Microzooplankton Grazing Research Articles

Microzooplankton grazing on the coccolithophore Emiliania huxleyi and its role in the global calcium carbonate cycle.

Identifying mechanisms driving the substantial dissolution of biogenic CaCO3 (60 to 80%) in surface and mesopelagic waters of the global ocean is critical for constraining the surface ocean's alkalinity and inorganic carbon budgets. We examine microzooplankton grazing on coccolithophores, photosynthetic calcifying algae responsible for a majority of open-ocean CaCO3 production, as a mechanism driving shallow dissolution. We show that microzooplankton grazing dissolves 92 ± 7% of ingested coccolith calcite, which may explain 50 to 100% of the observed CaCO3 dissolution in supersaturated surface waters. Microzooplankton grazing on coccolithophores is thus a substantial, previously unrecognized biological mechanism affecting the ballasting of organic carbon to deeper waters, the ecology and fitness of microzooplankton themselves due to buffering of food vacuole pH, and ultimately the continued ability of the surface ocean to take up atmospheric carbon dioxide.

Mortality partitioning between viral lysis and microzooplankton grazing in successive phytoplankton blooms using dilution and molecular methods

Phytoplankton play crucial roles in aquatic ecosystems, serving as the foundation of marine food webs and being responsible for ~50% of the world’s oxygen production. Predation by microzooplankton and viral lysis are the major sources of phytoplankton mortality, with the balance between these 2 processes affecting microbial food webs and biogeochemical cycles. However, determining the dominant mortality process in time and space remains an open question. This study investigated microzooplankton grazing and viral lysis rates during a mesocosm experiment in western Norway. High-resolution measurements were determined on phytoplankton groups using flow cytometry to observe changes in mortality rates and carbon flow during phytoplankton blooms. Digital droplet PCR was employed to detect Emiliania huxleyi and Micromonas spp. and associated viruses within environmental samples, to explore its use for determining mortality processes and understanding the impact on the phytoplankton community. The results showed that grazing and viral lysis dominated mortality at different times, with only one significant instance of both processes being observed. Microzooplankton grazing primarily affected picoplankton, while nanoeukaryotes and E. huxleyi were more susceptible to viral lysis. Molecular detection did not always match with abundances or rates determined by flow cytometry; however, it did provide insights into their dynamics throughout the mesocosm. These findings provide insights into the complex interactions between microzooplankton, viruses and phytoplankton communities. Understanding the balance between microzooplankton grazing and viral lysis can contribute to a more comprehensive understanding of carbon flow in aquatic ecosystems, which has significant implications for food webs and biogeochemical cycles.

Trophic flows to mesozooplankton support the conventional paradigm of pelagic food web structure in ocean ecosystems

Abstract The current conventional paradigm of ocean food web structure inserts one full level or more of microzooplankton heterotrophic consumption, a substantial energy drop, between phytoplankton and mesozooplankton. Using a dataset with contemporaneous measurements of primary production (PP), size-fractioned mesozooplankton biomass, and micro- and mesozooplankton grazing rates from 10 tropical to temperate ocean ecosystems, we examined whether the structural inefficiencies in this paradigm allow sufficient energy transfer to support active metabolism and growth of observed zooplankton standing stocks. Zooplankton carbon requirements (ZCR) were determined from allometric equations that account for ecosystem differences in temperature and size structure. ZCRs were relatively low (∼30% of PP or less) for both oligotrophic systems and bloom biomass accumulation in eutrophic coastal waters. Higher relative ZCRs (>30% PP) were associated with elevated mesozooplankton grazing scenarios (bloom declines, abundant salps), advective subsidies, and open-ocean upwelling systems. Microzooplankton generally dominated as grazers of PP but were equal or secondary to direct herbivory as nutritional support for mesozooplankton in five of eight regional studies. All systems were able to satisfy ZCR within the conventional food-web interpretation, but balanced open-ocean upwelling systems required the most efficient alignments of contributions from microzooplankton grazing, direct herbivory, and carnivory to do so.

Peculiarities of seasonal dynamics of net primary production and its microzooplankton grazing in the coastal waters of the Black Sea (Sevastopol region)

The studies of net primary production (NPP) seasonal dynamics, its microzooplankton grazing were conducted at two stations in the western Black Sea coastal waters near Sevastopol during a period from January 2020 to December 2022. Each year, one maximum of the phytoplankton NPP (280–820 mg C·m-3·day-1) was observed in July, when the water temperature ranged from 22 to 25 °C, and solar radiation intensity was 48–50 E·m-2·day-1. Large diatoms and dinoflagellates played a key role in its formation. At this time, the values of the net phytoplankton growth rate were 0.3–1.0 day-1, and the relative share of primary production consumed by microzooplankton (g/m) did not exceed 26%. In December – February, when the water temperature was 8.0–13.5 °C and the intensity of solar radiation was 10–15 E·m-2·day-1, the NPP decreased to a minimum (4–38 mg C·m-3·day-1). Under these conditions, the net phytoplankton growth rate decreased to 0–0.40 day-1, and g/m was 15–100%. For three years at station 2, the contribution of large algae with cell volumes from 4000 to 18000 µm3 in the NPP was significantly higher than at station 1. It was 61.1 % in 2020, 69.7 % in 2021 and 23% in 2022. The consumed NPP for this size group of algae was 54 % in 2020, 68.4 % in 2021 and 47.8 % in 2022.

Impact of ocean acidification on microzooplankton grazing dynamics

This study examines the potential impacts of projected atmospheric carbon dioxide (pCO2) levels reaching 800 ppm by the end of the century, focusing on ocean acidification effects on marine ecosystems in the coastal areas of Bohai. We investigated how acidification affects the grazing patterns of microzooplankton using dilution techniques and ecophysiological methods. Our findings indicate that acidic conditions shift the phytoplankton community structure, changing dominant species. Elevated CO2 concentrations reduced grazing pressure on phytoplankton, with less steep declines in growth rates at 800 ppm CO2 (spring: 2.43 d−1 vs. 2.16 d−1, summer: −0.46 d−1 vs. −0.73 d−1, autumn: −0.45 d−1 vs. −0.90 d−1) and significant decreases in grazing pressure percentages (%Pp from 0.84 to 0.58 and %Pi from 0.64 to 0.46). Short-term acid exposure significantly increased superoxide dismutase activity in both microplankton (from 0.03 to 0.08 U mg−1, p<0.01) and nanoplankton (from 0.05 to 0.09 U mg−1, p<0.001), indicating an adaptive response to oxidative stress. These results highlight that elevated CO2 levels primarily boost phytoplankton growth by reducing microzooplankton grazing pressure, resulting in higher growth rates and a shift towards smaller-sized phytoplankton, reflecting complex short-term ecological responses to acidification. Further research is needed to understand the long-term effects of ocean acidification on microzooplankton and their role in marine secondary productivity.

Viral Infection Leads to a Unique Suite of Allelopathic Chemical Signals in Three Diatom Host-Virus Pairs.

Ecophysiological stress and the grazing of diatoms are known to elicit the production of chemical defense compounds called oxylipins, which are toxic to a wide range of marine organisms. Here we show that (1) the viral infection and lysis of diatoms resulted in oxylipin production; (2) the suite of compounds produced depended on the diatom host and the infecting virus; and (3) the virus-mediated oxylipidome was distinct, in both magnitude and diversity, from oxylipins produced due to stress associated with the growth phase. We used high-resolution accurate-mass mass spectrometry to observe changes in the dissolved lipidome of diatom cells infected with viruses over 3 to 4 days, compared to diatom cells in exponential, stationary, and decline phases of growth. Three host virus pairs were used as model systems: Chaetoceros tenuissimus infected with CtenDNAV; C. tenuissimus infected with CtenRNAV; and Chaetoceros socialis infected with CsfrRNAV. Several of the compounds that were significantly overproduced during viral infection are known to decrease the reproductive success of copepods and interfere with microzooplankton grazing. Specifically, oxylipins associated with allelopathy towards zooplankton from the 6-, 9-, 11-, and 15-lipogenase (LOX) pathways were significantly more abundant during viral lysis. 9-hydroperoxy hexadecatetraenoic acid was identified as the strongest biomarker for the infection of Chaetoceros diatoms. C. tenuissimus produced longer, more oxidized oxylipins when lysed by CtenRNAV compared to CtenDNAV. However, CtenDNAV caused a more statistically significant response in the lipidome, producing more oxylipins from known diatom LOX pathways than CtenRNAV. A smaller set of compounds was significantly more abundant in stationary and declining C. tenuissimus and C. socialis controls. Two allelopathic oxylipins in the 15-LOX pathway and essential fatty acids, arachidonic acid (ARA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) were more abundant in the stationary phase than during the lysis of C. socialis. The host-virus pair comparisons underscore the species-level differences in oxylipin production and the value of screening more host-virus systems. We propose that the viral infection of diatoms elicits chemical defense via oxylipins which deters grazing with downstream trophic and biogeochemical effects.

Importance of microzooplankton for sustaining high mesozooplankton biomass during post-bloom period in the Oyashio region of the western subarctic Pacific

We investigated the plankton community structure and biomass during the post-bloom season in the Oyashio region of the western subarctic Pacific, including pico-, nano-, microplankton and mesozooplankton. We found that the nitrate, phosphate and silicic acid concentrations remained high at >4.2 μM, >0.77 μM and >7.1 μM, respectively, in the euphotic layer at almost all sampling stations, but that the chlorophyll a concentrations were low (<3 µg Chl. a l−1). These findings indicate high nutrient and low chlorophyll (HNLC)-like conditions. In the phytoplankton community, pennate diatoms, the larger subpopulation of pico-sized eukaryotic phytoplankton, and nano-flagellates substantially contributed to the low biomass of the chain-forming centric diatoms that mainly comprised the spring phytoplankton bloom. The microzooplankton biomass was 2.7–4.4 fold greater than the phytoplankton biomass in the surface layer. Naked ciliates substantially contributed to the microzooplankton community (40–87 %). The naked ciliate growth rates during our in situ bottle incubation experiments were significantly greater than the maximum growth rates as calculated from cell volume and water temperature. The mesozooplankton biomass was mainly composed of krill and copepods and was 5.9–9.3 fold higher than the microzooplankton biomass. This inverted biomass pyramid with relatively low microzooplankton and high mesozooplankton biomass may be explained by the high production and growth rates of the microzooplankton. The ratio of phytoplankton growth (µ, d−1) to grazing mortality (m, d−1) by microzooplankton were relatively low at 0.26–0.44 m/µ in our dilution experiments. These low values indicate that microzooplankton grazing does not regulate phytoplankton growth and suggests that microzooplankton feed on an alternative nutritional source, such as heterotrophic prey items, or mixotrophy to fulfill their growth needs. Additional research is needed during the post-bloom period to further evaluate the mechanisms that sustain microzooplankton dominance and production in the Oyashio region under the HNLC-like conditions, especially for naked ciliates.

Unusual Hemiaulus bloom influences ocean productivity in Northeastern US Shelf waters

Abstract. Because of its temperate location, high dynamic range of environmental conditions, and extensive human activity, the long-term ecological research site in the coastal Northeastern US Shelf (NES) of the northwestern Atlantic Ocean offers an ideal opportunity to understand how productivity shifts in response to changes in planktonic community composition. Ocean production and trophic transfer rates, including net community production (NCP), net primary production (NPP), gross oxygen production (GOP), and microzooplankton grazing rates, are key metrics for understanding marine ecosystem dynamics and associated impacts on biogeochemical cycles. Although small phytoplankton usually dominate phytoplankton community composition and Chl a concentration in the NES waters during the summer, in August 2019, a bloom of the large diatom genus Hemiaulus, with N2-fixing symbionts, was observed in the mid-shelf region. NCP was 2.5 to 9 times higher when Hemiaulus dominated phytoplankton carbon compared to NCP throughout the same geographic area during the summers of 2020–2022. The Hemiaulus bloom in summer 2019 also coincided with higher trophic transfer efficiency from phytoplankton to microzooplankton and higher GOP and NPP than in the summers 2020–2022. This study suggests that the dominance of an atypical phytoplankton community that alters the typical size distribution of primary producers can significantly influence productivity and trophic transfer, highlighting the dynamic nature of the coastal ocean. Notably, summer 2018 NCP levels were also high, although the size distribution of Chl a was typical and an atypical phytoplankton community was not observed. A better understanding of the dynamics of the NES in terms of biological productivity is of primary importance, especially in the context of changing environmental conditions due to climate processes.

Grazing by nano- and microzooplankton on heterotrophic picoplankton dominates the biological carbon cycling around the Western Antarctic Peninsula

Over the past 40 years, the significance of microzooplankton grazing in oceanic carbon cycling has been highlighted with the help of dilution experiments. The ecologically relevant Western Antarctic Peninsula (WAP) ecosystem in the Southern Ocean (SO), however, has not been well studied. Here we present data from dilution experiments, performed at three stations around the northern tip of the WAP to determine grazing rates of small zooplankton (hetero- and mixotrophic members of the 0.2–200 µm size fraction, SZP) on auto- and heterotrophic members of the < 200 µm plankton community as well as their gross growth. While variable impacts of SZP grazing on carbon cycling were measured, particulate organic carbon, not the traditionally used parameter chlorophyll a, provided the best interpretable results. Our results suggested that heterotrophic picoplankton played a significant role in the carbon turnover at all stations. Finally, a comparison of two stations with diverging characteristics highlights that SZP grazing eliminated 56–119% of gross particulate organic carbon production from the particulate fraction. Thus, SZP grazing eliminated 20–50 times more carbon from the particulate fraction compared to what was exported to depth, therefore significantly affecting the efficiency of the biological carbon pump at these SO sites.

Microzooplankton Grazing Exerts a Strong Top‐Down Control on Unicellular Cyanobacterial Diazotrophs

AbstractUnicellular cyanobacterial diazotrophs (UCDs) are important nitrogen (N) fixers in marine ecosystems. However, the top‐down control on UCDs is poorly understood, especially for microzooplankton grazing that is often considered to be the dominant loss factor for primary production in oligotrophic oceans. In this study, we investigated the growth and microzooplankton grazing rates of the major UCD taxa (UCYN‐A1, UCYN‐A2/3/4, UCYN‐B, and UCYN‐C) in the South China Sea and the Luzon Strait using a combination of dilution experiments and quantitative PCR. All UCD taxa were actively grazed by microzooplankton, with higher grazing rates occurring in deeper layers of the photic zone. Microzooplankton grazing rates of UCDs were significantly higher than those of phytoplankton as whole, indicating that UCDs might be preferentially grazed by microzooplankton assemblages. The growth and microzooplankton grazing rates of UCDs were almost balanced, suggesting that a substantial amount of the UCD‐derived N is transferred through the food web via microzooplankton in these regions. Our study highlights the importance of microzooplankton grazing in the fate of UCD‐derived N and indicates that UCDs contribute significantly to the plankton food web in oligotrophic oceans where new N is mainly originated from dinitrogen fixation.

Responses of Phytoplankton Communities to the Effect of Both River Plume and Coastal Upwelling

AbstractRiver plumes and coastal upwelling systems are both nutrient‐abundant habitats compared to the open ocean, and the mechanisms that structure the phytoplankton community when these occur together are unclear. In this study we investigated the dynamics and drivers of phytoplankton biomass and community composition in the Pearl River plume‐coastal upwelling system on the northern South China Sea shelf during summer. We found that phytoplankton biomass was lower in the plume than in upwelled water. Diatoms were the only dominant group in the upwelled water. In contrast, diatoms and Synechococcus were co‐dominant in the plume, and the negative correlation between the proportions of these two groups indicated that they occupied distinct niches. The lower phytoplankton biomass in plume waters was due to nutrient limitation, and there was a transition from limitation by phosphorus to limitation by nitrogen in the plume along its path. This bottom‐up control limited only diatoms, whereas Synechococcus was limited mainly by top‐down control via microzooplankton grazing. This difference led to niche differentiation of the dominant phytoplankton groups in the plume‐upwelling system. This discovery of niche differentiation enhances understanding of food web structure and will facilitate modeling of marine biogeochemical cycles.

Responses of Phytoplankton Communities to Internal Waves in Oligotrophic Oceans

AbstractUnderstanding the potential impacts of internal waves on phytoplankton communities in oligotrophic oceans remains an important research challenge. In this study, we elucidated the impact of internal waves on phytoplankton communities through a comprehensive 154‐hr time‐series of observations in the South China Sea (SCS). We identified distinctive variations in phytoplankton pigment biomass and composition across the upper, middle, and lower layers of the euphotic zone, which we attributed to the perturbations triggered by internal waves. Phytoplankton other than Prochlorococcus in the lower, nutrient‐replete layer likely benefitted from allochthonous nutrients introduced by internal waves, but their growth rates were constrained by light limitation, and their pigment biomass was held in check by microzooplankton grazing. In contrast, in the upper, nutrient‐depleted layer, the relative abundance of Prochlorococcus increased, likely because of the ammonium regenerated by zooplankton. The middle layer, characterized as the deep chlorophyll maximum layer, exhibited a dynamic equilibrium characterized by nutrient and light co‐limitation. This equilibrium resulted in high nitrate assimilation and growth by phytoplankton. The balancing of those rates by significant grazing losses maintained total chlorophyll a concentrations at a high level. Based on these findings, we proposed a three‐layer euphotic zone structure characterized by distinct physiological conditions, nutrient‐light dynamics, grazing pressure, and phytoplankton responses to internal waves. This three‐layer paradigm elucidated the intricate interplay between internal waves and phytoplankton communities and provided insights into the mechanisms that govern primary production and carbon cycling in oligotrophic oceanic ecosystems.

Decoupled growth and grazing rates of diatoms and green algae drive increased phytoplankton productivity on HNLC sub‐Antarctic plateaux

AbstractThe combination of iron limitation and microzooplankton grazing controls phytoplankton productivity and taxonomic composition in high‐nutrient low‐chlorophyll (HNLC) regions. While increased productivity and diatom contribution triggered by iron enrichment support this view, direct measurements of underpinning group‐specific growth and grazing rates are scarce for the Southern Ocean. To assess these rates, we conducted dilution experiments coupled to high‐performance liquid chromatography and flow‐cytometry in sub‐Antarctic waters on and off Campbell Plateau, southeast of Aotearoa‐New Zealand. Off the plateau, growth and grazing were closely balanced for all groups despite a two‐fold difference between slow‐ and fast‐growing groups. On Campbell Plateau, where HNLC conditions were alleviated, the balance was disrupted, mainly by the preferential growth of diatoms and green algae, which was stimulated beyond grazing. Our results expand the recognized ability of diatoms to escape grazing control to picoplanktonic green algae that also avoid grazing and contribute significantly to phytoplankton productivity and biomass accumulation.

Seasonal and spatial comparisons of microzooplankton grazing and phytoplankton growth in the Bohai Bay, China

The dilution experiment technique was used in two cruises in July-August (summer) and October-November (autumn) 2020, with a total of 14 stations. The grazing impact of microzooplankton on phytoplankton in the interior of Bohai Bay was comprehensively investigated. We compared phytoplankton growth rates (μ0) and microzooplankton grazing rates (m) spatially (distance between experimental stations and shore far vs. near) and seasonally (summer vs. autumn). Both m and μ0 values were significantly higher in summer than in autumn, and the phytoplankton growth rate μ0 was positively correlated to temperature. Offshore stations showed higher values. There is no significant spatial and seasonal differences in the ratio of microzooplankton grazing rate and phytoplankton growth rate (m/μ0) indicating that daily consumption of primary production by microzooplankton was similar in the two seasons. Therefore, our research showed a close coupling between microzooplankton grazing with phytoplankton growth in the Bohai Bay.

The Effect of Zooplankton on the Distributions of Dimethyl Sulfide and Dimethylsulfoniopropionate in the Bohai and Yellow Seas

AbstractDimethyl sulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) are ubiquitous sulfur compounds in the ocean. DMS is emitted into the atmosphere and has potential climatic effects. The distributions of DMS and DMSP are affected by various biological factors (i.e., bacterial catabolism and phytoplankton and zooplankton community composition). The horizontal and vertical distributions of DMSP, DMSP lyase activity (DLA), DMS, and the abundances of bacteria, DMSP‐consuming bacteria, dimethyl sulfoxide‐consuming bacteria, and picophytoplankton were investigated in the Bohai Sea (BS) and Yellow Sea (YS) during autumn 2020. DLA was significantly correlated with chlorophyll a, DMS, and dissolved DMSP concentrations. Our data show that bacteria Clade_I SAR11 was a significant contributor to DLA. A dilution experiment indicated that the highest microzooplankton grazing rate coincided with the highest DMS concentration and DMS production rate. A proportion of 16%–62% of the DMSPt was converted to DMS in the dilution experiment. Copepods dominated the mesozooplankton community. Calanus sinicus was the predominant copepod in the BS and YS. C. sinicus grazing stimulated DMS production. DMS concentration increased 299% after C. sinicus grazing on physically broken algal cells for 48 hr. These results will help with a better understanding of the control of DMS and DMSP concentrations by zooplankton and the DMS release mechanisms that occur through zooplankton grazing.

Phytoplankton Seasonal Dynamics under Conditions of Climate Change and Anthropogenic Pollution in the Western Coastal Waters of the Black Sea (Sevastopol Region)

The studies of seasonal phytoplankton dynamics, its growth rate, and microzooplankton grazing were conducted on two stations in the western Black Sea coastal waters near Sevastopol from January 2021 to December 2022. The phytoplankton species composition has remained relatively the same during recent years compared to the end of the last century and the beginning of the 2000s. However, significant changes have occurred in the ratio between different species of diatoms, and the proportion of dinoflagellates was increased, especially in the autumn. Large diatoms and dinoflagellates play a crucial role in forming the phytoplankton biomass seasonal peaks. The first central maximum was observed in July, and the second smaller one was in September–November. Whereas two decades ago, the small diatoms generated three peaks annually: in February, May, and September–October. The maximum values of the phytoplankton growth rate and the rate of its consumption by microzooplankton decreased 2–3 times compared to the beginning of the 2000s. The relative share of primary production consumed by microzooplankton annually averages 35%, two times lower than before.

Roles of phytoplankton, microzooplankton, and bacteria in DMSP and DMS transformation processes in the East China Continental Sea

The trace gas dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) are the most abundant oceanic biogenic organosulfur molecules. Phytoplankton, zooplankton, and bacteria are the most vital biotic factors controlling DMSP and DMS, but their relative effects on the transformation processes still remain uncertain due to their complex biogeochemistry. Here, we provided the relative transformation rates of DMSP and DMS, and their relationships with phytoplankton, microzooplankton, and bacteria in the East China Continental Sea in the summer of 2018. The phytoplankton was the major DMSP producer, contributing 89.8% of DMSP stock with the particulate DMSP production rate at 42.0 nmol L−1 d−1. The DMSP lyase activity was 140.5 nmol DMS L−1h−1, resulting in a short DMSP fate (0.8 d). The microzooplankton grazing resulted in DMSP loss and DMS production at the rates of −49.3 and 8.2 nmol L−1 d−1, equaling 60.0% of total DMSP loss and 71.6% of DMS microbial production. The dissolved DMSP degradation and DMS microbial production rates were at −23.4 and 11.5 nmol L−1 d−1, largely controlled by bacterial abundance. Microbial consumption, photolysis, and ventilation accounted for 41%, 43%, and 16% of surface DMS removal, indicating that most of the DMS was consumed within the surface. Finally, DMS was enriched in the sediment but not significantly enriched in the surface microlayer, with enrichment factors at 3.7 and 1.0, respectively. The budget model in this study could help better estimate the roles of phytoplankton, microzooplankton, and bacteria in DMSP and DMS transformation processes in the continental shelf seas.

Phytoplankton dynamics, growth and microzooplankton grazing across the subtropical frontal zone, east of New Zealand

Elevated but variable phytoplankton biomass and productivity is often associated with the subtropical front (STF) where nitrogen-limited subtropical and iron-limited subantarctic waters mix. To understand variability within the STF east of New Zealand, we assessed phytoplankton community structure, growth, and grazing dynamics in relation to physico-chemical conditions across 23 stations distributed along the Chatham Rise region during late autumn-early winter. Serial dilution experiments were coupled with size-fractionated chlorophyll a (Chla) analysis (Total and <20 μm) and flow-cytometry (Synechococcus and picoeukaryote numbers, <2 μm) to estimate phytoplankton growth and microzooplankton grazing rates. Within the broad STF zone, subantarctic influenced waters (SAIW), frontal zone (FZ), and subtropical influenced waters (STIW) were delimited based on salinity, temperature and nutrient gradients. The chlorophyll a biomass (TChla) of phytoplankton and the abundance of larger sized cells (>20 μm chlorophyll a) peaked in FZ waters but declined steadily southwards into the colder SAIW, and rapidly reduced north into the STIW. Chlorophyll a <2 μm peaked in the northern STIW. Phytoplankton growth (TChla) was higher in warmer STIW (μ = 0.49 ± 0.07 day−1) than in iron limited SAIW (μ = 0.29 ± 0.06 day−1) but was on average moderate (μ = 0.42 ± 0.05 day−1) when compared to previous studies in the region. Microzooplankton grazing on TChla was lower (m = 0.17 ± 0.04 day−1) than growth and accounted for half of daily primary production (m:μ, 0.47 ± 0.06). Growth in the <20 μm Chla size fraction was higher (μ = 0.52 ± 0.06 day−1) but a larger proportion was consumed by microzooplankton (m:μ = 0.64 ± 0.06). Picoeukaryotes showed the fastest growth (1.49 ± 0.13 day−1) and grazing (1.43 ± 0.11 day−1) rates on average, which peaked in the FZ but remained closely balanced across different waters (m:μ = 1.00 ± 0.02). Conversely, Synechococcus rates peaked in STIW and decreased southwards, with growth (μ = 0.42 ± 0.08 day−1) generally exceeding grazing (m = 0.28 ± 0.06 day−1) across all regions. Our results indicate differences in grazing together with nutrient (likely iron) availability were the primary factors controlling phytoplankton dynamics in the STF zone. These factors also affected the accumulation of larger phytoplankton biomass in the FZ, including the potential for export or transfer to higher trophic levels.

Role of nutrients and temperature in shaping distinct summer phytoplankton and microzooplankton population dynamics in the western North Pacific and Bering Sea

AbstractPhytoplankton growth and microzooplankton grazing are two critical processes in marine food webs, but they remain understudied in the vast area of the subarctic western Pacific and the Bering Sea. In this study, we measured phytoplankton growth and microzooplankton grazing rates in these less‐explored regions to demonstrate their spatial patterns and investigate underlying mechanisms driving the planktonic food web dynamics. Our results showed that the phytoplankton growth in these regions was determined by nutrient availability and temperature. In the high‐nutrient, low‐chlorophyll regions (HNLC), iron availability was the primary factor limiting phytoplankton growth. In contrast, phytoplankton growth in the Gulf of Anadyr and Kamchatka Strait was mainly limited by inorganic nitrogen exhausted by the summer blooms. We found that microzooplankton grazing rate was affected by temperature and prey availability, highlighting the positive effect of temperature. Strong top‐down control on phytoplankton by microzooplankton in the Gulf of Anadyr and Kamchatka Strait indicated an active microbial food web with high turnover rates. In contrast, the decoupling of phytoplankton growth and microzooplankton grazing in the HNLC regions illustrates a weak role of microzooplankton in the marine food web. These results indicated different food web structures in the areas with and without riverine iron input. By revealing the roles of temperature and nutrient or prey availability in regulating the spatial variability of plankton rates, we expect that the plankton will respond differently to ocean warming between the HNLC and coastal regions of the subarctic Pacific due to different nutrient conditions. Our study helps understand how marine plankton will respond to environmental changes at high latitudes.

The role of microzooplankton grazing in the microbial food web of a tropical mangrove estuary

In the Matang Mangrove Forest Reserve (MMFR), the role of microzooplankton (20–200 μm) grazing on phytoplankton as a channel for carbon transfer to higher trophic levels was investigated. Our results showed that primary production was higher during the southwest monsoon (SWM) (1905 ± 1478 μgCl−1d−1) than in the northeast monsoon (NEM) (708 ± 474 μgCl−1d−1). Microzooplankton grazing was tightly coupled to primary production in both SWM and NEM, accounting for 97% and 68% of primary production, respectively. Both primary production (activation energy: 1.29 ± 0.54 eV) and microzooplankton grazing (2.05 ± 0.63 eV) showed temperature dependency, and revealed a shift towards heterotrophy with seawater warming, specifically at temperatures above 32 °C. As a conclusion, the microbial food web in MMFR is characterised by high primary production that was efficiently grazed by microzooplankton.