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2018
Taylor, AG, Landry MR.  2018.  Phytoplankton biomass and size structure across trophic gradients in the southern California Current and adjacent ocean ecosystems. Marine Ecology Progress Series. 592:1-17.   10.3354/meps12526   AbstractWebsite

We used a combination of epifluorescence microscopy and flow cytometry to investigate variability of phytoplankton biomass and size structure with ocean richness, measured as autotrophic carbon (AC) or chlorophyll a (chl a). Samples were collected from mixed layers of 4 North Pacific ecosystems: the California Current (CCE), Equatorial Pacific, Costa Rica Dome, and subtropical gyre (station ALOHA). Both nano- (2-20 mu m) and microphytoplankton (20-200 mu m) in creased biomass as power functions with increasing richness, with a sharper slope leading to microphytoplankton dominance at high biomass. The AC: chl a ratio (mu g:mu g) was also well fit by a power function, varying from similar to 170 to 20 over the range of <0.06 to >11.7 mu g chl a l(-1). Prochlorococcus and diatoms were major biomass contributors at poorer and richer extremes, respectively, but mixed flagellates (prymnesiophytes, dinoflagellates, others) comprised a surprisingly consistent half of AC over the richness range. While pico- (<2 mu m) and nanophytoplankton co-dominated at low chl a, all picophytoplankton (Prochlorococcus, Synechococcus, picoeukaryotes) declined significantly in richer coastal waters. Their decrease was consistent with a previously proposed mechanism linking Prochlorococcus decline to increased productivity and grazing pressure on heterotrophic bacteria, termed here the enhanced microbial loop hypothesis. Generalized additive models further indicated that biotic variables explained more of picophytoplankton variability than abiotic variables in CCE coastal waters. Density-independent grazing may be a strong driver of picophytoplankton selection across trophic gradients, with implications for strategy trade-offs in growth rate and grazing resistance, and for representing mortality in marine ecosystem models. [GRAPHICS] Conceptual depiction of the Enhanced Microbial Loop hypo thesis at high chl a Conceptual depiction: Dennis Mc Thompson

2016
Goes, JI, Gomes HD, Selph KE, Landry MR.  2016.  Biological response of Costa Rica Dome phytoplankton to Light, Silicic acid and Trace metals. Journal of Plankton Research. 38:290-304.   10.1093/plankt/fbv108   AbstractWebsite

The Costa Rica Dome (CRD) is a unique open-ocean upwelling system, with picophytoplankton dominance of phytoplankton biomass and suppressed diatoms, yet paradoxically high export of biogenic silica. As a part of Flux and Zinc Experiments cruise in summer (June-July 2010), we conducted shipboard incubation experiments in the CRD to examine the potential roles of Si, Zn, Fe and light as regulating factors of phytoplankton biomass and community structure. Estimates of photosynthetic quantum yields revealed an extremely stressed phytoplankton population that responded positively to additions of silicic acid, iron and zinc and higher light conditions. Size-fractioned Chl a yielded the surprising result that picophytoplankton, as well as larger phytoplankton, responded most to treatments with added silicic acid incubated at high incident light (HL + Si). The combination of Si and HL also led to increases in cell sizes of picoplankton, notably in Synechococcus. Such a response, coupled with the recent discovery of significant intracellular accumulation of Si in some picophytoplankton, suggests that small phytoplankton could play a potentially important role in Si cycling in the CRD, which may help to explain its peculiar export characteristics.

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

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

Landry, MR, de Verneil A, Goes JI, Moffett JW.  2016.  Plankton dynamics and biogeochemical fluxes in the Costa Rica Dome: introduction to the CRD Flux and Zinc Experiments. Journal of Plankton Research. 38:167-182.   10.1093/plankt/fbv103   AbstractWebsite

The Costa Rica Dome (CRD) is an open-ocean upwelling system in the Eastern Tropical Pacific that overlies the ocean's largest oxygen minimum zone (OMZ). The region has unique characteristics, biomass dominance by picophytoplankton, suppressed diatoms, high biomass of higher consumers and presumptive trace metal limitation, but is poorly understood in terms of pelagic stock and process relationships, including productivity and production controls. Here, we describe the goals, project design, physical context and major findings of the Flux and Zinc Experiments cruise conducted in June-July 2010 to assess trophic flux relationships and elemental controls on phytoplankton in the CRD. Despite sampling during a year of suppressed summertime surface chlorophyll, cruise results show high productivity (similar to 1 g C m(-2) day(-1)), high new production relative to export, balanced production and grazing, disproportionate biomass-specific productivity of large phytoplankton and high zooplankton stocks. Zinc concentrations are low in surface waters relative to phosphorous and silicate in other regions, providing conditions conducive to picophytoplankton, like Synechococcus, with low Zn requirements. Experiments nonetheless highlight phytoplankton limitation or co-limitation by silicic acid, driven by a strong silica pump that is linked to low dissolution of biogenic silica in the cold shallow thermocline of the lower euphotic zone.

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.

2014
Taniguchi, DAA, Landry MR, Franks PJS, Selph KE.  2014.  Size-specific growth and grazing rates for picophytoplankton in coastal and oceanic regions of the eastern Pacific. Marine Ecology Progress Series. 509:87-101.   10.3354/meps10895   AbstractWebsite

Estimates of growth and grazing mortality rates for different size classes and taxa of natural picophytoplankton assemblages were measured in mixed-layer experiments conducted in 3 regions of the eastern Pacific: the California Current Ecosystem, Costa Rica Dome, and equatorial Pacific. Contrary to expectation, size-dependent rates for cells between 0.45 and 4.0 mu m in diameter showed no systematic trends with cell size both in and among regions. For all size classes, mean +/- SD growth rates ranged from -0.70 +/- 0.17 to 0.83 +/- 0.13 d(-1) and grazing rates between -0.07 +/- 0.13 and 1.17 +/- 0.10 d(-1). Taxon-specific growth rates for Prochlorococcus ranged from 0.17 +/- 0.12 to 0.59 +/- 0.01 d(-1), for Synechococcus from 0.68 +/- 0.03 to 0.97 +/- 0.04 d(-1), for picoeukaryotes from 0.46 +/- 0.13 to 1.03 +/- 0.06 d(-1), and for all cells combined between 0.45 +/- 0.03 and 0.65 +/- 0.02 d(-1). For grazing, Prochlorococcus rates ranged between 0.02 +/- 0.12 and 0.66 +/- 0.02 d(-1), Synechococcus rates between 0.24 +/- 0.08 and 0.92 +/- 0.05 d(-1), for picoeukaryotes between 0.19 +/- 0.10 and 0.78 +/- 0.09 d(-1), and for all cells between 0.16 +/- 0.05 and 0.75 +/- 0.02 d(-1). When comparing rates among taxa, only Prochlorococcus had consistently lower rates than Synechococcocus in all regions. No other trends were apparent. Temperature relationships based on the Metabolic Theory of Ecology were able to explain more of the variability among grazing rates than among growth rates for each taxon considered.

2012
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).

2009
Bidigare, RR, Chai F, Landry MR, Lukas R, Hannides CCS, Christensen SJ, Karl DM, Shi L, Chao Y.  2009.  Subtropical ocean ecosystem structure changes forced by North Pacific climate variations. Journal of Plankton Research. 31:1131-1139.   10.1093/plankt/fbp064   AbstractWebsite

Biological responses to basin-scale climate forcing in the subtropical North Pacific Ocean are assessed based on temporal variations in plankton community structure observed at Station. ALOHA and results of a coupled plysical-biogeochemical model. Observational data and model simulations for the period 1990-2004 reveal distinct temporal patterns, with significant increases in net primary productivity, modeled nitrate flux into the euphotic zone and the measured downward of particulate nitrogen during 1999-2004. Concurrent increases in microalgae, cyanobacteria and modeled and measured zooplankton biomass were also observed during this period. We provide evidence that these responses were a consequence of climate forcing that destratified the upper ocean, making it more susceptible to mixing events and nutrient entrainment. These findigs underscore the importance of nitrate flux and plankton community structure, as modulated by climate forcing, in regulating particle export over interannual and decadal time scales.

1999
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.

1998
Chung, SP, Gardner WD, Landry MR, Richardson MJ, Walsh ID.  1998.  Beam attenuation by microorganisms and detrital particles in the equatorial Pacific. Journal of Geophysical Research-Oceans. 103:12669-12681.   10.1029/98jc00608   AbstractWebsite

The transmissometer has been actively used to monitor the variability of particulate matter in the surface ocean. Attenuation due to water (c(w)) is constant at a given wavelength, so variability in the signal is due primarily to particles (beam c(p)) Most of the beam c signal appears to originate from particles < 20 mu m, which, in the euphotic zone, are most likely to be microorganisms. However, how much of the beam c(p) (lambda = 660 nm) is attributable to any given organism category or the detrital component is poorly known. To answer this question, at least numerical abundances and optical (scattering) cross sections of each category are needed. During the two transect cruises (TT007 and TT011) of the Joint Global Ocean Flux Study (JGOFS) Equatorial Pacific (EqPac) program (12 degrees N - 12 degrees S, 140 degrees W), particulate matter attenuation (beam c(p)) and abundances of four microorganism categories (heterotrophic bacteria, Prochlorococcus, Synechococcus, and small autotrophic eukaryotes) with their forward angle light scattering (FALS) information (flow cytometer) were simultaneously measured from the same water samples. The bulk scattering coefficients of each population and total scattering coefficients (b(p)) of these four picoplankton populations (b(p)(PICO)) were calculated and compared with beam c(p). In the equatorial Pacific, heterotrophic bacteria and Prochlorococcus were the most significant contributors to the beam c(p) (16 and 7% of beam c(p), respectively) via scattering, and autotrophic eukaryotes and Synechococcus were less important contributors (2 and 1%, respectively) in the equatorial Pacific. If absorption was also included, similar to 30% of the beam c(p) could be accounted for by these four populations in the euphotic zone, supporting the argument that a transmissometer at 660 nm is an effective tool in measuring the net biological processes in the open ocean. A rough estimate for the beam c(p) signal from nonliving detrital particles was also made in surface waters of the region.

Liu, H, Campbell L, Landry MR, Nolla HA, Brown SL, Constantinou J.  1998.  Prochlorococcus and Synechococcus growth rates and contributions to production in the Arabian Sea during the 1995 Southwest and Northeast Monsoons. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 45:2327-2352.   10.1016/s0967-0645(98)00073-3   AbstractWebsite

We investigated the growth rates of Prochlorococcus and Synechococcus spp. and their relative contributions to carbon production at five stations in the Arabian Sea during the late Southwest and early Northeast Monsoon seasons in 1995. Estimates of Prochlorococcus growth rates were based on diel cell cycle analysis. Accurate determination of the duration of the cell cycle terminal event, e.g., t(S +G2) was not possible for Synechococcus because of its highly variable cell cycle patterns (e.g., imperfect phasing, multiple DNA-replication peaks, and dark-arrested division). Consequently, growth and mortality rates of Synechococcus were estimated from diel variations in population abundance. The assumptions of this approach were validated by observations that Synechococcus cell division occurred only during the daytime as well as good agreement between growth rate estimates for Prochlorococcus compared to cell cycle analyses. Prochlorococcus growth rates were typically less than 1 doubling per day, although higher rates occurred in the surface waters at an offshore site (Stn. N7) during the SW Monsoon and at a coastal station (S2) during the NE Monsoon. For Synechococcus, maximum growth rates >2 d(-1) were observed at mesotrophic (nitrate concentration 0.1-3 mu m) onshore stations during both seasons. Synechococcus spp. grew much faster than Prochlorococcus in the upper water column at almost every station during both seasons, but the depth range of its maximum growth rate was shallower and its growth and abundance decreased sharply in deeper waters. In addition, growth rates of Synechococcus increased with nutrient availability whereas Prochlorococcus growth rates did not vary dramatically with nutrients. Although there was no significant difference in Synechococcus growth rates between the late SW and early NE Monsoon seasons, the estimated carbon production and relative contribution to primary production were greater during the early NE Monsoon due to the larger biomass of Synechococcus and lower total primary production. Maximum Prochlorococcus production was found only in the most oligotrophic regions, and Prochlorococcus was not a major contributor of primary production for the most part of the Arabian Sea during the SW and NE Monsoons. Overall, Prochlorococcus and Synechococcus were inversely related in terms of their relative contributions to primary production. (C) 1998 Elsevier Science Ltd. All rights reserved.

Campbell, L, Landry MR, Constantinou J, Nolla HA, Brown SL, Liu H, Caron DA.  1998.  Response of microbial community structure to environmental forcing in the Arabian Sea. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 45:2301-2325.   10.1016/s0967-0645(98)00072-1   AbstractWebsite

The effect of environmental forcing on microbial community structure was investigated in the Arabian Sea during four seasonal cruises during 1995: late Northeast Monsoon (January); Spring Intermonsoon (March-April); late Southwest Monsoon (August-September); and early Northeast Monsoon (December). The distributions of picoplankton populations-heterotrophic bacteria (HBac), Prochlorococcus (Pro), Synechococcus spp. (Syn) and picoeucaryotic algae (Peuc)-were determined by flow cytometric analysis. Seasonal variations in abundance maxima, vertical profiles, integrated abundance (0-200 m), and estimated carbon biomass were contrasted along two transects from the coast of Oman to 1500 km offshore. HBac were numerically dominant in surface waters in all regions (1-3 x 10(6) cells ml(-1)), with higher maximum abundances in coastal waters than at offshore stations. Conversely, Pro were most abundant at the oligotrophic offshore stations, and 100-fold lower, or absent, at coastal stations, except during the Spring Intermonsoon when abundances were extremely high along the entire southern transect. Syn abundances were highly variable, with no consistent trend between coastal and offshore stations. This variability may be explained by the prevalence of mesoscale eddies, but also could be due to overlapping distributions of multiple Syn populations distinguished by pigment type. Syn with a low phycourobilin (PUB) to phycoerythrobilin (PEB) ratio pigment type were more abundant at coastal stations, whereas Syn with a high PUB:PEB ratio increased in abundance offshore. Average depth profiles for Pro, Syn, and HBac displayed uniform abundance in the surface mixed layer, with a rapid decrease below the surface mixed layer depth; however, during the Spring Intermonsoon most profiles had a peak at the base of the surface mixed layer. Distributions of Peuc typically displayed a subsurface maximum near the base of the surface mixed layer, except during the SW Monsoon when abundance peaked near the surface. This report is the first to describe the seasonal and spatial variation in microbial community structure in the Arabian Sea over a complete monsoon cycle. Overall, the eucaryotic component was more important at coastal stations, and the procaryotic components were predominant at offshore stations. Pro abundance was restricted to warm oligotrophic waters and was inversely related to surface nitrate concentrations; thus, an increase in the % Pro as a fraction of total procaryote abundance was also indicative of oligotrophic conditions. The effects of SW Monsoonal forcing on microbial community structure resulted in an increase of Peuc, but this response was limited to coastal stations. Pro and Syn remained dominant at offshore stations. (C) 1998 Elsevier Science Ltd. All rights reserved.

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.

1995
Liu, HB, Campbell L, Landry MR.  1995.  Growth and mortality rates of Prochlorococcus and Synechococcus measured with a selective inhibitor technique. Marine Ecology-Progress Series. 116:277-287.   10.3354/meps116277   AbstractWebsite

A selective metabolic inhibitor method has been developed to estimate growth rates and mortalities due to protozoan grazing of the photoautotrophic prokaryotic picoplankton Prochlorococcus and Synechococcus. Laboratory and field experiments show that 1 mg ml(-1) (final concentration) kanamycin inhibits the growth of Prochlorococcus and Synechococcus effectively and does not significantly affect protozoan grazing. At Station ALOHA (22 degrees 45' N, 158 degrees W) 100 km north of Oahu, Hawaii, USA, growth rates of Prochlorococcus ranged from 0.4 to 0.5 d(-1) within the surface mixed layer to about 0.1 d(-1) at the bottom of the euphotic zone. Synechococcus grew faster, with a daily growth rate of up to 1.0 d(-1) in surface waters. Grazing mortalities varied for Prochlorococcus and Synechococcus from 20 to 116% and 43 to 87% of growth rates, respectively. Growth generally exceeded grazing. Because of its high abundance (up to 2 x 10(5) cells ml(-1) in the upper 100 m), Prochlorococcus contributes significantly to phytoplankton biomass and primary production in the subtropical North Pacific Ocean. At Station ALOHA in October 1993, integrated (0 to 175 m) carbon production due to Prochlorococcus was 382.2 mg C m(-2) d(-1). In contrast, Synechococcus produced only 14.6 mg C m(-2) d(-1).

Landry, MR, Kirshtein J, Constantinou J.  1995.  A refined dilution technique for measuring the community grazing impact of microzooplankton, with experimental tests in the central Equatorial Pacific. Marine Ecology-Progress Series. 120:53-63.   10.3354/meps120053   AbstractWebsite

The standard dilution technique can provide unbiased estimates of phytoplankton growth and microzooplankton grazing rates only when certain restrictive assumptions are met. The most important of these assumptions - that grazing impact Varies in direct proportion to the dilution of grazer population density - can be easily violated when clearance rate of individual grazers and/or growth response of the grazer population vary significantly with food concentration over the course of the incubation. We have developed a modified protocol which now allows the dilution technique to be applied unambiguously, even when its original assumptions may be violated. The new protocol uses flow-cytometry measured disappearance of fluorescently labeled tracer cells (FLB or FLA) as an internal measure of 'relative grazing activity' in each dilution treatment. Coefficients of phytoplankton growth and mortality rates are determined from Model II regression analyses of 'net growth' versus 'relative grazing', rather than the usual Model I regressions of 'net growth' versus 'dilution factor'. Tests of this hybrid experimental design in the central equatorial Pacific during an EQPAC cruise in August 1992 gave results essentially identical to the standard dilution interpretation.

1992
Monger, BC, Landrey MR.  1992.  Size-selective grazing by heterotrophic nanoflagellates: An analysis using live-stained bacteria and dual-beam flow cytometry. Arch. Hydrobiol. Beih. Ergebn. Limnol.. 37( Bjoernsen PK, Riemann B, Monger BC, Eds.).:173-185.: SCHWEIZERBART'SCHE VERLAGSBUCHHANDLUNG, STUTTGART (FGR) AbstractWebsite

The effect of prey size on clearance rates of a direct-contact feeding chrysomonad (HNAN-1) was examined using dual-beam flow cytometry (FCM) and fluorescently-labelled prey prepared from living and heat-killed cultures. Over the range in prey sizes used in these experiments (0.7 to 1.4 mu m diameter, clearance rate increased approximately linearly with prey diameter. These results are consistent with model predictions based on the balance of hydrodynamic (repulsive) and van der Waals (attractive) forces. The high precision of FCM analysis allows unequivoval rejection of size-dependencies with exponents greater than 1.8. HNAN-1 does not discriminate between living Synechococcus cells or polystyrene micospheres of comparable size, or between living and heat-killed preparations of the heterotrophic bacteria Vibrio damsela and Pseudomonas diminuta . Significantly higher clearance rates were observed for HNAN-1 feeding on a small, living bacterial isolate from Kaneohe Bay, Hawaii, USA compared to heat-killed cells from the same culture or any of the larger prey used in our experiments.