Microzooplankton Grazing Rates Research Articles

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.

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.

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 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.

Spatial variability of microzooplankton grazing on phytoplankton in coastal southern Florida, USA.

Microzooplankton are considered the primary consumers of phytoplankton in marine environments. Microzooplankton grazing rates on phytoplankton have been studied across the globe, but there are still large regions of the ocean that are understudied, such as sub-tropical coastal oceans. One of these regions is the coastal area around south Florida, USA. We measured microzooplankton grazing rates in two distinct environments around south Florida; the oligotrophic Florida Keys and the mesotrophic outflow from the Everglades. For 2-years from January 2018 to January 2020, we set up 55 dilution and light-dark bottle experiments at five stations to estimate the microzooplankton community grazing rate, instantaneous phytoplankton growth rate, and primary production. Our results suggest that microzooplankton are consuming a higher proportion of the primary production near the Everglades outflow compared to the Florida Keys. We also found that changes in phytoplankton growth rates are disconnected from changes in the microzooplankton grazing rates in the Florida Keys. Overall, the data from the Everglades outflow is what would be expected based on global patterns, but factors other than microzooplankton grazing are more important in controlling phytoplankton biomass in the Florida Keys.

De-coupled phytoplankton growth and microzooplankton grazing in a simulated oil spill event in mesocosms

Microzooplankton (<200 μm) are essential intermediates between primary production and organisms at the higher trophic levels. Their ecological functions could be substantially affected by crude oil pollution. A natural plankton community was exposed to 10 μL L−1 of chemically dispersed crude oil (DOil) in outdoor mesocosms for 7 days, with control (Ctrl) mesocosms set up for comparison. Dilution experiments were conducted to estimate the grazing rates of microzooplankton on the 2nd and 6th days of the pollutants exposure. Results showed 0.36–2.28 d−1 microzooplankton grazing rates in the Ctrl mesocosms on both days but negative rates in the DOil mesocosms. A significant linear relationship between in situ phytoplankton growth and microzooplankton grazing rates was found in the Ctrl treatment but not in the DOil treatment. This suggests a de-coupling between phytoplankton growth and microzooplankton and the potential for the formation of phytoplankton blooms in seawater after an oil spill event.

Assessing the effect of oil spills on the dynamics of the microbial plankton community using a NPZD model

A mesocosms approach was used to assess the seasonal variability and the effect of oil spills on the structure and dynamics of marine microbial plankton communities in a coastal upwelling system. To this aim, four experiments were conducted during the main characteristic periods of the seasonal cycle in the Ría de Vigo (NW of Spain): spring phytoplankton bloom, summer stratification, autumn upwelling and winter mixing. In each of these experiments, enclosed communities in control and oiled bags were monitored and key variables and rates were measured during 9 days. Temporal changes in community structure and function were interpreted applying a simple NPZD (Nutrients-Phytoplankton-Zooplankton-Detritus) model in which the strategy was to maintain invariant as many model parameters as possible. The analysis of those parameters that were modified to accurately simulate the time evolution of the microbial communities in the mesocosms, provided important information about the main processes influencing the plankton community structure and dynamics in the bags as well as about the effect of oil. This modelling approach allowed assessing the dynamics of the four communities just varying phytoplankton maximum growth rates, bacterial growth efficiencies, microzooplankton grazing rates and phytoplankton sinking rates. Comparison of parameters in control and oiled bags in each period allowed to infer the negligible effect of oil spills on the structure and dynamics of the microbial plankton communities in this coastal upwelling system.

Phytoplankton Growth Rate and Microzooplankton Grazing under Conditions of Climatic Changes and Anthropogenic Pollution in the Coastal Waters of the Black Sea (Sevastopol Region)

In the coastal waters of the Black Sea near Sevastopol, a gradual temperature elevation and an increase in anthropogenic pressure since the early 2000s have caused significant structural and functional changes in phytoplankton. Currently, there is a significant decrease in the contribution of small diatom species (Skeletonema sp. and Chaetoceros socialis H.S.Lauder as well as coccolithophorids Emiliania huxleyi (Lohmann) W.W.Hay and H.P.Mohler) to the total phytoplankton biomass in these waters. Previously these species caused regular weak blooms. In the warm periods (from May to October), during which the main phytoplankton biomass is formed, large diatom species Pseudosolenia calcar-avis (Schultze) B.G.Sundström, 1986, Proboscia alata (Brightwell) Sundström and dinoflagellates predominate. Therefore, the maximum values of the phytoplankton community’s specific growth rate are about two times lower than in the preceding periods and do not exceed 1.10–1.40 day−1. There was also a decrease observed in the microzooplankton grazing rate, which, during the year, was no higher than 0.70–1.20 day−1. This is primarily conditioned by the increased role of large algae in phytoplankton, which means a decline in nutrition quality for microzooplankton. As a result of the joint influence of nutrition quality and water pollution, the relative share of net primary production consumed by microzooplankton in the warm periods of the year averaged only 32%, which is two times lower than the average values generally accepted for marine ecosystems. This means that the transfer of matter and energy from phytoplankton to higher trophic levels is significantly decreased.

Microzooplankton grazing and its key group composition in subtropical eutrophic coast of Southern China: in relation to environmental changes.

Microzooplanton play a crucial role in marine ecosystems, as they transfer matter and energy from pico- and nano-phytoplankton to mesozooplankton. In this study, we explored the seasonal variations of microzooplankton grazing derived from dilution experiments in a typical eutrophic coast of Southern China, as well as the abundance and biodiversity of its key group (ciliate), to further understand its function in the subtropical coastal food web associated with potential regulation factors. A total of 29 ciliate species belonging to 18 genera were identified, with the dominating species of Mesodinium rubrum, Strombidium globosaneum and Strombidium conicum. The spatial difference of ciliates abundance was attributed by the changes of temperature and salinity. Phytoplankton growth rate (μ) and microzooplankton grazing rate (m) ranged from 0.03 to 1.36 d-1 and 0.10 to 1.57 d-1, respectively, and both μ and m showed the highest values in summer and the lowest in winter. Moreover, microzooplankton grazing pressure on the phytoplankton standing stocks and potential primary production ranged from 10% to 79% and 58% to 471%, respectively. Our results indicated that temperature is the main environmental driving force for the seasonal changes of μ and m, and that the impacts of run-offs from the Pearl River and offshore seawater intrusion from the South China Sea are responsible for the spatial-temporal variations of phytoplankton growth and microzooplankton grazing.

Grazing behaviour of tropical calanoid copepods and its effect on phytoplankton community structure.

The Netravathi estuary is a micro- to mesotidal, tropical, shallow estuary with an abundant density of phytoplankton. The impact of the zooplankton grazing rate and the selected environmental variables on the phytoplankton community structure was studied seasonally from 2017 to 2019. Microplanktons of centric diatoms such as Skeletonema costatum and Cosinodiscus radiatus dominated during monsoon seasons, whereas dinoflagellates such as Peridinium sp. and Ceratium furca were observed in high numbers at the estuary during non-monsoon seasons. Water temperature showed a strong association with grazing rates of microzooplankton (r = 0.910). Canonical correspondence analysis studies established an intensification in grazing rate during non-monsoon seasons by calanoid copepods which in turn occasioned the reduction of microplanktons belonging to diatoms. Moreover, during monsoon seasons, diminution in grazing rates of calanoid copepods coupled with the presence of micro larvae as a secondary consumer of zooplankton resulted in the proliferation of centric diatoms. The grazing behaviour of microzooplankton affected the phytoplankton species diversity of the estuary in a negative manner (D = 0.29 in 2017 Pre-M, D = 0.76 in 2019 Pre-M). Selective grazing behaviour of calanoid copepods concerning temperature alteration exerted a strong influence in determining the phytoplankton community structure of the estuary to a great extent.

Geographical and Seasonal Thermal Sensitivity of Grazing Pressure by Microzooplankton in Contrasting Marine Ecosystems

Grazing pressure, estimated as the ratio between microzooplankton grazing and phytoplankton growth rates (g:μ), is a strong determinant of microbial food-web structure and element cycling in the upper ocean. It is generally accepted that g is more sensitive to temperature than μ, but it remains unknown how the thermal dependence (activation energy, Ea) of g:μ varies over spatial and temporal scales. To tackle this uncertainty, we used an extensive literature analysis obtaining 751 paired rate estimates of μ and g from dilution experiments performed throughout the world’s marine environments. On a geographical scale, we found a stimulatory effect of temperature in polar open-ocean (∼0.5 eV) and tropical coastal (∼0.2 eV) regions, and an inhibitory one in the remaining biomes (values between −0.1 and −0.4 eV). On a seasonal scale, the temperature effect on g:μ ratios was stimulatory, particularly in polar environments; however, the large variability existing between estimates resulted in non-significant differences among biomes. We observed that increases in nitrate availability stimulated the temperature dependence of grazing pressure (i.e., led to more positive Ea of g:μ) in open-ocean ecosystems and inhibited it in coastal ones, particularly in polar environments. The percentage of primary production grazed by microzooplankton (∼56%) was similar in all regions. Our results suggest that warming of surface ocean waters could exert a highly variable impact, in terms of both magnitude and direction (stimulation or inhibition), on microzooplankton grazing pressure in different ocean regions.

Microbial food web dynamics in the oceanic Gulf of Mexico

Abstract Phytoplankton growth and microzooplankton grazing rates were measured in repeated profiles of dilution experiments incubated in situ on a drift array in order to assess microbial production and food web characteristics in the oligotrophic bluefin tuna spawning habitat of the Gulf of Mexico (May peak spawning seasons, 2017–2018). Grazing often exceeded growth with the processes more balanced overall in the surface mixed layer, but biomass accumulated in the mid-euphotic zone. Community production estimates (260–500 mg C m−2 day−1) were low compared to similar open-ocean studies in the Pacific Ocean. Prochlorococcus was a consistent major contributor (113–204 mg C m−2 day−1) to productivity, while diatoms and dinoflagellates (2–10 and 4–13 mg C m−2 day−1, respectively) were consistently low. Prymnesiophytes, the most dynamic component (34–134 mg C m−2 day−1), co-dominated in 2017 experiments. Unexpected imbalances in grazing relative to production were observed for all picoplankton populations (Prochlorococcus, Synechococcus and heterotrophic bacteria), suggesting a trophic cascade in the absence of mesozooplankton predation on large microzooplankton. Study sites with abundant larval tuna had the shallowest deep chlorophyll maxima and significant net positive phytoplankton growth below the mixed layer.

What controls microzooplankton biomass and herbivory rate across marginal seas of China?

AbstractMicrozooplankton are the primary herbivores and nutrient regenerators in the marine food web, but their importance is often underestimated, and the quantitative relationships between environmental factors and the biomass and herbivory rate of microzooplankton remain obscure. To fill this gap, we conducted 224 dilution experiments to measure microzooplankton biomass and herbivory rate across a vast area of the marginal seas of China. To gain the potential mechanisms controlling microzooplankton herbivory, we also use a model that combines the Metabolic Theory of Ecology and the functional responses of grazing to quantify the effects of temperature, phytoplankton biomass, and microzooplankton biomass on microzooplankton grazing rate. We estimate an activation energy of 0.51 eV of microzooplankton and found that the Holling III function best described the functional response of microzooplankton grazing with a maximal ingestion rate of 4.76 d−1 at 15°C and a half‐saturation constant of 0.27 μM N. We also find that microzooplankton biomass scales with phytoplankton biomass with an exponent of 0.77, consistent with the general 3/4 scaling law found in other ecosystems. This scaling relationship is accompanied by a shift from ciliates to heterotrophic dinoflagellates with increasing phytoplankton biomass. Our results provide empirical patterns that will be vital to parameterize and validate marine ecosystem models, particularly in China seas.

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.

Contrasting fine‐scale distributional patterns of zooplankton driven by the formation of a diatom‐dominated thin layer

AbstractAlthough plankton thin layers have been described and modeled in a variety of environments, the physical structure surrounding the layer, associated biological rates, and distributions of multiple trophic levels are rarely examined simultaneously. Similar combinations of measurements, such as growth, mortality, and spatial relationships among plankton, are key to understanding how physical processes generating thin layers can influence abundances, composition, and predator‐prey interactions. An in situ imaging system was deployed along a southward‐oriented transect to describe the full ~ 2.3‐km extent of a thin layer arching from 8 to 4 m deep, with Chlorophyll a enhanced by an order of magnitude inside the layer (23.9 mg m−3 peak concentration). Physical oceanographic measurements, distributions of different plankton groups, and output from a high‐resolution model indicated that surface convergence and vertical shear drove the formation of the layer, which was dominated by Odontella sp. diatoms that were relatively scarce in the broader study region. Phytoplankton apparent growth (0.64 d−1) balanced microzooplankton grazing rates (0.52 d−1) within the layer, whereas grazing mortality (2.23 d−1) greatly exceeded phytoplankton growth (−0.39 d−1) outside the thin layer. Mesozooplankton had starkly differing distributions; copepods aggregated south of the layer near the surface, and doliolids followed the thin layer trajectory. A physical oceanographic model run over a 1‐month time period, including the time of sampling, indicated that similar surface convergences occurred frequently. Thin layers driven by convergence and shear may be common, with behavioral and buoyancy differences among plankton likely contributing to community structure and modification of trophic transfer.

The Importance of Mesozooplankton Diel Vertical Migration for Sustaining a Mesopelagic Food Web

We used extensive ecological and biogeochemical measurements obtained from quasi-Lagrangian experiments during two California Current Ecosystem Long-Term Ecosystem Research cruises to analyze carbon fluxes between the epipelagic and mesopelagic zones using a linear inverse ecosystem model (LIEM). Measurement constraints on the model include 14C primary productivity, dilution-based microzooplankton grazing rates, gut pigment-based mesozooplankton grazing rates (on multiple zooplankton size classes), 234Th:238U disequilibrium and sediment trap measured carbon export, and metabolic requirements of micronekton, zooplankton, and bacteria. A likelihood approach (Markov Chain Monte Carlo) was used to estimate the resulting flow uncertainties from a sample of potential flux networks. Results highlight the importance of mesozooplankton active transport (i.e., diel vertical migration) for supplying the carbon demand of mesopelagic organisms and sequestering carbon dioxide from the atmosphere. In nine water parcels ranging from a coastal bloom to offshore oligotrophic conditions, mesozooplankton active transport accounted for 18% - 84% (median: 42%) of the total carbon supply to the mesopelagic, with gravitational settling of POC (12% - 55%; median: 37%) and subduction (2% - 32%; median: 14%) providing the majority of the remainder. Vertically migrating zooplankton contributed to downward carbon flux through respiration and excretion at depth and via consumption loses to predatory zooplankton and mesopelagic fish (e.g. myctophids and gonostomatids). Sensitivity analyses showed that the results of the LIEM were robust to changes in nekton metabolic demands, rates of bacterial production, and mesozooplankton gross growth efficiency. This analysis suggests that prior estimates of zooplankton active transport based on conservative estimates of standard (rather than active) metabolism should be revisited.

Marine phytoplankton in subtropical coastal waters showing lower thermal sensitivity than microzooplankton

AbstractTemperature sensitivity of plankton in terms of activation energy (Ea, eV) in the Arrhenius equation is critical for predicting how marine productivity and carbon export will respond to ocean warming. In this study, we quantified the temperature responses of phytoplankton growth rate and microzooplankton grazing rate by conducting short‐term temperature modulation experiments on natural communities at two subtropical sites with contrasting nutrient conditions. Our results showed that the activation energy of phytoplankton growth rate (Ea = 0.36 eV, 95% confidence interval [CI] = 0.28–0.44 eV) at each station was less than that of microzooplankton grazing rate (Ea = 0.53 eV, 95% CI = 0.47–0.59 eV), indicating an increasing grazing pressure on phytoplankton under warming conditions. Although the difference is consistent with that reported in previous studies, it is very likely to arise from another reason, i.e., differential proximities of the optimal temperature (Topt in nonlinear temperature responses of rates) of phytoplankton and microzooplankton to the environmental temperature, as we found that the environmental temperature is closer to the optimal temperature of phytoplankton growth than to that of microzooplankton grazing in this subtropical environment. Our results suggest that nonlinear temperature responses of plankton should be considered when evaluating and predicting the effects of ocean warming on ecosystem productivity and food web dynamics, especially in subtropical and tropical waters.