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Steinberg, DK, Landry MR.  2017.  Zooplankton and the ocean carbon cycle. Annual Review of Marine Sciences, Vol 9. 9:413-444., Palo Alto: Annual Reviews   10.1146/annurev-marine-010814-015924   Abstract

Marine zooplankton comprise a phylogenetically and functionally diverse assemblage of protistan and metazoan consumers that occupy multiple trophic levels in pelagic food webs. Within this complex network, carbon flows via alternative zooplankton pathways drive temporal and spatial variability in production-grazing coupling, nutrient cycling, export, and transfer efficiency to higher trophic levels. We explore current knowledge of the processing of zooplankton food ingestion by absorption, egestion, respiration, excretion, and growth (production) processes. On a global scale, carbon fluxes are reasonably constrained by the grazing impact of microzooplankton and the respiratory requirements of mesozooplankton but are sensitive to uncertainties in trophic structure. The relative importance, combined magnitude, and efficiency of export mechanisms (mucous feeding webs, fecal pellets, molts, carcasses, and vertical migrations) likewise reflect regional variability in community structure. Climate change is expected to broadly alter carbon cycling by zooplankton and to have direct impacts on key species.

Gutierrez-Rodriguez, A, Selph KE, Landry MR.  2016.  Phytoplankton growth and microzooplankton grazing dynamics across vertical environmental gradients determined by transplant in situ dilution experiments. Journal of Plankton Research. 38:271-289.   10.1093/plankt/fbv074   AbstractWebsite

The Costa Rica Dome (CRD) represents a classic case of the bloom-forming capacity of small phytoplankton. Unlike other upwelling systems, autotrophic biomass in the CRD is dominated by picocyanobacteria and small eukaryotes that outcompete larger diatoms and reach extremely high biomass levels. We investigated responses of the subsurface phytoplankton community of the CRD to changes associated with vertical displacement of water masses, coupling in situ transplanted dilution experiments with flow cytometry and epifluorescence microscopy to assess group-specific dynamics. Growth rates of Synechococcus (SYN) and photosynthetic picoeukaryotes (PEUK) were positively correlated with light (R-pearson_SYN = 0.602 and R-pearson_PEUK = 0.588, P<0.001). Growth rates of Prochlorococcus (PRO), likely affected by photoinhibition, were not light correlated (R-pearson_PRO = 0.101, P = 0.601). Overall, grazing and growth rates were closely coupled in all picophytoplankton groups (R-spearman_PRO = 0.572, R-spearman_SYN = 0.588, R-spearman_PEUK = 0.624), and net growth rates remained close to zero. Conversely, the abundance and biomass of larger phytoplankton, mainly diatoms, increased more than 10-fold in shallower transplant incubations indicating that, in addition to trace-metal chemistry, light also plays a significant role in controlling microphytoplankton populations in the CRD.

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

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

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.

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

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

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).

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.

Twining, BS, Baines SB, Fisher NS, Landry MR.  2004.  Cellular iron contents of plankton during the Southern Ocean Iron Experiment (SOFeX). Deep-Sea Research Part I-Oceanographic Research Papers. 51:1827-1850.   10.1016/j.dsr.2004.08.007   AbstractWebsite

Iron (Fe) availability limits phytoplankton biomass and production in large regions of the Southern Ocean and influences community composition and size structure, which may affect C export and other system-level functions. To improve our understanding of Fe partitioning within communities and the response of different components to fertilization, we assessed the cellular Fe contents of individual diatoms, autotrophic flagellates, and heterotrophic flagellates during the recent Southern Ocean Fe Experiment using synchrotron-based X-ray fluorescence (SXRF). Dual Fe-55/C-14 radioisotope incubations were also conducted to estimate Fe:C ratios in size-fractionated plankton. Cellular Fe quotas determined by the two techniques were in close agreement when low amounts of Fe-55 (0.2 nM) were added, but Fe-55 additions of 2 nM resulted in 2-3-fold higher quotas. SXRF assessments of cellular Fe quotas (normalized to C) were generally in good agreement with prior bulk analyses of natural assemblages, but revealed compositional differences among protistan taxa not previously detected. Mean Fe:C ratios for diatoms, autotrophic flagellates, and heterotrophic flagellates from unfertilized waters were 6.0, 8.7, and 14.1 mumol mol C-1, respectively. Smaller cells had higher Fe:C ratios than larger cells. Fertilization enhanced Fe quotas in all cell types, with mean Fe:C ratios increasing approximately 4-fold (from about 10 to about 40 mumol mol C-1) after two Fe additions. This study provides some of the first measurements of Fe quotas in phytoplankton cells from natural communities and the first measurements of Fe quotas in natural protozoa. (C) 2004 Elsevier Ltd. All rights reserved.

Landry, MR, Brown SL, Neveux J, Dupouy C, Blanchot J, Christensen S, Bidigare RR.  2003.  Phytoplankton growth and microzooplankton grazing in high-nutrient, low-chlorophyll waters of the equatorial Pacific: Community and taxon-specific rate assessments from pigment and flow cytometric analyses. Journal of Geophysical Research-Oceans. 108   10.1029/2000jc000744   AbstractWebsite

[1] Phytoplankton growth and microzooplankton grazing rates were investigated using the seawater dilution technique during a French Joint Global Ocean Flux Study cruise focusing on grazing processes in the high-nutrient, low-chlorophyll equatorial Pacific at 180degrees ( Etude du Broutage en Zone Equatoriale, October - November, 1996). Raw rate estimates based on spectrofluorometric and high-performance liquid chromatography pigment analyses were typically in close agreement, but most showed substantial imbalances in growth and grazing. Flow cytometric (FCM) analyses were used both as an alternate approach for distinguishing populations and as a means for adjusting pigmentbased growth estimates for changes in cellular chlorophyll content and biovolume. Total chlorophyll a (Tchl a) gave mean community growth rates of 0.76 d(-1) at 30 m and 0.27 d(-1) at 60 m. Grazing rates averaged 0.56 and 0.15 d(-1) at the two depths, respectively, and 69% of phytoplankton growth overall. For the prokaryotic picophytoplankter, Prochlorococcus ( PRO), rate estimates from dv-chl a and FCM cell counts generally indicated balanced growth and grazing and therefore close grazing control by microzooplankton. At the equator, rate estimates from dv-chl a averaged 0.6 - 0.7 d(-1) at 30 m and 0.25 - 0.26 at 60 m and were consistent with inferences based on diel pigment variations in the 30 - 70 m depth range. Phytoplankton production estimates from experimentally determined rates and microscopical assessments of autotrophic carbon at 30 m ( mean = 19 mg C m(-3) d(-1)) agreed well with contemporaneous measurements by 14 C uptake. Diatom growth rate estimates (1.0 - 1.6 d(-1)), constrained by contemporaneous measurements of silicate uptake, implied a relatively small biomass (10 - 45 nmol C L-1) with high rates of turnover and recycling.

Landry, MR, Selph KE, Brown SL, Abbott MR, Measures CI, Vink S, Allen CB, Calbet A, Christensen S, Nolla H.  2002.  Seasonal dynamics of phytoplankton in the Antarctic Polar Front region at 170 degrees W. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 49:1843-1865.   10.1016/s0967-0645(02)00015-2   AbstractWebsite

Phytoplankton dynamics in the region of 55-70degreesS, 170degreesW were investigated using Sea-viewing Wide Field-of-View Sensor satellite imagery, shipboard sampling and experimental rate assessments during austral spring and summer, 1997-1998. We used image-analysis microscopy to characterize community biomass and composition, and dilution experiments to estimate growth and microzooplankton grazing rates. Iron concentrations were determined by flow-injection analysis. The phytoplankton increase began slowly with the onset of stratification at the Polar Front (PF) (60-61degreesS) in early November. Seasonally enhanced levels of chlorophyll were found as far north as 58degreesS, but mixed-layer phytoplankton standing stock was highest, approaching 200 mg C m(-3), in the region between the receding ice edge and a strong silicate gradient, which migrated from similar to62degreesS to 65degreesS during the study period. The most southern stations sampled on four cruises were characterized by small pennate diatoms and Phaeocystis. From the PF to the Southern Antarctic circumpolar current front (similar to65degreesS), this ice margin assemblage was seasonally replaced by a community dominated by large diatoms. The large diatom community developed only in waters where measured iron concentrations were initially high (greater than or equal to0.2 nM), and crashed when dissolved silicate was depleted to low levels. Phytoplankton growth rates were highest (0.5-0.6 d(-1)) between the PF and silicate front (60degreesS and 63degreesS) in December. In January, growth rates were lowest (0.1 d(-1)) near the PF, and the highest rates (0.34.4 d(-1)) were found in experiments between 64.8degreesS and 67.8degreesS. Phytoplankton production estimates were highest south of the PF through December and January, averaging 2.2-2.4 mmol C m(-3) d(-1) and reaching levels of 5 mmol cm(-3) d(-1) (64.8degreesS and 67.8degreesS in January). Microzooplankton grazers consumed 54-95% of production for experiments conducted on four AESOPS cruises. They were less efficient in balancing growth rates during the time of highest phytoplankton growth and increase in December, and most efficient in February-March, after the large diatom bloom had collapsed. The diatom bloom region in the present study is in an upwelling zone for Antarctic circumpolar deep water with high iron content. This may explain why this marginal ice zone differs from others where blooms have not been observed. (C) 2002 Published by Elsevier Science Ltd.

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.

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

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

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.

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.

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, Gifford DJ, Kirchman DL, Wheeler PA, Monger BC.  1993.  Direct and indirect effects of grazing by Neocalanus plumchrus on plankton community dynamics in the subarctic Pacific. Progress in Oceanography. 32:239-258.   10.1016/0079-6611(93)90016-7   AbstractWebsite

The effects of grazing of Neocalanus plumchrus C5 copepodids on plankton trophic coupling in the subarctic Pacific were examined in shipboard microcosm experiments during June 1987. Mixed-layer seawater was incubated for 5d in 601 containers under simulated in situ conditions and copepod densities ranging from 0 (control) to 0.75 copepods l-1. Direct grazing effects were determined from temporal changes in abundances of chlorophyll, diatoms, and ciliates. Indirect effects were evaluated from measured rates of primary production (C-14-bicarbonate uptake), bacterial secondary production (H-3-thymidine incorporation), and N-15-ammonium uptake and regeneration. Phytoplankton grew to higher than natural levels in all microcosms over the course of the incubations, but copepods reduced the rates of increase by factors suggesting time-averaged clearance rates of 120, 420, 450 and 170ml copepod-1d-1 for chlorophyll, Nitzschia spp., centric diatoms and ciliates, respectively. Of the rates measured, those largely attributable to phytoplankton growth (i.e. primary production and ammonium uptake) declined with increasing macrozooplankton grazing, in proportion to phytoplankton standing stock measured as chlorophyll a. In contrast, rates associated with microbial loop activity (thymidine incorporation and ammonium regeneration) were enhanced by macrozooplankton grazing. Consequently, increased copepod grazing resulted in a larger fraction of phytoplankton production being processed through the microbial loop.

Landry, MR, Monger BC, Selph KE.  1993.  Time-dependency of microzooplankton grazing and phytoplankton growth in the subarctic Pacific. Progress in Oceanography. 32:205-222.   10.1016/0079-6611(93)90014-5   AbstractWebsite

Dilution experiments were conducted on SUPER Program cruises in June 1987 and May and August 1988 to assess the role of microzooplankton in controlling phytoplankton stocks in the subarctic Pacific. Net growth rates of chlorophyll a varied in individual experiments from -0.4 to +0.7d-1. Experiments incubated for 48h gave higher net estimates than 24h incubations (0.01 to 0.22 d-1 for different cruises), ''aggerating the imbalance between growth and grazing. Specific growth rates (mu) and grazing mortality (m) for 24h incubations were approximately balanced for the June and May cruises, and net growth estimates from the dilution experiments predicted changes in chlorophyll concentrations for May that closely matched those observed in the field, A major decline in phytoplankton abundance in the middle of May coincided with a high abundance of ciliates. Cell counts indicated that Synechococcus and small autotrophic nonflagellates were always kept in check by microzooplankton grazing, even when chlorophyll indicated uncontrolled phytoplankton growth in August 1988 experiments. Diatoms showed high growth potential in most incubations and dominated among the cells that bloomed in August. Our results support the hypotheses that micrograzers are major consumers of phytoplankton in the subarctic Pacific and that their grazing can control some elements of the phytoplankton community. However, growth limitation, presumably from iron deficiency, remains essential to the explanation of phytoplankton control in mid to late summer.