Microzooplankton Grazing Research Articles

Dynamical study of infochemical influences on tropic interaction of diffusive plankton system

We propose a model for tropic interaction among the infochemical-producing phytoplankton and non-info chemical-producing phytoplankton and microzooplankton. Volatile information-conveying chemicals (infochemicals) released by phytoplankton play an important role in the food webs of marine ecosystems. Microzooplankton is an ecologically important grazer of phytoplankton for coexistence of a large number of phytoplankton species. Here, we discuss how information transferred by dimethyl sulfide shapes the interaction of phytoplankton. Phytoplankton deterrents may lead to propagation of IPP bloom. The interaction between IPP and microzooplankton follows the Beddington–DeAngelis-type functional response. Analytically, we discuss boundedness, stability and Turing instability of the model system. We perform numerical simulation for temporal (ODE model) as well as a spatial model system. Our numerical investigation shows that microzooplankton grazing refuse of IPP leads to oscillatory dynamics. Increasing diffusion coefficient of microzooplankton shows Turing instability. Time evolution also plays an important role in the stability of system dynamics. The results obtained in this paper are useful to understand the dominance of algal bloom in coastal and estuarine ecosystem.

Phytoplankton growth and grazing mortality through the oligotrophic subtropical North Pacific

The subtropical North Pacific has been historically considered as a stable and homogenous oligotrophic marine ecosystem. The consistently low phytoplankton biomass has been attributed to a close balance between phytoplankton growth and grazing mortality. However, phytoplankton summer blooms were frequently observed in the central North Pacific near the Hawaiian Islands. To determine whether this is a result of unbalanced phytoplankton growth and microzooplankton grazing, we conducted a trans-Pacific cruise through the subtropical North Pacific. The growth and microzooplankton grazing mortality rates of the phytoplankton community and specific groups in the surface layer (10 m), were examined by dilution experiments. Positive phytoplankton net growth rates (0.34 ± 0.29 day−1) were observed under the depletion of dissolved inorganic nitrogen (DIN, < 40 nM) throughout the study area. However, considering the grazing of micro- and mesozooplankton collectively, the phytoplankton growth was largely consumed (net growth rate of 0.08 ± 0.15 day−1), except in the central North Pacific (net growth rate of 0.42 ± 0.11 day−1). Phytoplankton biomass accumulation in this area was also demonstrated by the satellite-observed Chl a, although this was just sustained for a few days. The high phytoplankton growth rate in the central North Pacific (0.84 ± 0.26 day−1) was a result of the dominant Prochlorococcus, which contributed 88% of the community Chl a. The weak response of Prochlorococcus growth to ammonium addition indicates their growth (1.14 ± 0.55 day−1) was not limited by the ambient DIN and was likely a result of their advantage of utilizing various dissolved organic nitrogen.

Modeling the seasonal variability of phytoplankton in the subarctic northeast Pacific Ocean

The subarctic northeast Pacific Ocean (SNEP) is a high-nitrate, low-chlorophyll region where, due to iron (Fe) limitation, chlorophyll a (chl a) concentration is persistently low throughout the year despite abundant nitrate. Observations from a Biogeochemical-Argo float (BGC-Argo) deployed in the SNEP in 2012 show prominent variations in chl a in the upper ocean (0-150 m). Chl a peaks in late summer and autumn at a value &gt;3�times that in winter; these observations at a fine resolution can help us disentangle the mechanisms driving the full seasonal cycle of phytoplankton. A one-dimensional physical-biogeochemical coupled model was applied at OSP to investigate the factors driving phytoplankton dynamics in the SNEP. Seasonal variability of chl a in the upper layer is reproduced by the model; results are reasonably comparable to the BGC-Argo observations in 2012 near OSP. Phytoplankton growth is limited in winter by the deep mixed layer, which reduces light availability; light gradually increases later in the year due to shoaling of the mixed layer and increasing solar radiation. In spring and early summer, Fe limitation restricts phytoplankton growth, especially diatoms, and grazing pressure on small phytoplankton from overwintering microzooplankton inhibits accumulation of phytoplankton biomass. Chl a accumulates throughout the summer, peaking in late summer and autumn as mesozooplankton consume microzooplankton, which reduces microzooplankton grazing on phytoplankton. Although the model study was restricted to the seasonal pattern of BGC-Argo observations in 2012, it nevertheless fully describes seasonal variability in chl a in the SNEP and is crucial to understanding mechanisms underlying phytoplankton dynamics in high-nitrate, low-chlorophyll regions.

The Possession of Coccoliths Fails to Deter Microzooplankton Grazers

Phytoplankton play a central role in the regulation of global carbon and nutrient cycles, forming the basis of the marine food webs. A group of biogeochemically important phytoplankton, the coccolithophores, produce calcium carbonate scales that have been hypothesized to deter or reduce grazing by microzooplankton. Here, a meta-analysis of mesocosm-based experiments demonstrates that calcification of the cosmopolitan coccolithophore, Emiliania huxleyi, fails to deter microzooplankton grazing. The median grazing to growth ratio for E. huxleyi (0.56 ± 0.40) was not significantly different among non-calcified nano- or picoeukaryotes (0.71 ± 0.31 and 0.55 ± 0.34, respectively). Additionally, the environmental concentration of E. huxleyi did not drive preferential grazing of non-calcified groups. These results strongly suggest that the possession of coccoliths does not provide E. huxleyi effective protection from microzooplankton grazing. Such indiscriminate consumption has implications for the dissolution and fate of CaCO3 in the ocean, and the evolution of coccoliths.

Phytoplankton adaptation strategies under the influence of climatic changes and anthropogenic pressure on the Black Sea coastal ecosystems on the example Sevastopol Bay

Some ways of the Black Sea coastal waters phytoplankton community adaptation to changes in water temperature, nutrients concentration and anthropogenic pollution have been identified on the example of the Sevastopol Bay. The increase in water temperature and decrease in nutrient content in the studied waters during 2000 – 2014 caused a gradual decrease in the chlorophyll a concentrations, total phytoplankton biomass and its re-composition, predominantly in the summer and autumn periods. The phytoplankton restructuring was predominantly reflected by a decrease in relative diatoms contribution in the total phytoplankton biomass and an increase in dinoflagellates contribution. Among the dominant diatoms species, the share of resistant species to high temperatures, pollution, low nitrate content in the water and microzooplankton grazing was increasing. An increase in nitrate concentration in the studied waters in 2020 led to increase in total phytoplankton biomass and a predominance of diatoms species, which under the stated conditions did not lead to bloom emergence as were regularly observed earlier in the Sevastopol region.

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.

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.

Grazing by microzooplankton and copepods on the microbial food web in spring in the southern Yellow Sea, China

Assessment of microzooplankton and copepods grazing pressure on picoplankton is a key requirement for resolving the microbial food web efficiency. Although microzooplankton grazing on picoplankton has been extensively studied, the impact of microzooplankton on different groups of picoplankton, i.e., heterotrophic bacteria, Synechococcus and picoeukaryotes have rarely been compared. Furthermore, in the very few existing studies there is no consistent evidence of an enhancing or restraining effect of copepods on picoplankton. More studies are needed to improve our understanding of the influence of microzooplankton and copepod on picoplankton. Dilution incubations and copepod addition incubations were performed during a cruise to the southern Yellow Sea on May 16–29, 2007. The bulk grazing of microzooplankton and the calanoid copepod Calanus sinicus on phytoplankton, flagellates and picoplankton was estimated. Stations were divided into either eutrophic or oligotrophic according to the nutrient and biological parameters. Picoplankton comprised a large part of the diet of microzooplankton in the central oligotrophic area, while phytoplankton was the main food of microzooplankton in the coastal eutrophic area. In the central oligotrophic area, microzooplankton preferred grazing on Synechococcus. After copepod addition, ciliate abundance decreased while Synechococcus abundance increased (382%, 64% and 64% at three experimental stations, respectively), indicating strong grazing pressure of microzooplankton on Synechococcus. Our results suggest that Synechococcus might be an essential carbon source the planktonic food web in the oligotrophic waters of southern Yellow Sea.

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.

Mesozooplankton grazing minimally impacts phytoplankton abundance during spring in the western North Atlantic.

The impacts of grazing by meso- and microzooplankton on phytoplankton primary production (PP) was investigated in the surface layer of the western North Atlantic during spring. Shipboard experiments were performed on a latitudinal transect at three stations that differed in mixed layer depth, temperature, and mesozooplankton taxonomic composition. The mesozooplankton community was numerically dominated by Calanus finmarchicus at the northern and central station, with Calanus hyperboreus also present at the northern station. The southern station was >10 °C warmer than the other stations and had the most diverse mesozooplankton assemblage, dominated by small copepods including Paracalanus spp. Microzooplankton grazing was detected only at the northern station, where it removed 97% of PP. Estimated clearance rates by C. hyperboreus and C. finmarchicus suggested that at in-situ abundance these mesozooplankton were not likely to have a major impact on phytoplankton abundance, unless locally aggregated. Although mesozooplankton grazing impact on total phytoplankton was minimal, these grazers completely removed the numerically scarce > 10 µm particles, altering the particle-size spectrum. At the southern station, grazing by the whole mesozooplankton assemblage resulted in a removal of 14% of PP, and its effect on net phytoplankton growth rate was similar irrespective of ambient light. In contrast, reduction in light availability had an approximately 3-fold greater impact on net phytoplankton growth rate than mesozooplankton grazing pressure. The low mesozooplankton grazing impact across stations suggests limited mesozooplankton-mediated vertical export of phytoplankton production. The constraints provided here on trophic transfer, as well as quantitative estimates of the relative contribution of light and grazer controls of PP and of grazer-induced shifts in particle size spectra, illuminate food web dynamics and aid in parameterizing modeling-frameworks assessing global elemental fluxes and carbon export.

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.

Lagrangian Studies of Marine Production: A Multimethod Assessment of Productivity Relationships in the California Current Ecosystem Upwelling Region

AbstractA multimethod process‐oriented investigation of diverse productivity measures in the California Current Ecosystem (CCE) Long‐Term Ecological Research study region, a complex physical environment, is presented. Seven multiday deployments covering a transition region from high to low productivity were conducted over two field expeditions (spring 2016 and summer 2017). Employing a Lagrangian study design, water parcels were followed over several days, comparing 24‐h in situ measurements (14C and 15NO3 ‐uptake, dilution estimates of phytoplankton growth, and microzooplankton grazing) with high‐resolution productivity measurements by fast repetition rate fluorometry (FRRF) and equilibrium inlet mass spectrometry (EIMS), and integrated carbon export measuremnts using sediment traps. Results show the importance of accounting for temporal and fine spatial scale variability when estimating ecosystem production. FRRF and EIMS measurements resolved diel patterns in gross primary and net community production. Diel productivity changes agreed well with comparably more traditional measurements. While differences in productivity metrics calculated over different time intervals were considerable, as those methods rely on different base assumptions, the data can be used to explain ecosystem processes which would otherwise have gone unnoticed. The processes resolved from this method comparison further understanding of temporal and spatial coupling and decoupling of surface productivity and potential carbon burial in a gradient from coastal to offshore ecosystems.

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.

Varying Biological Activity and Wind Stress Affect the DMS Response during the SAGE Iron Enrichment Experiment

Surface dissolved dimethylsulfide (DMS) and depth-integrated dimethylsulfoniopropionate (DMSP) measurements were made from March to April 2004 during the SOLAS Air–Sea Gas Exchange Experiment (SAGE), a multiple iron enrichment experiment in subantarctic waters SE of New Zealand. During the first two iron enrichments, chl a and DMS production were constrained, but during the third enrichment, large pulses of DMS occurred in the fertilised IN patch, compared with the unfertilised OUT patch. During the third and fourth iron infusions, total chl a concentrations doubled from 0.52 to 1.02 µg/L. Hapto8s and prasinophytes accounted for 50%, and 20%, respectively, of total chl a. The large pulses of DMS during the third iron enrichment occurred during high dissolved DMSP concentrations and wind strength; changes in dinoflagellate, haptophyte, and cyanobacteria biomass; and increased microzooplankton grazing that exerted a top down control on phytoplankton production. A further fourth iron enrichment did cause surface waters to increase in DMS, but the effect was not as great as that recorded in the third enrichment. Differences in the biological response between SAGE and several other iron enrichment experiments were concluded to reflect microzooplankton grazing activities and the microbial loop dominance, resulting from mixing of the MLD during storm activity and high winds during iron enrichment.

Protistal Grazers Increase Grazing on Unicellular Cyanobacteria Diazotroph at Night

In this study, we have for the first time analysed diel microzooplankton grazing selectivity on unicellular cyanobacterial diazotroph (i.e. Crocosphaera watsonii WH8501) and non-diazotrophic unicellular microalga (i.e. Chlorella autotrophica). A mixed diet consisting of these two algae was supplied to 4 species of protistan microzooplankton at daytime and night-time, respectively. C. watsonii fixes nitrogen at night-time and showed a stronger diel pattern of cellular C:N ratio than C. autotrophica. All four grazers ingested more C. watsonii at night-time than in daytime, suggesting the diazotroph became more nutritious (inferred by C:N ratio) and thus a preferred prey for grazers when it fixes nitrogen. In particular, Oxyrrhis marina changed from preferring C. autotrophica in daytime to preferring C. watsonii at night-time. The rest grazers showed species-specific grazing preferences, which could be explained by extracellular polysaccharide production of C. watsonii , feeding mode, cingulum size and cell size of grazers.

Decoupling Between Phytoplankton Growth and Microzooplankton Grazing Enhances Productivity in Subantarctic Waters on Campbell Plateau, Southeast of New Zealand

AbstractThe Subantarctic zone is one of the largest High‐Nutrient Low‐Chlorophyll zones of the Southern Ocean. Despite widespread iron limitation, phytoplankton accumulation (chlorophyll a (chla) &gt; 0.3 mg m−3) often occurs near islands and bathymetric features such as on the Campbell Plateau, southeast of New Zealand. To investigate the processes responsible for localized increases in chla commonly observed by satellites, we characterized phytoplankton biomass structure, production, and microzooplankton grazing on Campbell Plateau and surrounding waters in austral autumn (March 2017). Chla on the plateau tended to be higher, more variable (0.52 ± 0.38 mg chla m−3, mean ± standard deviation), and characterized by larger phytoplankton forms (22 ± 27%chla &gt; 20 μm) than surrounding waters (0.29 ± 0.12 mg chla m−3, 5 ± 2%). The increased contribution of diatoms, together with higher photosystem II maximum photochemical efficiency (Fv/Fm = 0.45 ± 0.05) and lower effective absorption cross‐section (σPSII = 774 ± 90 Å RCII−1) on the plateau, suggests an alleviation of iron stress relative to surrounding waters (Fv/Fm = 0.37 ± 0.04, σPSII = 974 ± 89 Å RCII−1). Phytoplankton growth (μ0 = 0.42 ± 0.20 day−1) and production rates (6.1 ± 3.2 mg C m−3 day−1) were also higher compared to surrounding waters (0.27 ± 0.04 day−1, 3.5 ± 1.9 mg C m−3 day−1). While microzooplankton grazing (g = 0.28 ± 0.18 day−1) balanced phytoplankton growth off the plateau (g:μ0 = 1.13 ± 0.18), the imbalance observed on Campbell Plateau (g = 0.25 ± 0.25 day−1) allowed a substantial proportion of primary production to escape microzooplankton grazing control (g:μ0 = 0.48 ± 0.31). Overall, the degree of coupling tended to decrease with the depth of the mixed layer (R2 &gt; 0.6, p &lt; 0.001). We hypothesize that the entrainment of deeper water into the mixed layer regulates the onset and fate of the autumn bloom by altering nutrient supply and microzooplankton grazing pressure.

Bottom-up and top-down control in a multitrophic system: the role of nutrient limitation and infochemical-mediated predation in a plankton food-web model

Chemicals released following herbivore grazing on primary producers can promote multitrophic interactions by influencing the foraging behaviour of higher order predators. In particular, chemicals released during microzooplankton grazing on phytoplankton can act as infochemical cues that elicit foraging responses and improve search efficiency in carnivorous copepods. Models investigating such infochemical-mediated multitrophic interactions in the plankton are typically based on top-down control, where phytoplankton concentrations are controlled through predation and grazing from higher trophic levels. However, in marine environments nutrient limitation is an important factor that influences a food-web from below, and earlier models of this system only indirectly account for this by assuming predator-free growth is logistic with a fixed carrying capacity. Here we consider the dynamics of infochemical-mediated interactions in a marine system where nutrient limitation is modelled directly through an extended NPZ-style model. We show the one-parameter bifurcation behavior of the top-down model to change when the total nutrient availability is changed, and hence demonstrate phytoplankton bloom formation to be a function of both top-down and bottom-up processes.

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.

Plankton community responses to pulsed upwelling events in the southern Taiwan Strait

AbstractWe used field observations (mapping and time-series observations) and satellite data to investigate the changes of environmental factors and plankton community structure during four pulsed upwelling events in the southern Taiwan Strait (STWS) during August 2004 and July 2005. Based on the surface temperature, salinity, and concentrations of nitrate, oxygen, and chlorophyll a (Chl a), we identified three stages of upwelling: recent, mature, and aged upwelled water. Diatoms and Synechococcus dominated during the first two and third stages of the upwelling, respectively. In recently upwelled water, phytoplankton grew rapidly, and nutrient concentrations were relatively high. Diatoms accounted for &amp;gt;69% of the total Chl a in recently upwelled and mature water. As the upwelled water aged, the phytoplankton community shifted to coexistence of diatoms and Synechococcus. The microzooplankton community was dominated by aloricate ciliates and tintinnids during upwelling, but the abundance of heterotrophic dinoflagellates increased as the upwelled water matured. Microzooplankton grazing reached a maximum during the mature stage and approximately balanced phytoplankton growth in aged, upwelled water. Overall, our study revealed rapid changes in the plankton community during the different stages of upwelling that reflected the complex and dynamic nature of upwelling systems.

An empirical model of carbon flow through marine viruses and microzooplankton grazers.

Viruses and microzooplankton grazers represent major sources of mortality for marine phytoplankton and bacteria, redirecting the flow of organic material throughout the world's oceans. Here, we investigate the use of nonlinear population models of interactions between phytoplankton, viruses and grazers as a means to quantitatively constrain the flow of carbon through marine microbial ecosystems. We augment population models with a synthesis of laboratory-based estimates of prey, predator and viral life history traits that constrain transfer efficiencies. We then apply the model framework to estimate loss rates in the California Current Ecosystem (CCE). With our empirically parameterized model, we estimate that, of the total losses mediated by viruses and microzooplankton grazing at the focal CCE site, 22 ± 3%, 46 ± 27%, 3 ± 2% and 29 ± 20% were directed to grazers, sloppy feeding (as well as excretion and respiration), viruses and viral lysate respectively. We identify opportunities to leverage ecosystem models and conventional mortality assays to further constrain the quantitative rates of critical ecosystem processes.