Microzooplankton Grazing Impact Research Articles

Trophic interactions and diel feeding rhythms of microzooplankton in a productive Swedish Fjord

Abstract Microzooplankton play a pivotal role in the energy transfer between lower and upper trophic levels in marine planktonic food webs. While laboratory data suggest that microzooplankton exhibit higher feeding rates during the daytime, evidence from the field is scarce and contradictory. In this study, we first characterized the nano- and microplanktonic communities of the Gullmar Fjord (Sweden) and its environmental conditions during July and August 2017. Then, we explored the grazing impact of microzooplankton on the phytoplankton community of this ecosystem and assessed their diel grazing activity using the dilution technique. Finally, we evaluated the impact of mesozooplankton at natural concentrations during the experiments. Microzooplankton removed 26% of the phytoplankton standing stock and 96% of the primary production daily, while mesozooplankton did not exert a significant impact on microplankton activity. We did not detect significant diel microzooplankton grazing rhythms during the first experimental period; however, during the second part of the study, after an upwelling event, grazing rates were significantly higher during the night. Therefore, the microzooplankton grazing rhythm in natural systems may vary according to the species composition and abundances of both microzooplankton and prey communities.

Impact of microzooplankton grazing on the phytoplankton community in the Kuroshio of the East China sea: A major trophic pathway of the Kuroshio ecosystem

Many foraging fishes spend their vulnerable life stages with low food availability in the oligotrophic Kuroshio. Although they are likely dependent on microbial production as their energy source, there is limited knowledge of the trophodynamics of the planktonic food web in the Kuroshio of the East China Sea (ECS-Kuroshio). Here, we demonstrate energy sources and feeding impacts of microzooplankton on the phytoplankton community based on size-fractionated dilution experiments in the ECS-Kuroshio. Naked ciliates were the most predominant among micro-sized heterotrophs biomass throughout the study area. Pico- to nano-phytoplankton such as Prochlorococcus and haptophytes contributed to total chlorophyll a concentrations. The apparent growth rates in non-diluted bottles without nutrient enrichment were significantly negative for Prochlorococcus and haptophytes. Microzooplankton grazing rates were higher for nano-phytoplankton (0.17–1.45 d−1) compared with those for pico- (0.18–1.23 d−1) and micro-phytoplankton (0.06–0.92 d−1). The community grazing impacts accounted for up to 122% of primary production. These results suggest that one major pathway at lower trophic levels in the ECS-Kuroshio is from pico- and nano-phytoplankton to microzooplankton. This pathway could support energy transfer to higher trophic levels through food resources of mesozooplankton and fish larvae.

Phytoplankton growth and microzooplankton grazing in the central and northern South China Sea in the spring intermonsoon season of 2017

Phytoplankton growth rates and mortality rates were experimentally examined at 21 stations during the 2017 spring intermonsoon (April to early May) in the northern and central South China Sea (SCS) using the dilution technique, with emphasis on a comparison between the northern and central SCS areas which had different environmental factors. There had been higher temperature but lower nutrients and chlorophyll a concentrations in the central SCS than those in the northern SCS. The mean rates of phytoplankton growth (μ0) and microzooplankton grazing (m) were (0.88±0.33) d−1 and (0.55±0.22) d−1 in the central SCS, and both higher than those in the northern SCS with the values of μ0 ((0.81±0.16) d−1) and m ((0.30±0.09) d−1), respectively. Phytoplankton growth and microzooplankton grazing rates were significantly coupled in both areas. The microzooplankton grazing impact (m/μ0) on phytoplankton was also higher in the central SCS (0.63±0.12) than that in the northern SCS (0.37±0.06). The microzooplankton abundance was significantly correlated with temperature in the surface. Temperature might more effectively promote the microzooplankton grazing rate than phytoplankton growth rate, which might contribute to higher m and m/μ0 in the central SCS. Compared with temperature, nutrients mainly affected the growth rate of phytoplankton. In the nutrient enrichment treatment, the phytoplankton growth rate (μn) was higher than μ0 in the central SCS, suggesting phytoplankton growth in the central SCS was nutrient limited. The ratio of μ0/μn was significantly correlated with nutrients concentrations in the both areas, indicating the limitation of nutrients was related to the concentrations of background nutrients in the study stations.

Trophic interactions of micro- and mesozooplankton in the Amundsen Sea polynya and adjacent sea ice zone during austral late summer

To elucidate the trophic link between micro- and mesozooplankton in the Amundsen Sea polynya (ASP) and adjacent sea ice zone, we estimated the impact of microzooplankton grazing on major phytoplankton groups, as well as the diet composition, ingestion rate, and prey selectivity of two copepods and Euphausia crystallorophias larvae on microbial communities during the late summer. Phaeocystis antarctica, which was ingested by micro- and mesozooplankton, comprised most phytoplankton biomass. Herbivorous microzooplankton consumed at least half of the phytoplankton production, but the microzooplankton grazing may not contribute strongly to the decline of the phytoplankton bloom. Three mesozooplankton species (Calanoides acutus, Metridia gerlachei, and E. crystallorophias larvae) preferentially grazed on ciliates and heterotrophic dinoflagellates (HDF) with sizes from 20 to >100 µm over phytoplankton. Although microzooplankton comprised only an average of 41.7 ± 3.2% o the total carbon available in the natural prey pool, they accounted for an average of 75.4 ± 2.9% of total carbon ingested by copepods and krill larvae. Heterotrophic food sources made up a substantial proportion of mesozooplankton diets, with strong positive selection for microzooplankton at most locations regardless of phytoplankton size and type. In particular, HDF comprised the major dietary component of mesozooplankton in the study area. The presence of mesozooplankton reduced the grazing pressure on P. antarctica and diatoms through predation on herbivorous microzooplankton. Approximately half of the primary production capacity may have indirectly reached mesozooplankton through microzooplankton consumption. Thus, strongly selective feeding behavior and higher grazing pressure on microzooplankton indicated the importance of microheterotrophic pathways through strong trophic coupling between mesozooplankton and the microbial food web during the decline of phytoplankton bloom. In the highly productive ASP system, food web structure can be classified as multivorous, whereby herbivorous and omnivorous modes both play significant roles in carbon export, enhancing the efficiency of the pelagic food web.

Beyond Eutrophication: Vancouver Lake, WA, USA as a Model System for Assessing Multiple, Interacting Biotic and Abiotic Drivers of Harmful Cyanobacterial Blooms

Eutrophication of lakes and reservoirs has contributed to an increase in the magnitude and frequency of harmful cyanobacterial blooms; however, the interactive effects of nutrient availability (eutrophication) and other abiotic and biotic drivers have rarely been comprehensively studied in the field. We undertook an eight-year (2005–2013) research program that assessed the interaction of multiple factors driving cyanobacterial blooms in Vancouver Lake, a large, shallow eutrophic lake in Washington, USA. Our program consisted of nearly continuous monthly or weekly monitoring of water quality and plankton community composition over eight years, as well as multiple zooplankton grazing experiments over three years. We found a relatively consistent seasonal succession of phytoplankton and zooplankton assemblages, and a pattern of interacting factors influencing cyanobacterial bloom dynamics. Typically, a combined effect of decreased dissolved inorganic nitrogen (N), a sudden increase of dissolved inorganic phosphorus (P), and a cascading effect of zooplankton grazing created a ‘perfect storm’ of conditions that promoted the rapid proliferation of cyanobacteria over the two to three weeks before a bloom. At the blooms’ peaks, cyanobacterial carbon biomass reached as high as 20 µg L−1, with total [chl a] often exceeding 750 µg L−1. In the weeks following the blooms’ peaks, [PO4-P] and [NH4-N] dropped and copepod feeding rates fell to near zero, whereas microzooplankton grazing rates reached their maxima. Microzooplankton grazing impact, combined with low nutrient availability, then drove down cyanobacteria abundance. Vancouver Lake serves as a model for understanding multiple, interacting drivers of cyanobacterial bloom dynamics in shallow, temperate lakes, and is therefore an important system in which to investigate new questions related to the science and management of harmful algal blooms.

Microzooplankton grazing impact on the phytoplankton community at a coastal upwelling

Experiments were carried out at a coastal upwelling site (ENSENADA station) off northern Baja California (México) during autumn 2015 (OCT-15) and spring 2016 (APR-16) to estimate phytoplankton daily growth (µo) and mortality (m) rates and to assess microzooplankton grazing impact (m:µo) on the phytoplankton community and specific autotrophic groups. In accordance with regional seasonality and under an environmental warming scenario due to the El Niño 2015–2016 event, significant differences in both hydrographic conditions and the growth–mortality dynamics of the phytoplankton community were observed between the 2 study periods. The µo and m estimates were, respectively, 0.120 ± 0.012 d–1 and 1.145 ± 0.049 d–1 for OCT-15 and 1.186 ± 0.002 d–1 and 0.409 ± 0.086 d–1 for APR-16. The results of this study suggest that the effects of the anomalous warming on the phytoplankton community were more evident in OCT-15. During that period, growth of the larger autotrophic components (diatoms) was severely controlled by the environmental limitation of nutrients caused by the sinking of the thermocline that resulted from the entrance of warm water to the region. Furthermore, microzooplankton exerted active grazing pressure on phytoplankton biomass (72% of chlorophyll a [Chla]) and primary production (PP = 0.20 µg Chla·L–1·d–1), with grazing impact >100% of PP. In APR-16, when the ecosystem apparently started returning to the spring conditions, a high value for PP (3.73 µg Chla·L–1·d–1) was estimated, with only one third of it being consumed by microzooplankton (34% of PP). The results of this research evidence the high dynamism of multivorous food webs coupled to the seasonal and interannual variability of coastal upwelling systems.

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.

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 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 (> 3 μm) phytoplankton than pico-phytoplankton (< 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 (< 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.

Microzooplankton herbivory and community structure in the Amundsen Sea, Antarctica

We examined microzooplankton abundance, community structure, and grazing impact on phytoplankton in the Amundsen Sea, Western Antarctica, during the early austral summer from December 2010 to January 2011. Our study area was divided into three regions based on topography, hydrographic properties, and trophic conditions: (1) the Oceanic Zone (OZ), with free sea ice and low phytoplankton biomass dominated by diatoms; (2) the Sea Ice Zone (SIZ), covered by heavy sea ice with colder water, lower salinity, and dominated by diatoms; and (3) the Amundsen Sea Polynya (ASP), with high phytoplankton biomass dominated by Phaeocystis antarctica. Microzooplankton biomass and communities associated with phytoplankton biomass and composition varied among regions. Heterotrophic dinoflagellates (HDF) were the most significant grazers in the ASP and OZ, whereas ciliates co-dominated with HDF in the SIZ. Microzooplankton grazing impact is significant in our study area, particularly in the ASP, and consumed 55.4–107.6% of phytoplankton production (average 77.3%), with grazing impact increasing with prey and grazer biomass. This result implies that a significant proportion of the phytoplankton production is not removed by sinking or other grazers but grazed by microzooplankton. Compared with diatom-based systems, Phaeocystis-based production would be largely remineralized and/or channeled through the microbial food web through microzooplankton grazing. In these waters the major herbivorous fate of phytoplankton is likely mediated by the microzooplankton population. Our study confirms the importance of herbivorous protists in the planktonic ecosystems of high latitudes. In conclusion, microzooplankton herbivory may be a driving force controlling phytoplankton growth in early summer in the Amundsen Sea, particularly in the ASP.

Decrease in diatom palatability contributes to bloom formation in the Western English Channel

The aim of this paper is to investigate the role of phytoplankton nutritional status in the formation of the spring bloom regularly observed at the station L4 in the Western English Channel. Using a modelling approach, we tested the hypothesis that the increase in light from winter to spring induces a decrease in diatom nutritional status (i.e., an increase in the C:N and C:P ratios), thereby reducing their palatability and allowing them to bloom. To this end, a formulation describing the Stoichiometric Modulation of Predation (SMP) has been implemented in a simplified version of the European Regional Seas Ecosystem Model (ERSEM). The model was coupled with the General Ocean Turbulence Model (GOTM), implemented at the station L4 and run for 10years (2000–2009). Simulated carbon to nutrient ratios in diatoms were analysed in relation to microzooplankton biomass, grazing and assimilation efficiency. The model reproduced in situ data evolutions and showed the importance of microzooplankton grazing in controlling the early onset of the bloom. Simulation results supported our hypothesis and provided a conceptual model explaining the formation of the diatom spring bloom in the investigated area. However, additional data describing the microzooplankton grazing impact and the variation of carbon to nutrient ratios inside phytoplanktonic cells are required to further validate the proposed mechanisms.

Does microzooplankton grazing contribute to the pico-phytoplankton dominance in subtropical and tropical oligotrophic waters?

Microzooplankton grazing has been proposed as an important factor regulating the phytoplankton size composition, particularly in oligotrophic waters, but it has not been well examined. To examine the role of microzooplankton grazing in the pico-phytoplankton dominance in subtropical and tropical oligotrophic waters, dilution experiments with size-fractionated chl-a were conducted in the South China Sea (SCS) in August–September 2008 and May 2009. Total phytoplankton growth rate (µ) was 0.34–1.98 d−1(0.96 ± 0.49 d−1). The corresponding microzooplankton grazing rate (m) was 0.30–1.99 d−1 (0.83 ± 0.54 d−1). The microzooplankton grazing impact (m/µ) on total phytoplankton was 83.5% ± 76.3%. The growth rates of pico-phytoplankton (<3 µm) (1.02 ± 0.55 d−1) were higher than those of nano- (3–20 µm) (0.95 ± 0.58 d−1) and micro-cells (20–200 µm) (0.60 ± 0.71 d−1), while the microzooplankton grazing rates on pico-phytoplankton (0.77 ± 0.58 d−1) were lower than those on nano-cells (1.00 ± 0.79 d−1), resulting in the higher grazing impact on larger cells. Increase of the pico-cell proportion and decrease of the nano- and micro-cell proportions after the incubation coincided well with the varied grazing impacts. Our results indicate that the high microzooplankton grazing impact keeps the phytoplankton standing stock at a low level, and the selective grazing of microzooplankton on nano-phytoplnakton as well as the low growth rates of micro-phytoplankton contributes to the pico-phytoplankton dominance in subtropical and tropical oligotrophic waters of the SCS. Variation in the phytoplankton-size-dependent grazing impact in different studies has been discussed.

Microzooplankton grazing and selective feeding during bloom periods in the Tolo Harbour area as revealed by HPLC pigment analysis

Dilution experiments were conducted to investigate microzooplankton grazing impact on phytoplankton of different taxonomic groups and size fractions (<5, 5–20, 20–200μm) during spring and summer bloom periods at two different sites (inner Tolo Harbour and Tolo Channel) in the Tolo Harbour area, the northeastern coastal area of Hong Kong. Experiments combined with HPLC pigment analysis in three phytoplankton size fractions measured pigment and size specific phytoplankton growth rates and microzooplankton grazing rates. Pigment-specific phytoplankton growth rates ranged between 0.08 and 3.53d−1, while specific grazing rates of microzooplankton ranged between 0.07 and 2.82d−1. Highest specific rates of phytoplankton growth and microzooplankton grazing were both measured in fucoxanthin in 5–20μm size fraction in inner Tolo Harbour in summer, which coincided with the occurrence of diatom bloom. Results showed significant correlations between phytoplankton growth and microzooplankton grazing rates. Microzooplankton placed high grazing pressure on phytoplankton community. High microzooplankton grazing impact on alloxanthin (2.63–5.13) suggested strong selection toward cryptophytes. Our results provided no evidence for size selective grazing on phytoplankton by microzooplankton.

北部湾北部海域浮游动物若干生态学研究V：夏季微型浮游动物对浮游植物的摄食压力

PDF HTML阅读 XML下载 导出引用 引用提醒 北部湾北部海域夏季微型浮游动物对浮游植物的摄食压力 DOI: 10.5846/stxb201207201034 作者: 作者单位: 厦门大学海洋与地球学院,厦门大学海洋与地球学院,厦门大学海洋与地球学院,厦门大学海洋与地球学院,厦门大学海洋与地球学院,厦门大学海洋与地球学院,厦门大学海洋与地球学院 作者简介: 通讯作者: 中图分类号: 基金项目: 海湾公益项目（201005012） An ecological study on zooplankton in the northern Beibu Gulf V: the effects of microzooplankton grazing on phytoplankton in summer Author: Affiliation: College of ocean,College of ocean,,,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:2011年8月份于北部湾北部海域5个观测站位获得的分层水样，分析了表层叶绿素a含量和表层微型浮游动物丰度以及类群组成；同时于现场采用稀释培养法研究了该海域浮游植物生长率（μ）和微型浮游动物的摄食率（g）。分析和测定结果表明：调查海区的微型浮游动物丰度400-1167 个/L，类群组成以无壳纤毛虫为主；浮游植物的生长率为-1.50-1.13 d-1，微型浮游动物摄食率为0.33-1.08 d-1；推算微型浮游动物对浮游植物现存量以及初级生产力的摄食压力分别为28.1%-66.0%和-7.4%-438.4%。相对于中国其他海区，8月份北部湾北部海域微型浮游动物摄食速率处于中等水平。调查期间，广西沿海高生产力海区，浮游植物生长率大于微型浮游动物动物的摄食率，浮游植物生物量处于积累期；涠洲岛以南海域，浮游植物生产力较低，微型浮游动物摄食作用是控制浮游植物生长的重要因素。 Abstract:In August 2011, we quantified the impact of microzooplankton grazing on phytoplankton production in the surface waters of the northern Beibu Gulf, Vietnam. Shipboard dilution incubation experiments were carried out at five stations to calculate phytoplankton specific growth rates and the specific rates of grazing losses to microzooplankton. Dilution experiments using chlorophyll a (Chl a) as a tracer were used to estimate daily rates in two size fractions; image-analyzed microscopy provided quantitative estimates of microzooplankton standing stock (including ciliate and vertebrate larvae). PFW (particle-free water) was used to dilute seawater to five target dilutions of 0%, 20%, 40%, 60%, and 80%. Microzooplankton grazing and phytoplankton growth rates were estimated by the linear regression of AGR (apparent growth rate) versus dilution factor. We estimated the grazing impact of microzooplankton on phytoplankton by calculating the percentage of phytoplankton standing stock and potential primary production ingested. Average total chlorophyll a concentration was (0.67±0.58) μg/L. Small-celled phytoplankton dominated this particular community. A notable characteristic of the phytoplankton community structure was that nano-and picoplankton made up a significant portion. Aloricate ciliates dominated the microzooplankton samples, in particular Strombidium, which accounted for 27.2% of total aloricate ciliate abundance. In addition, loricate ciliates and copepod nauplii were recorded. Microzooplankton density varied from 400 to 1167 ind/L. The horizontal distribution of microzooplankton was greater in nearshore than in offshore waters. Abundance did not differ significantly with chlorophyll a concentration in this study; however, there was a positive correlation between microzooplankton abundance and temperature. The phytoplankton growth rate (<200 μm) ranged between -1.50 and 1.13 d-1, with the highest values being associated with nearshore waters. Meanwhile phytoplankton mortality due to microzooplankton grazing (0.33-1.08 d-1) was highest at the same nearshore sites; however, growth rates were low at these sites. Consequently, microzooplankton consumed 28.1%-66.0% of chlorophyll a standing stocks and -8.1%-438.4% of the primary production per day. However, the grazing rate of microzooplankton on nanoplankton (2-20 μm) varied from 1.87 to 0.52 d-1, with an average of (0.94±0.57) d-1. Given that the rates of grazing on phytoplankton of <200 μm were lower than those on nanoplankton, we conclude that microzooplankton prefer to graze on nanoplankton. There was a positive correlation between phytoplankton growth rate and grazing rate; in addition, significant differences between grazing rate and chlorophyll a were found. This suggests that microzooplankton grazing activity is the main factor regulating the community structure and restraining the growth of phytoplankton. During the investigation period, microzooplankton grazing effectively controlled the growth of phytoplankton, consuming 176.2% of primary production on average. When grazing rates are far greater than phytoplankton growth rates, energy is transferred from lower to higher trophic levels more effectively. In the surface waters of the northern Beibu Gulf, microzooplankton largely control the growth of phytoplankton by grazing offshore rather than in nearshore waters where primary production is high. The growth and grazing rates combined promote planktonic ecosystem stability in this area. These results indicate that grazing by microzooplankton is a key process controlling the growth of phytoplankton in this area. 参考文献 相似文献 引证文献

Microzooplankton grazing along the Western Antarctic Peninsula

The significance of microzooplankton as grazers in pelagic ecosystems has been established, yet relatively few studies of microzooplankton grazing, compared to that of macro- zooplankton, have been conducted in the Southern Ocean. We report phytoplankton and bacterial growth and grazing mortality rates along the Western Antarctic Peninsula (WAP), a region of rapid climate change. Growth and grazing rates were determined by dilution experiments at select stations along the WAP in January of 2009 to 2011 and in the nearshore waters near Palmer Station in February and March 2011. Microzooplankton exerted higher grazing pressure on bac- teria compared to phytoplankton along the WAP and also selectively grazed on smaller phyto- plankton (picoautotrophs and nanophytoplankton) and on the more actively growing (high nucleic acid) bacterial cells. Among all phytoplankton size classes, growth rates ranged from undetectable (i.e. not significant; NS) to 0.99 d �1 , grazing mortality rates were NS to 0.56 d �1 , and microzooplankton removed 100% of daily bacterial produc- tion. There was a significant (albeit weak) exponential relationship between temperature and phytoplankton mortality, although the range of experimental temperatures was small. The present study provides a reference point of microzooplankton grazing impact along the WAP in the sum- mer and contributes valuable information to studies modeling the flow of carbon through the WAP food web, improving our ability to predict climate-induced changes in the WAP ecosystem.

Effects of trophic cascades in dilution grazing experiments: from artificial saturated feeding responses to positive slopes

Dilution grazing experiments are the most commonly used method for quantifying microzooplankton grazing rates on phytoplankton. However, in some cases, the outcomes of these experiments yield non-significant grazing impacts, even when microbial grazers are numerous, or result in unexpected non-linear functions or positive slopes. Here, we argue that some of these situations and others that may go unnoticed can be explained by trophic cascades occurring in the community confined within the incubation experiments. When trophic cascades are present, simple linear models relating dilution levels to phytoplankton net growth rates can be distorted into artificial saturation curves, V-shaped responses or a non-significant fit and may even invert the sign of the slope. These artefacts have minor consequences for true estimates of phytoplankton mortality rates when the dilution level is sufficiently high (ca. 5–10%), but they may result in significant underestimation of the microzooplankton grazing impact when working at lower, typical maximum dilution

Microzooplankton grazing impact in the Bering Sea during spring sea ice conditions

Microzooplankton grazing impact on phytoplankton in the Bering Sea during spring was assessed in 2008, 2009 and 2010 using two-point dilution assays. Forty-nine experiments were completed in a region encompassing shelf to slope waters, including the 70m line along the edge of the shelf. A variety of conditions were encountered, with a concomitant range of trophic states, from pre-bloom low chlorophyll-a (Chl-a)<3μgl−1 during heavy ice cover to late spring open water diatom blooms with Chl-a up to 40μgl−1. Microzooplankton biomass was dominated by large heterotrophic dinoflagellates and ciliates. Both athecate and thecate dinoflagellates, as well as some species of ciliates, fed on diatom cells and chains. Other types of protists, notably thecate amoebae and parasitoid flagellates, were also observed preying on diatoms. Total microzooplankton biomass ranged from 0.1 to 109μgCl−1 and was positively related to Chl-a concentration. Significant rates of microzooplankton herbivory were found in 55% of dilution experiments. Maximum grazing rate was 0.49d−1, and average grazing rate, including experiments with no significant grazing, was 0.09±0.10d−1. Phytoplankton intrinsic growth rates varied from slightly negative growth to >0.4d−1. Microzooplankton grazing was significant in both non-bloom and bloom conditions, averaging 46±75% of phytoplankton daily growth. Based on the amount of phytoplankton carbon consumed, we estimated potential microzooplankton community growth rates of up to 1.3d−1. Our results confirm the importance of protist grazers in planktonic food webs of high latitude ecosystems. We also conclude that our finding of significant grazing by microzooplankton on spring blooms in the Bering Sea does not support theories about phytoplankton bloom formation based on escape from grazing, due either to predation resistance or to slow growth of herbivorous protists at cold temperature.

No microzooplankton grazing during a Mediopyxis helysia dominated diatom bloom

The new diatom species Mediopyxis helysia was described to science from clones found in 2003 in the North Sea, northern Wadden Sea, and the Gulf of Maine. Seven years after its first occurrence, we observed Mediopyxis to contribute up to almost 50% of the biovolume of the diatoms during a diatom spring bloom in the western Wadden Sea. Grazing experiments based on the dilution technique could not detect any microzooplankton grazing impact on the bloom community. Mediopyxis is now also well established in the western Wadden Sea and does have the potential to become a dominant species. The reasons for its success remain largely unresolved but avoidance of being grazed might be one factor. Future research on this new species is needed to understand the success and forecast the ecological footprint of this large diatom species arriving in the western European Seas.

Selectivity and grazing impact of microzooplankton on phytoplankton in two subtropical semi-enclosed bays with different chlorophyll concentrations

Microzooplankton grazing rates were compared between two sites (S1 and S2) in the coastal seas of eastern Hong Kong with similar physio-chemical parameters, but different chlorophyll concentrations. During the period from March 2007 to January 2008, six sets of dilution experiments, combined with high performance liquid chromatography and phytoplankton size fractionation (<200μm, <20μm and <5μm), were carried out to study the microzooplankton grazing rate on phytoplankton of different taxonomic groups and sizes. Although total chlorophyll a concentrations were much higher in S1 (4.98–18.42μgl−1) than in S2 (0.29–1.68μgl−1), size composition of phytoplankton was relatively similar between the two sites. Measured as chlorophyll a, phytoplankton growth rates (−0.84–1.91d−1 in S1; 0.03–2.85d−1 in S2) and microzooplankton grazing rates (0.00–2.26d−1 in S1; 0.00–1.49d−1 in S2) for all three size fractions were similar between the two bays. Phytoplankton growth rates and microzooplankton grazing rates measured as other pigments for phytoplankton of different size fractions did not show strong variations. Microzooplankton grazing impact, expressed as the ratio of microzooplankton grazing rate to phytoplankton growth rate, was generally higher in S1 than in S2, although the difference was not statistically significant. High microzooplankton grazing impact on alloxanthin (1.00–45.85) suggested strong selection toward cryptophytes. Our results provided no evidence for size selective grazing on phytoplankton by microzooplankton.

Changes in phytoplankton and microzooplankton populations during grazing experiments at a Mediterranean coastal site

Microzooplankton grazing on phytoplankton populations was investigated by means of eight dilution experiments in different seasons and trophic conditions at station Mare Chiara, a long-term study site in the Gulf of Naples, Tyrrhenian Sea. To check for changes in prey and predator populations, size-fractionated chlorophyll and HPLC analyses and microzooplankton counts were performed before and after the incubations. On average, 68% of daily phytoplankton production was consumed by the microzooplankton in spring and summer, and in winter, more than 100% of phytoplankton daily production was consumed. Nutrients were added in four of our experiments. Diatoms (as recorded by fucoxanthin concentrations) increased significantly in the nutrient-enriched experiments, and unexpectedly, also heterotrophic dinoflagellate growth was enhanced by nutrient additions. Remarkable changes in grazer abundance were observed in several experiments; a substantial difference was found in microzooplankton grazing pressure (g/k × 100) as calculated by regression of apparent chlorophyll a growth against nominal dilution factor or against average predator concentrations, 72 ± 19% and 60 ± 23% of phytoplankton daily production, respectively. However, our observations suggest that if growth and mortality of grazers in the dilution series are not considered, microzooplankton grazing impact may be either underestimated, at low phytoplankton growth rates, or overestimated in more productive conditions.