Microzooplankton Grazing Research Articles

Allochthonous carbon is a major driver of the microbial food web – A mesocosm study simulating elevated terrestrial matter runoff

Climate change predictions indicate that coastal and estuarine environments will receive increased terrestrial runoff via increased river discharge. This discharge transports allochthonous material, containing bioavailable nutrients and light attenuating matter. Since light and nutrients are important drivers of basal production, their relative and absolute availability have important consequences for the base of the aquatic food web, with potential ramifications for higher trophic levels. Here, we investigated the effects of shifts in terrestrial organic matter and light availability on basal producers and their grazers. In twelve Baltic Sea mesocosms, we simulated the effects of increased river runoff alone and in combination. We manipulated light (clear/shade) and carbon (added/not added) in a fully factorial design, with three replicates. We assessed microzooplankton grazing preferences in each treatment to assess whether increased terrestrial organic matter input would: (1) decrease the phytoplankton to bacterial biomass ratio, (2) shift microzooplankton diet from phytoplankton to bacteria, and (3) affect microzooplankton biomass. We found that carbon addition, but not reduced light levels per se resulted in lower phytoplankton to bacteria biomass ratios. Microzooplankton generally showed a strong feeding preference for phytoplankton over bacteria, but, in carbon-amended mesocosms which favored bacteria, microzooplankton shifted their diet towards bacteria. Furthermore, low total prey availability corresponded with low microzooplankton biomass and the highest bacteria/phytoplankton ratio. Overall our results suggest that in shallow coastal waters, modified with allochthonous matter from river discharge, light attenuation may be inconsequential for the basal producer balance, whereas increased allochthonous carbon, especially if readily bioavailable, favors bacteria over phytoplankton. We conclude that climate change induced shifts at the base of the food web may alter energy mobilization to and the biomass of microzooplankton grazers.

Disentangling niche competition from grazing mortality in phytoplankton dilution experiments.

The dilution method is the principal tool used to infer in situ microzooplankton grazing rates. However, grazing is the only mortality process considered in the theoretical model underlying the interpretation of dilution method experiments. Here we evaluate the robustness of mortality estimates inferred from dilution experiments when there is concurrent niche competition amongst phytoplankton. Using a combination of mathematical analysis and numerical simulations, we find that grazing rates may be overestimated—the degree of overestimation is related to the importance of niche competition relative to microzooplankton grazing. In response, we propose a conceptual method to disentangle the effects of niche competition and grazing by diluting out microzooplankton, but not phytoplankton. Our theoretical results suggest this revised “Z-dilution” method can robustly infer grazing mortality, regardless of the dominant phytoplankton mortality driver in our system. Further, we show it is possible to independently estimate both grazing mortality and niche competition if the classical and Z-dilution methods can be used in tandem. We discuss the significance of these results for quantifying phytoplankton mortality rates; and the feasibility of implementing the Z-dilution method in practice, whether in model systems or in complex communities with overlap in the size distributions of phytoplankton and microzooplankton.

Micro-zooplankton grazing as a means of fecal bacteria removal in stormwater BMPs.

A priority for environmental managers is control of stormwater runoff pollution, especially fecal microbial pollution. This research was designed to determine if fecal bacterial grazing by micro-zooplankton is a significant control on fecal bacteria in aquatic best management practices (BMPs); if grazing differs between a wet detention pond and a constructed wetland; and if environmental factors enhance grazing. Both 3-day grazing tests and 24-h dilution assays were used to determine grazing differences between the two types of BMP. Micro-zooplankton grazing was a stronger bacteria removal mechanism in stormwater wetlands rich in aquatic vegetation compared to a standard wet detention pond, although grazing was important in detention ponds as well. Our experiments indicated that the majority of grazers that fed on fecal bacteria were <20 μm in size. Grazing rates were positively correlated with fecal coliform abundance and increased water temperatures. Enumeration of grazers demonstrated that protozoans were significantly more abundant among wetland vegetation than in open water, and open wetland waters contained more flagellates and dinoflagellates than open wet detention pond waters. Grazing on fecal bacteria in BMPs is enhanced by aquatic vegetation, and grazing in aquatic BMPs in warmer climates should be greater than in cooler climates.

Recovering growth and grazing rates from nonlinear dilution experiments

AbstractBiological rate measurements provide critical information for understanding key processes and modeling future states of marine ecosystems. Experimentally derived rates can be challenging to interpret when methodological assumptions are untested or potentially violated under variable natural conditions, such as the assumed linear grazing response of the dilution technique for estimating rates of phytoplankton growth and microzooplankton grazing impact. Here, we show that grazing nonlinearity can be related to the ratio of initial phytoplankton biomass to the half‐saturation parameter in the Holling II model, while not being affected by varying grazer biomass during dilution experiments. From this, we present theory to recover growth and grazing rates from multi‐treatment dilution experiments with nonlinear grazing results. We test our analyses with data collected during the California Current Ecosystem‐Long‐Term Ecological Research (CCE‐LTER) program. We show that using a linear regression in 2‐treatment dilution experiments may lead to underestimates of microzooplankton grazing rates, particularly in high‐phytoplankton‐biomass coastal regions where grazing can be saturated. Using the Holling II grazing model and a correction factor, growth and grazing rates from 2‐treatment experiments can also be estimated, as illustrated by application to Lagrangian water‐tracking studies of growth and grazing dynamics in the CCE.

Сезонная динамика основных первично-продукционных характеристик фитопланктона и его потребление микрозоопланктоном в поверхностном слое прибрежных вод Черного моря

Seasonal variability of phytoplankton biomass, “pure” primary production and its share consumed micro¬zooplankton, as well as the specific microzooplankton grazing rate of phytoplankton for the surface layer (0-1 m) in coastal waters of the Black Sea was studied. Almost 80% of the total annual primary’ produc¬tion of phytoplankton is created by phytoplankton with the average cell volume .from 150 to 4000 mm3. The main share of annual primary production, eaten by microzooplankton (86%) is also associated with these algae sized groups.

Microzooplankton grazing rate in Ensenada of La Paz, B.C.S., Mexico

Phytoplankton growth and microzooplankton grazing rates in situ, were studied in a fixed sampling station from October 2007 to August 2008, at the Ensenada de La Paz, Mexico, using the dilution method. The apparent growth rate of phytoplankton was higher in December (1.33 d-1) and lower in October (0.35 d-1). The grazing rate was highest in April (0.86 d-1) and minimum in February and August (0.38 d-1). Grazing pressure, measured as the percentage of potential production removed (% PPR) varied between 28 and 76%. The relationship between phytoplankton growth and grazing rate indicates that chlorophyll-a production is consumed by approximately 50%, involving other processes that cause mortality of phytoplankton, as sedimentation rate and mesozooplankton grazing. The finding of moderate grazing in autumn and a clear growth and consumption in the winter are features that require monitoring in larger temporal and spatial scales.

Phytoplankton community structure and dynamics in the North Atlantic subtropical gyre

Phytoplankton fuel epipelagic ecosystems and affect global biogeochemical cycles. Nevertheless, there is still a lack of quantitative information about the factors that determine both phytoplankton community structure and dynamics, particularly in subtropical gyres. Here, we estimated size fractionated phytoplankton growth (μ) and microzooplankton grazing rates (m) along a transect in the subtropical North Atlantic, from the island of Hispaniola to the Iberian Peninsula, by conducting dilution experiments and fitting mixed models. We also examined the relationship between nutrient availability and the differences in both phytoplankton community structure and size fractionated phytoplankton growth rates at two spatial scales (i.e. subtropical gyre and within-province spatial scale). Our results revealed high values for both phytoplankton growth and microzooplankton grazing rates. Phytoplankton growth (0.00–1.19d−1) displayed higher variability among stations, biogeochemical provinces and size fractions than the microzooplankton grazing rate (0.32–0.74d−1). Differences in phytoplankton community structure were associated with dissolved inorganic nitrogen (0.72–5.85μM; R2=0.19) and squared Brunt-Väisälä frequency (R2=0.21) at the whole gyre scale. Conversely, the differences in phytoplankton growth rate showed a weak relationship with those properties (R2⩽0.05) at that scale, but a stronger relationship at the within province scale (R2⩾0.07). These results support the idea that phytoplankton grow at high rates in oligotrophic subtropical gyres, this is likely due to the selection of phytoplankton groups with functional traits suited to exploit low nutrient availability. Thus, shedding new, multi-scale knowledge on the commonly misunderstood “ocean deserts”.

Processes driving seasonal variability in DMS, DMSP, and DMSO concentrations and turnover in coastal Antarctic waters

AbstractThis study presents new measurements of the concentrations and turnover rates of dimethyl sulfide (DMS), dimethylsulfoniopropionate (DMSP), and dimethyl sulfoxide (DMSO) in coastal waters near Palmer Station, Antarctica, during the spring and summer of 2012–2013. Using several novel analytical and experimental techniques, we document variability in DMS, DMSP, and DMSO (DMS/P/O) concentrations and quantify dominant production and removal terms in the mixed layer DMS budget. Our results demonstrate considerable seasonal variability in the concentration of DMS (range 0–20 nM), total DMSP (8–160 nM), and total DMSO (4–160 nM). Over the seasonal cycle, dissolved DMSP concentrations were well correlated with total DMSP concentrations and the abundance of Phaeocystis antarctica, while DMSO concentrations (total and dissolved) were well correlated with DMS concentrations. DMSP cleavage from the dissolved pool (mean rate = 5.5 nM d−1) and release from microzooplankton grazing (mean 5.6 nM d−1) were the dominant sources of DMS, with smaller DMS production rates associated with DMSO reduction from the dissolved pool (mean 2.6 nM d−1) and krill grazing (mean 0.82 nM d−1). Specific rate constants for DMSP cleavage were inversely related to net primary production. Bacterial uptake was a primary contributor to DMS removal (mean −12 nM d−1), and we observed a significant correlation between bacterial production and gross DMS loss rate constants. Estimated sea‐air flux and photo‐oxidation constituted secondary DMS sinks. Our experimental and analytical methods provide insight into the DMS/P/O cycle at Palmer Station, and a starting point for future studies examining inter‐annual DMS/P/O variability in coastal Antarctic waters.

Effects of eutrophication on the planktonic food web dynamics of marine coastal ecosystems: The case study of two tropical inlets

We studied the plankton dynamics of two semi-enclosed marine coastal inlets of the north of Jurong Island separated by a causeway (SW Singapore; May 2012–April 2013). The west side of the causeway (west station) has residence times of ca. one year and is markedly eutrophic. The east side (east station) has residence times of one month and presents lower nutrient concentrations throughout the year. The higher nutrient concentrations at the west station did not translate into significantly higher concentrations of chlorophyll a, with the exception of some peaks at the end of the South West Monsoon. Microzooplankton were more abundant at the west station. The west station exhibited more variable abundances of copepods during the year than did the east station, which showed a more stable pattern and higher diversity. Despite the higher nutrient concentrations at the west station (never limiting phytoplankton growth), the instantaneous phytoplankton growth rates there were generally lower than at the east station. The phytoplankton communities at the west station were top-down controlled, largely by microzooplankton grazing, whereas those of the east station alternated between top-down and bottom-up control, with mesozooplankton being the major grazers. Overall, the trophic transfer efficiency from nutrients to mesozooplankton in the eutrophic west station was less efficient than in the east station, but this was mostly because a poor use of inorganic nutrients by phytoplankton rather than an inefficient trophic transfer of carbon. Some hypotheses explaining this result are discussed.

Size-selective microzooplankton grazing on the phytoplankton in the Curonian Lagoon (SE Baltic Sea)

Summary Dilution experiments were performed to estimate phytoplankton growth and microzooplankton grazing rates at two sites: freshwater (Nida) and brackish water (Smiltyne) in the Curonian Lagoon (SE Baltic Sea). Using the size-fractionation approach and dilution experiments, we found that the microzooplankton community was able to remove up to 78% of nanophytoplankton (2–20 μm) standing stock and 130% of the total daily primary production in the brackish waters of the lagoon, and up to 83% of standing stock and 76% of the primary production of picophytoplankton (0.2–2 μm) in the freshwater part. The observed differences were attributed to the changes in ciliate community size and trophic structure, with larger nano-filterers (30–60 μm) dominating the brackish water assemblages and pico-nano filterers (

Environmental and biological factors controlling the spring phytoplankton bloom at the Patagonian shelf-break front – Degraded fucoxanthin pigments and the importance of microzooplankton grazing

The aim of this study was to investigate the biotic and abiotic factors controlling the spring phytoplankton blooms at the Patagonian shelf-break front (PSBF). Using a CHEMTAX analysis of HPLC pigment data and other methods, the biomass and spatial variability of plankton communities were studied in four sections (39–48°S) across the PSBF during October 2005. Environmental factors and the biomass and composition of plankton communities exhibited a marked spatial heterogeneity. The latitudinal and cross-shelf progression in the timing of the spring bloom initiation and the nutritive properties of the water masses (Subantarctic Shelf Waters and Malvinas Current Waters) seemed to be the key factors. Three plankton regions were distinguished: (a) Outer shelf (OS), (b) Shelf-break front (SBF) and (c) Malvinas Current (MC). At the highly stratified OS region, the post-bloom community showed low-biomasshigh-phytoplankton diversity formed mainly by small cells (haptophytes 30–62%, diatoms 17–49%, chlorophytes 0–34%, and prasinophytes 0–21% of total Chl a). High amounts of degraded fucoxanthin were found associated with the heterotrophic dinoflagellate, Protoperidinium capurroi. Grazing by this microheterotroph on the diatom population seemed to be the most important factor for the spring bloom decay at the OS. A remarkable quasi monospecific bloom (∼90%) of a nanodiatom (Thalassiosira bioculata var. raripora) associated with high Chl a (up to 20mgm−3) occurred along (∼1000km) the SBF and in the most northern extension of the MC. In the southern region, the bloom was developed under absent or incipient density stratification, increasing solar irradiance, high nitrate and phosphate availability, and low numbers of phytoplankton grazers. The average mixedlayer PAR irradiance (<2.0mol quanta PAR m−2d−1) and Si:N ratios (<0.2) were low, suggesting a diatom population limited by light and under progressive silicate limitation. The more stratified northern region of the SBF showed a later stage of the bloom development, but the large population of diatoms under Si limitation was not in senescence and losses from microzooplankton grazing were minor. The observed high proportion of Chl a below a shallow upper mixed layer (up to 85%) could directly reach the bed, favoring the development of epibenthic communities and the formation of seed diatom banks and organic iron-rich sediments. The upwelling along the SBF provides a large source of macronutrients and probably the dissolved iron needed to sustain the intense diatom bloom, but also diatom resting stages that could act as seeds for the next spring bloom. The macronutrient-rich MC region showed low chlorophyll (Chl a<0.8mgm−3) and a highly diverse phytoplankton community, mainly composed of small cells (diatoms 20–70%, haptophytes 20–40%, chlorophytes 2–25%, prasinophytes 2–18%, and cryptophytes 3–12% of total Chl a).

Control of nanoflagellate abundance by microzooplankton and viruses in a coastal ecosystem of the subtropical western Pacific

Little is known about the factors controlling variations in nanoflagellate abundance in oceanic environments. In this study, combined size-fractionation with dilution experiments were performed to measure nanoflagellate growth rates, microzooplankton grazing, and virus-mediated mortality collected three times during the summer of 2013. Heterotrophic and pigmented nanoflagellate growth rates varied from 0.39 to 0.49d−1 and 0.39 to 0.52d−1, respectively. Moreover, PNF and HNF loss rates varied from 0.19 to 0.23d−1 and 0.17 to 0.25d−1, respectively. In these cases, a significant impact of microzooplankton grazing was detected, highlighting the important role of microzooplanktons in transferring carbon bound in nanoflagellates to higher trophic levels in this study area. Nanoflagellate net growth rate was not increased in the virus-diluted treatments, suggesting that there was no impact of viral lysis on the mortality of nanoflagellates during summer periods. Furthermore, these results showed a decrease of Synechococcus spp. and bacterial abundance with removal of viruses, and a subsequent decrease in nanoflagellate growth. This result implies that viral infection is an important mechanism in nutrient recycling under oligotrophic oceanic conditions in summer.

Seasonal changes in microzooplankton feeding behavior under varying eutrophication level in the Bahía Blanca estuary (SW Atlantic Ocean)

Dilution experiments were conducted during a two-year survey in the Bahía Blanca estuary (SW Atlantic Ocean) to evaluate seasonal variations of microzooplankton grazing (m) and phytoplankton growth (μ). The trophic interactions between these groups were investigated in relation to anthropogenic eutrophication. For this purpose, two sites exposed differently to sewage input in the estuary were selected. The percentage of primary production grazed by microzooplankton averaged 79%, although this percentage showed high seasonal variability. The lowest consumer control was found in winter and summer, and was associated with high density of chain-forming diatoms. Conversely, the microzooplankton grazing exerted heavy control on phytoplankton biomass during spring and fall, in concurrence with the highest density of nanoflagellates. Results showed average differences between μ and m close to zero during most of the annual cycle, suggesting microzooplankton control of autotrophic prey in the two sites. Under high exposure to sewage, however, experimental results evidenced the prevalence of non-linear feeding response, poor trophic phasing at low phytoplankton growth rate and lower reactivity of microzooplankton grazing. Regardless their exposure to sewage input, microzooplankton was an important loss term of phytoplankton in the estuarine system and appear as a primary factor defining phytoplankton biomass accumulation.

Temperature influence on phytoplankton community growth rates

AbstractA large database of field estimates of phytoplankton community growth rates in natural populations was compiled and analyzed to determine the apparent temperature effect on phytoplankton community growth rate. We conducted an ordinary least squares regression to optimize the parameters in two commonly used growth‐temperature relations (Arrhenius and Q10 models). Both equations fit the observational data equally with the optimized parameter values. The optimum apparent Q10 value was 1.47 ± 0.08 (95% confidence interval, CI). Microzooplankton grazing rates closely matched the temperature trends for phytoplankton growth. This likely reflects a dynamic adjustment of biomass and grazing rates by the microzooplankton to match their available food source, illustrating tight coupling of phytoplankton growth and microzooplankton grazing rates. The field‐measured temperature effect and growth rates were compared with estimates from the satellite Carbon‐based Productivity Model (CbPM) and three Earth System Models (ESMs), with model output extracted at the same month and sampling locations as the observations. The optimized, apparent Q10 value calculated for the CbPM was 1.51, with overestimation of growth rates. The apparent Q10 value in the Community Earth System Model (V1.0) was 1.65, with modest underestimation of growth rates. The GFDL‐ESM2M and GFDL‐ESM2G models produced apparent Q10 values of 1.52 and 1.39, respectively. Models with an apparent Q10 that is significantly greater than ~1.5 will overestimate the phytoplankton community growth response to the ongoing climate warming and will have spatial biases in estimated growth rates for the current era.

Meso‐ and microzooplankton grazing in the Amazon River plume and western tropical North Atlantic

AbstractLargely due to size differences, mesozooplankton are important exporters of carbon and prey for larger organisms, while microzooplankton are important recyclers of nutrients, dominant grazers of phytoplankton, and a key link in the microbial loop. We investigated the relative importance of meso‐ and microzooplankton grazing in the western tropical North Atlantic Ocean (WTNA) and Amazon River plume. Sampling as part of the ANACONDAS project occurred in spring (May–June) 2010 during the peak outflow of the Amazon River and in fall (September–October) 2011 during the plume seasonal retroflection. Mesozooplankton grazing rates decreased with increasing salinity in both seasons, but during the fall both day and nighttime grazing rates were significantly negatively correlated with salinity. Mesozooplankton grazing was highest in plume‐influenced surface waters (0–25 m), and usually dominated by smaller size classes (0.2–0.5 mm and 0.5–1.0 mm). Microzooplankton grazing accounted for approximately 68% of bulk phytoplankton growth across all stations. Comparison of meso‐ and microzooplankton grazing suggests a transition in food web dynamics from a mesozooplankton dominated “export” structure in the plume transitioning to a microzooplankton dominated “retention” structure at mesohaline and oceanic stations above sea surface salinity of 33. Comparison between the seasons suggests a seasonal planktonic succession of low mesozooplankton grazing during the spring peak discharge followed by higher grazing rates and impact by mesozooplankton during the fall retroflection. These results provide important baseline information required for examining effects of climate change on the planktonic food web of the WTNA and for use in biogeochemical models of the region.

Phytoplankton dynamics driven by vertical nutrient fluxes during the spring inter-monsoon period in the northeastern South China Sea

Abstract. A field survey from the coastal ocean zones to the offshore pelagic zones of the northeastern South China Sea (nSCS) was conducted during the inter-monsoon period of May 2014 when the region was characterized by prevailing low-nutrient conditions. Comprehensive field measurements were made for not only hydrographic and biogeochemical properties but also phytoplankton growth and microzooplankton grazing rates. We also performed estimations of the vertical turbulent diffusivity and diffusive nutrient fluxes using a Thorpe-scale method and the upwelling nutrient fluxes by Ekman pumping using satellite-derived wind stress curl. Our results indicated a positive correlation between the integrated phytoplankton chlorophyll a and vertical nutrient fluxes in the offshore region of the nSCS during the study period. We generally found an increasing role of turbulent diffusion but a decreasing role of curl-driven upwelling in vertical transport of nutrients from the coastal ocean zones to the offshore pelagic zones. Elevated nutrient fluxes near Dongsha Islands supported high new production leading to net growth of the phytoplankton community, whereas the low fluxes near the southwest of Taiwan had resulted in a negative net community growth leading to decline of a surface phytoplankton bloom. Overall, phytoplankton dynamics in the large part of the nSCS could be largely driven by vertical nutrient fluxes including turbulent diffusion and curl-driven upwelling during the spring inter-monsoon period.

Phytoplankton pigment specific growth and losses due to microzooplankton grazing in a northern Gulf of Mexico estuary during winter/fall

Microzooplankton dilution grazing experiments were carried out on 6 dates, over a 3 month period at 2 locations in the Bay of St. Louis, MS (BSL) to determine phytoplankton pigment specific growth rates under natural (µ0) and replete (µn) nutrient conditions and microzooplankton grazing. We hypothesized that diatoms would be the largest portion of the phytoplankton composition due to the winter/fall season and that these organisms would have the highest growth/grazing rates. We suspected that river flow from the Jourdan River would adversely affect growth and grazing rates of all phytoplankton classes. Growth rates of 5 phytoplankton accessory pigments (peridinin, fucoxanthin, alloxanthin, zeaxanthin, chlorophyll b) were identified. Intrinsic growth rates (µ0) were often zero or negative (range: -0.46–0.56/d) at the location nearest the Jourdan River, particularly for alloxanthin (e.g. cryptophytes) and peridinin (e.g. dinoflagellates). Significant grazing of chlorophyll a was observed on 3 of 6 dates while grazing on marker pigments was variable. The phytoplankton community appeared nutrient limited during all but one experiment (µ0<µn). Intrinsic growth and grazing rates were correlated (p < 0.05, Spearman Rank Order correlation). Peridinin- and alloxanthin-based growth and grazing rates were correlated positively with salinity suggesting a river influence on these 2 phytoplankton pigment classes. We conclude that in the BSL microzooplankton preferentially grazed on the phytoplankton class which had the highest intrinsic growth rate. We show that this is greatly affected by riverine input into the estuary and nutrient limitation.

Seasonal and size-dependent variations in the phytoplankton growth and microzooplankton grazing in the southern South China Sea under the influence of the East Asian monsoon

Abstract. To examine seasonal and size-dependent variations in the phytoplankton growth and microzooplankton grazing in oligotrophic tropical waters under the influence of seasonal reversing monsoon, dilution experiments were conducted during the summer of 2009 (21 May to 9 June) and winter 2010 (9 to 18 November) in the southern South China Sea (SSCS). The results showed that environmental variables, phytoplankton biomass, phytoplankton growth rate (μ), microzooplankton grazing rate (m), and correlationship (coupling) between the μ and m, rather than the microzooplankton grazing impact on phytoplankton (m/μ) significantly varied between the two seasons. Higher relative preference index (RPI) for the larger-sized (&gt; 3 μm) phytoplankton than pico-phytoplankton (&lt; 3 μm), indicating significant size-selective grazing by microzooplankton on the larger-sized phytoplankton, were also observed. The μ and m were significantly correlated with seawater salinity and temperature, and phytoplankton biomass, which indicated that salient seasonal variations in the phytoplankton growth and microzooplankton grazing in the SSCS were closely related to the environmental variables under the influence of the East Asian monsoon. We propose that intermittent arrivals of the northeast winter monsoon could lead to the low μ and m, and the decoupling between the μ and m in the SSCS, through influencing nutrient supply to the surface water, and inducing surface seawater salinity decrease. The low m/μ (&lt; 50 % on average) indicates low remineralization of organic matter mediated by microzooplankton and mismatch between the μ and m, and thus probably accounts for part of the high vertical biogenic particle fluxes in the prevailing periods of the monsoons in the SSCS. The size-selective grazing suggests that microzooplankton grazing partially contributes to the pico-phytoplankton dominance in the oligotrophic tropical waters such as that of the SSCS.

Phytoplankton growth and microzooplankton grazing dynamics across vertical environmental gradients determined by transplant in situ dilution experiments.

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 (Rpearson_SYN = 0.602 and Rpearson_PEUK = 0.588, P < 0.001). Growth rates of Prochlorococcus (PRO), likely affected by photoinhibition, were not light correlated (Rpearson_PRO = 0.101, P = 0.601). Overall, grazing and growth rates were closely coupled in all picophytoplankton groups (Rspearman_PRO = 0.572, Rspearman_SYN = 0.588, Rspearman_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.

Seasonal variations in the effect of microzooplankton grazing on phytoplankton in the East China Sea

The effect of microzooplankton grazing on phytoplankton in the East China Sea (ECS) was investigated in summer 2009 and winter 2009/2010 using the dilution technique. There were no significant differences in phytoplankton growth rates and microzooplankton grazing rates between coastal (influenced by the Changjiang River plume) and offshore (influenced by the Kuroshio) waters in either season. The mean rates of phytoplankton growth (0.77±0.53d−1) and microzooplankton grazing (0.69±0.42d−1) in summer (n=26) were significantly higher than those in winter (0.39±0.18d−1 and 0.21±0.08d−1 for mean rates of growth and grazing, respectively) (n=24). In both seasons, phytoplankton growth and microzooplankton grazing rates were significantly higher at the surface than at the depth of 5% surface irradiance. Aloricate ciliates were abundant and dominated microzooplankton in the ECS. There were no significant differences in microzooplankton abundance and biomass between seasons or depths. The grazing per microzooplankton was higher in summer than in winter. Phytoplankton growth rates were positively correlated with temperature in the surface waters in summer. Microzooplankton grazing balanced the primary production in summer, while the grazing was low in winter. In winter, the low picophytoplankton and temperature may have been responsible for the low microzooplankton grazing in the ECS.