at a biologically and physically mature state, with an 80-100m doming of isopycnal surfaces in its central region and a deep biomass maximum of large diatoms. Depth-profile experimentation defined three main zones. The upper (mixed) zone (0-40m), showed little compositional or biomass response to eddy nutrient enrichment, but growth, grazing and production rates were significantly enhanced in this layer relative to the ambient community outside of the eddy. Prochlorococcus spp. dominated the upper mixed layer, accounting for 50-60% of its estimated primary production both inside and outside of Opal. In contrast, the deep zone of 70-90 m showed little evidence of growth rate enhancement and was principally defined by a similar to 100-fold increase of large (> 20-mu m) diatoms and a shift from Prochlorococcus to diatom dominance (similar to 80%) of production. The intermediate layer of 50-60 m marked the transition between the upper and lower extremes but also contained an elevated biomass of physiologically unhealthy diatoms with significantly depressed growth rates and proportionately greater grazing losses relative to diatoms above or below. Microzooplankton grazers consumed 58%, 65% and 55%, respectively, of the production of diatoms, Prochlorococcus and the total phytoplankton community in Cyclone Opal. The substantial grazing impact on diatoms suggests that efficient recycling was the major primary fate of diatom organic production, consistent with the low export fluxes and selective export of biogenic silica, as empty diatom frustules, in Cyclone Opal. (C) 2008 Elsevier Ltd. All rights reserved.

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

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

2000
Landry, MR, Ondrusek ME, Tanner SJ, Brown SL, Constantinou J, Bidigare RR, Coale KH, Fitzwater S.  2000.  Biological response to iron fertilization in the eastern equatorial Pacific (IronEx II). I. Microplankton community abundances and biomass. Marine Ecology-Progress Series. 201:27-42.   10.3354/meps201027   AbstractWebsite

During the IronEx II experiment in the eastern equatorial Pacific (May to June 1995), the response of the microplankton community to mesoscale iron fertilization was followed using a combination of marker-pigment, microscopical and flow cytometric techniques. Phytoplankton standing stock bloomed dramatically over a period of 6 d following 3 iron additions of 2 and 1 nM, respectively. Carbon biomass in the patch increased by a factor of 4, chlorophyll a by about a factor of 16 and diatoms by >70-fold relative to contemporaneous levels in the ambient community. The bloom then plateaued sharply and remained at a more or less constant level for 4 d, despite the addition of more iron (1 nM) and physiological indices (low C:chl. a ratio and elevated photochemical quantum efficiency) suggesting that the cells were healthy and growing rapidly. Relatively large pennate diatoms (Nitzschia spp., median length 20 to 24 mu m) dominated the patch bloom, with smaller pennate species and centric diatoms declining in relative importance. Heterotrophic bacteria increased at a slow rate (0.08 d(-1)) for >10 d during the experiment, as did heterotrophic nanoflagellates. There were also indications of enhanced cell size, cellular pigment content and possibly growth rates of small phytoplankton. Nonetheless, Little difference was observed between the ambient community and the peak patch bloom with respect to the size composition of auto- and heterotrophic populations <10 mu m in cell size. The relative constancy of the smaller size fractions, the sharp curtailment of net growth of the bloom after 6 d, and >3-fold increase in large heterotrophic dinoflagellates and ciliates suggest that protistan grazers may have played an active role in controlling the phytoplankton response to increased iron availability.

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.

1999
Liu, HB, Landry MR, Vaulot D, Campbell L.  1999.  Prochlorococcus growth rates in the central equatorial Pacific: An application of the f(max) approach. Journal of Geophysical Research-Oceans. 104:3391-3399.   10.1029/1998jc900011   AbstractWebsite

Minimum daily growth rates of Prochlorococcus were estimated for the central equatorial Pacific (12 degrees S-12 degrees N, 140 degrees W) during EI Nino (February-March 1992) and normal upwelling (August-September 1992) conditions. Growth rate estimates were based on the percentages of cells in the S and G(2) division phases at dawn (similar to 0700 LT) and dusk (similar to 1800 LT) as approximate values for f(min) and f(max) respectively. During both environmental conditions, depth-integrated growth rates of Prochlorococcus were higher in oligotrophic waters at the northern end of the transect (0.36-0.52 d(-1)) and decreased to a minimum at the equator. The lowest growth rates were found at the equator during El Nino and at 2 degrees N during normal upwelling, where a large biomass of buoyant diatoms had accumulated in the vicinity of a convergent front. Prochlorococcus growth rate reached a high of 0.64 d(-1) at 1 degrees S and maintained a moderate rate (0.36-0.49 d(-1)) throughout the southern end of the transect. An inverse relationship was found between the contribution of Prochlorococcus to the total primary production and nitrate concentration as well as total primary production measured by the C-14 method. While Prochlorococcus is a dominant primary producer in tropical and subtropical, open-ocean ecosystems generally, it is relatively more important in oligotrophic waters than in the nutrient rich equatorial upwelling zone.

Liu, HB, Bidigare RR, Laws E, Landry MR, Campbell L.  1999.  Cell cycle and physiological characteristics of Synechococcus (WH7803) in chemostat culture. Marine Ecology-Progress Series. 189:17-25.   10.3354/meps189017   AbstractWebsite

The marine cyanobacterium Synechococcus spp. is one of the most abundant picoplanktonic photoautotrophs in the world's oceans. When grown in nitrogen-limited chemostat culture under continuous light, Synechococcus strain WH7803 displays a bimodal DNA histogram. Consequently, the duration of each growth phase can be computed from the proportion of cells in each cell cycle phase. We measured cellular DNA distribution, carbon and nitrogen content. and pigment composition of Synechococcus WH7803 in a series of chemostat experiments with growth rate ranging from 0.1 to 0.9 d(-1). Flow cytometric data showed that the duration of each cell cycle phase increased linearly with doubling time, and the fraction of cells in the S and G(2) phases, f(S+G2), was relatively constant at growth rates of less than 1 doubling d(-1). Cellular carbon and nitrogen contents were about 200 and 40 fg cell(-1), respectively, but were much higher for slowly growing cells. The carbon to nitrogen ratio. the carbon to chlorophyll a ratio. and the ratio of zeaxanthin to chlorophyll a all decreased with increasing growth rate. These observed relationships are useful for applying cell cycle analysis to estimate growth rate, or for using cell abundance or pigment concentrations to estimate biomass and productivity of Synechococcus in the sea.

1997
Latasa, M, Landry MR, Schluter L, Bidigare RR.  1997.  Pigment-specific growth and grazing rates of phytoplankton in the central equatorial Pacific. Limnology and Oceanography. 42:289-298.   10.4319/lo.1997.42.2.0289   AbstractWebsite

Dilution experiments were performed during two transects across the central equatorial Pacific (3 degrees N-3 degrees S, 140 degrees W) to estimate growth and mortality rates of select phytoplankton groups distinguished by their characteristic pigments. The first transect was conducted in February-March 1992 during a moderate El Nino event; the second transect took place in August-September 1992, under normal upwelling conditions. Experiments with and without added nutrients (N, P, Fe, and Mn) indicated that growth rates were nutrient limited during El Nino conditions. Nevertheless, the enhanced growth rates with added nutrients during El Nino were less than rates without added nutrients during the normal upwelling period; therefore, nutrients alone did not account for all of the differences between cruises, Growth rates were different for the various algal groups. in August-September, diatoms grew at 1.7 d(-1), and prochlorophytes and prymnesiophytes at similar to 0.5 d(-1). Grazing by microzooplankton balanced growth for algal groups exhibiting the lowest growth rates (i.e. prymnesiophytes and prochlorophytes). Although microzooplankton grazing on diatoms and pelagophytes was also significant, a substantial fraction of their growth escaped consumption, accounting for most of the net chlorophyll production of the phytoplankton community.