Zooplankton Grazing Rates Research Articles

Model exploration of microplastic effects on zooplankton grazing reveal potential impacts on the global carbon cycle

Amongst the increasing number of anthropogenic stress factors threatening ocean equilibrium, microplastics (MP; < 5 mm) have emerged as particularly worrisome. In situ observations have shown that MP accumulate in large areas at the surface ocean where it may threaten the functioning marine species. In particular, experimental evidence has shown that the grazing rates of several zooplankton species may be significantly altered by MP. These direct impacts on zooplankton may alter nutrient and carbon cycling. However, how these laboratory results may translate into impacts on the global ocean is yet unknown. Here, we use a global coupled physical-biogeochemical model including MP (NEMO/PISCES-PLASTIC) to investigate the impacts of MP exposure on zooplankton grazing rates. Drawing from experimental results, we use varying water contamination impact thresholds to explore the biogeochemical consequences of MP impacts on short (10 years) and long timescales (100 years). Our simulations show that the geographical extent of MP impacts on zooplankton remains restricted to about 10% of the global ocean surface, even after 100 years of constant MP contamination. However, in the most contaminated regions (e.g. the sub-tropical gyres), [MP] has surged from a few mg m−3 to > 50 mg m−3. Despite their oligotrophic nature and limited contribution to the overall ocean carbon cycle, MP impacts on zooplankton grazing could disrupt carbon cycling in these highly contaminated regions (up to 50% reduction in yearly primary production, carbon export fluxes and organic matter remineralisation after 100 years). Our research suggests that persistent MP pollution in the ocean could diminish primary production by 4%. In spite of the large sensitivity of our results to the water contamination impact threshold, we suggest MP impacts on zooplankton grazing may cause an annual loss of 1 Gt yr−1 of exported carbon after 100 years, if MP inputs remain constant globally.

Zooplankton exposure to microplastics at global scale: Influence of vertical distribution and seasonality

Microplastics (MPs) are ubiquitous contaminants in the ocean. Zooplankton is thus widely exposed to MP ingestion. Here, we use a global coupled physical–biogeochemical model enriched with a 3D representation of MPs to assess the global zooplankton exposure to MPs. As expected, our results indicate that water MP concentration is the highest in the surface layers of subtropical gyres and coastal areas close to major MP sources, which is mostly due to floating MPs, while neutral MPs contaminate the mesopelagic zone. Additionally, we showed that floating MPs may be also transported to the mesopelagic waters during the seasonal deepening of the mixed layer depth. We then estimate zooplankton exposure to MPs based on water MP concentrations, plankton biomass, and zooplankton grazing rate. Two main drivers lead to high zooplankton exposure to MPs: 1) high water MP contamination and 2) intense grazing activity. Seasonally, re-stratification of surface waters may lead to MP vertical concentration coinciding with planktonic blooms, thus increasing contamination risk.

Selective grazing of zooplankton on phytoplankton defines rapid algal succession and blooms in oceans

In a model plankton ecosystem containing phytoplankton and zooplankton, the later usually avoids toxic phytoplankton and prefers nontoxic species by modulating its grazing rate. The toxins released by toxic phytoplankton can lead to a decreased grazing rate of zooplankton on both phytoplankton, and the presence of nontoxic phytoplankton in the system also reduces the grazing pressure of zooplankton on the toxic phytoplankton. In this study, a set of functions were given that model the selective grazing of zooplankton on various phytoplankton. Preference coefficient and avoidance coefficient were introduced in the selective grazing functions to measure the level of zooplankton preference of nontoxic phytoplankton and avoidance of toxic phytoplankton, respectively. A dynamics model was established in a nontoxic phytoplankton-toxic phytoplankton-zooplankton system based on the grazing functions of zooplankton to study the existence and stability of an interior equilibrium and Hopf-bifurcation. Our results show that zooplankton selective grazing promotes both zooplankton and toxic phytoplankton biomass, and contributes to maintain the coexistence of all species. Low grazing selectivity, i.e., low values of preference coefficient and avoidance coefficient, decreases the zooplankton population and increases the opportunity for phytoplankton blooms. A moderate level of grazing selectivity helps keep the system in balance. With high grazing selectivity on phytoplankton, toxic phytoplankton becomes the dominant species, resulting in a rapid algal succession and eventual toxic phytoplankton blooms. When the preference coefficient and avoidance coefficient in the dynamics model crossed critical values respectively, the ecosystem enters into a Hopf-bifurcation around the interior equilibrium, inducing oscillations of these populations.

Spatial and Temporal Variations of Chlorophyll a and Primary Productivity in the Hangzhou Bay

The Hangzhou Bay (HZB) is an important part of the Zhoushan fishing ground, the most productive region in the Eastern China Seas. Although HZB remains eutrophication all year round, its chlorophyll a (Chl) and primary productivity (PP) are usually significantly lower than those in the adjacent waters. In the present study, we presented the Chl and PP distributions in the HZB and analyzed their correlations with environmental factors in four seasons. The field observation showed that Chl and PP had significant seasonal variations, and was highest in the summer (1.66 ± 0.61 mg·m−3 and 12.11 ± 12.25 mg C·m−3·h−1, respectively). Total suspended matters (TSM) concentration was the key environmental factor that constrains PP in the study area. High concentration of TSM reduced light exposure (LE, the annual mean value was 0.92 ± 0.81 Einstein·m−2·day−1) in the mixed layer of the HZB, which was much lower than the saturated light intensity of phytoplankton growth, and thus caused a strong light limitation in the HZB. However, the seasonal variations in the photosynthesis rates (PB) and Chl did not coincide. This fact suggested that the growth rate was not the only factor controlling seasonal variations of phytoplankton biomass. In winter, the very high TSM and strong mixing might reduce the zooplankton grazing rate, and lead to a relatively high concentration of Chl during the very low LE and PB period. These results implied that, in the HZB, the extremely turbid water could affect both phytoplankton growth and loss, which was probably the major mechanism responsible for the complex phytoplankton spatial and temporal variations.

Linking population dynamics models with empirically derived models through phytoplankton primary production.

There are alternative methods for estimation of phytoplankton primary production (PP) that are fundamentally different in the calculation approach. The process‐oriented PP model is a mechanistic, empirically derived method based on the photosynthesis–light relationships. The population dynamics‐based PP calculation, which is a synthetic method, provides a production estimate based on population dynamics of phytoplankton. These alternative methods were here compared with regard to production estimates and linked to enhance the performance of the existing models of population dynamics applied to a wide variety of lakes worldwide in terms of morphometry, nutrient status, and light environments. Estimates of PP were shown to be sensitive to changes in phytoplankton sinking and zooplankton grazing rates in both methods. Production estimates in the process‐oriented PP model were also sensitive to light‐associated parameters such as day length. Although the production estimated from the population dynamics‐based PP calculation tended to be lower than that from the process‐oriented PP model irrespective of lake morphometry, production estimates calculated from both methods with standard parameterization were comparable when production was estimated on an annual timescale. However, it was also shown that the alternative methods could produce different production estimates when estimated on shorter timescales such as cyanobacterial blooms in summer. Cyanobacteria with low mortality due to grazing and sinking losses have been considered as trophic bottlenecks, but there is increasing evidence that their mortality is, to a considerable extent, due to parasitic pathogens. In the case of cyanobacterial blooms, an addition of parasite‐related loss term (19%–33% of standing stock) resulted in a resolution of the difference in production estimates between the methods. These analyses theoretically support the critical role of parasitism and resolve the bottleneck problem in aquatic ecosystem metabolism.

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.

Temperature Dependence of Freshwater Phytoplankton Growth Rates and Zooplankton Grazing Rates

Phytoplankton growth rates and zooplankton grazing rates were estimated on 16 occasions over a period of 17 months in University Lake, a highly eutrophic lake on the campus of Louisiana State University. Phytoplankton growth rates and chlorophyll a concentrations averaged 1.0 ± 0.2 d−1 and 240 ± 120 mg m−3, respectively. Chlorophyll a concentrations were at or above the inflection point of the Holling type I curve that described the relationship between zooplankton grazing rates and chlorophyll a concentrations. In most cases, it was necessary to dilute lake water by more than a factor of 4 before zooplankton grazing rates became sensitive to chlorophyll a concentrations. Chlorophyll a concentrations were positively correlated with temperature and were roughly fourfold higher at 30 °C than at 15 °C. An analysis of the temperature dependence of the growth rates and grazing rates in this study and 87 other paired estimates of limnetic phytoplankton growth rates and zooplankton grazing rates revealed virtually identical temperature dependences of growth rates and grazing rates that were very similar to the temperature dependence predicted by the metabolic theory of ecology. Phytoplankton growth rates exceeded zooplankton grazing rates by 0.13 ± 0.05 d−1 at all temperatures over a temperature range of 8.5–31.5 °C. The Q10 for both phytoplankton growth rates and zooplankton grazing rates was 1.5 over that temperature range.

An investigation of delay induced stability transition in nutrient-plankton systems

Abstract In this paper, a nutrient-plankton interaction model is proposed to explore the characteristic of plankton system in the presence of toxic phytoplankton and discrete time delay. Anti-predator efforts of phytoplankton by toxin liberation act as a prominent role on plankton dynamics. Toxicity controls the system dynamics and reduces the grazing rate of zooplankton. The toxic substance released by phytoplankton is not an instantaneous process, it requires some time for maturity. So, a discrete time delay is incorporated in the toxin liberation by the phytoplankton. The choice of functional response is important to understand the toxin liberation and it depends on the nonlinearity of the system, which follows the Monod-Haldane type functional response. Theoretically, we have studied the boundedness condition along with all the feasible equilibria analysis and stability criteria of delay free system. We have explored the local stability conditions of delayed system. The existence criterion for stability and direction of Hopf-bifurcation are also derived by using the theory of normal form and center manifold arguments. The essential features of time delay are studied by time series, phase portrait and bifurcation diagram. We perform a global sensitivity analysis to identify the important parameters of the model having a significant impact on zooplankton. Our numerical investigation reveals that the toxin liberation delay switches the stability of the system from stable to limit cycle and after a certain interval chaotic dynamics is observed. High rate of toxic substances production shows extinction of zooplankton. Further, the negative and positive impacts of other control parameters are studied. Moreover, to support the occurrence of chaos, the Poincare map is drawn and the maximum Lyapunov exponents are also computed.

Diagnosing Mechanisms of Ocean Carbon Export in a Satellite-Based Food Web Model

The net primary productivity (NPP) of marine phytoplankton is ~ 50 Pg C yr-1, and roughly 10-20% of this NPP is exported out of the surface ocean as sinking particulate organic carbon (POC). Numerous mechanisms are hypothesized to control POC export out of the surface ocean but the relative importance of the various mechanisms remains poorly quantified on global scales. Here, we use a previously published satellite-based mechanistic model of POC export to examine the effects on global POC export of size-specific physical aggregation, size-specific and temperature-dependent zooplankton fecal pellet production, and size-specific and temperature-dependent non-grazing phytoplankton mortality. We test these mechanisms in different model configurations to determine if these processes improve the ability of the model to match POC export observations, and to assess the role of each process in controlling global POC export. We find that all model configurations predict that over 60% of the global POC export is from small zooplankton fecal pellets. All model configurations predict similar total POC export, and we find only small differences in the magnitude, timing, and geographical variations of total POC export. However, the fraction of total POC export due to sinking phytoplankton aggregates, and that due to the fecal pellets of large zooplankton, vary by more than a factor of two across the different model configurations. The POC export in all models is most sensitive to parameters controlling zooplankton fecal fluxes and non-grazing phytoplankton mortality. We compared zooplankton grazing rates predicted by the models to results of experimental data, and found that some models match the experimental grazing rates better than others, although data uncertainties remain large. More field measurements of bulk ecosystem rates (i.e., phytoplankton aggregation and zooplankton grazing), as well as explicit determinations of of the proportion of fecal matter to phytoplankton aggregation, will help to better constrain mechanistic models of global POC export.

Biodiversity‐mediated effects on ecosystem functioning depend on the type and intensity of environmental disturbances

Environmental disturbances affect ecosystem functioning through changes in organisms’ metabolism (direct effect) and biodiversity loss (indirect or biodiversity‐mediated effect). It is still a challenge to separate direct and biodiversity‐mediated effects of environmental changes on ecosystem functioning due to the difficulties in isolating ‘true’ biodiversity loss effects. Furthermore, it is still unclear whether biodiversity‐mediated effects are as important as direct effects. In this study, we performed an experiment in artificial microcosms to disentangle biodiversity‐mediated and direct effects of two major environmental disturbances on the functioning of aquatic ecosystems: increases in temperature and salinity. The ecosystem function analyzed was the microalgae predation by the zooplankton community (zooplankton grazing rates). Temperature and salinity increases affected the zooplankton grazing rates due to changes in community composition and abundance, as well as organism performance. The impact of salinity changes on community structure was higher than that of temperature; however, the importance of biodiversity‐mediated and direct effects was similar to regulating the ecosystem functioning, albeit they have presented different directions and magnitude across the treatments. At a moderate level of temperature increase, we observed that the biodiversity‐mediated effect was more relevant than the direct effect, with negative effects on the overall grazing rates. Our results suggest that disturbances can affect the functioning of aquatic environments through a set of complex biological mechanisms that balance direct and biodiversity‐mediated effects. We concluded that the relative importance of biodiversity‐mediated effects depends on the type and intensity of the disturbance.

In situ ingestion of Microcystis is negatively related to copepod abundance in the upper San Francisco Estuary

AbstractAlthough zooplankton grazing on cyanobacteria is a key process regulating the structure and function of bloom‐dominated waters, few studies have measured in situ ingestion and none have linked it to zooplankton abundance. This study measured in situ zooplankton grazing rate on Microcystis via quantitative polymerase chain reaction (qPCR) amplification of prey DNA from the guts of the copepod Pseudodiaptomus forbesi, an abundant and important prey species for endangered fish in the upper San Francisco Estuary (SFE). We calibrated the qPCR assay in laboratory grazing experiments with Eurytemora affinis, a similar sized copepod abundant in the SFE during winter, due to higher ingestion of Microcystis by this copepod compared to Pseudodiaptomus forbesi. In the field, Pseudodiaptomus forbesi ingested Microcystis during blooms, including strains capable of producing microcystin toxins. Although the Microcystis content in Pseudodiaptomus forbesi guts increased with bloom intensity, estimated ingestion rates were low and suggested copepods avoided this cyanobacterium. Moreover, Pseudodiaptomus forbesi abundance was inversely related to its gut Microcystis content, but not related to other factors including temperature, salinity, turbidity, and gut fluorescence of chlorophyll a. Results demonstrate that the gut Microcystis content of zooplankton may be a key factor explaining their abundance in nature. Our results also highlight the utility of molecular methods for understanding in situ trophic interactions in bloom‐dominated waters. Increased blooms under future climate scenarios (e.g., extreme drought and higher temperatures) may decrease the abundance of key zooplankton like Pseudodiaptomus forbesi through increased ingestion of Microcystis, with potentially negative effects on endangered fishes in the SFE.

Temperature dependent growth rate of phytoplankton and salinity induced grazing rate of zooplankton as determinants of realistic multi-delayed food chain model

Impact of temperature dependent growth rate of phytoplankton, salinity induced grazing rate of zooplankton and gestation delay of phytoplankton and zooplankton differently are considered in the present study. Reproduction of predator after consuming prey is not instantaneous, but mediated by some time lag required for gestation. Here first gestation delay is considered in the numerical response term of phytoplankton and second one is at the numerical response term of zooplankton respectively. There is a very interesting interplay between these two delays. In accordance with previous studies, it is observed that delay destabilizes the system, in general and stability loss occurs via Hopf-bifurcation. In particular, we show that there exists critical values of the delay parameters below which the coexistence equilibrium is stable and above which it is unstable. Hopf bifurcation occurs when the delay parameters cross their critical values. All the stability results of the system significantly depend upon the temperature and salinity of the environment. Numerical examples are also support the model assumption and analytical results.

An Optimal Control Problem Related to a Hybrid Aquatic System

The increasing interest for ecological water quality models has gained a lot of importance during the last few decades and has arisen as a result of the eutrophication of lakes and reservoirs throughout the world. In this talk, we consider the analysis and numerical implementation of an optimal control problem associated with a nutrient-phytoplankton-zooplankton-fish model introduced in [2]. In this model, the parameter for nutrient level and fish predation acts as a distributed control variable. A similar optimal control problem, but slightly different from this approach with respect to the grazing rate of zooplankton on phytoplankton, was treated in [1]. We establish the existence of optimal solutions and derive the optimality system. We also provide some numerical simulations that support the theoretical results. ...

Photosynthesis rate, salinity induced grazing rate and gestation delay as determinants of the dynamics of a realistic food chain model: a stochastic approach

Impact of photosynthesis rate of phytoplankton, salinity induced grazing rate of zooplankton and gestation delay of zooplankton and fish population are considered in the present study. Models with delay are much more realistic, as in reality time delays occur in almost every biological situation and assume to be one of the reasons of regular fluctuations in population density. Reproduction of predator after consuming prey is not instantaneous, but mediated by some time lag required for gestation. It is observed that there is stability switches and Hopf bifurcation occurs when the delay crosses some critical value. It is observed that the quantitative level of abundance of system population depends crucially on the delay parameter if the gestation period exceeds some critical value. Also, environmental stochasticity in the form of Gaussian white-noise, plays a significant role to describe the system and its values. Numerical examples are also support the model assumption and analytical results.

Inferred support for disturbance‐recovery hypothesis of North Atlantic phytoplankton blooms

AbstractAnalyses of satellite‐derived chlorophyll data indicate that the phase of rapid phytoplankton population growth in the North Atlantic (the “spring bloom”) is actually initiated in the winter rather than the spring, contradicting Sverdrup's critical depth hypothesis. An alternative disturbance‐recovery hypothesis (DRH) has been proposed to explain this discrepancy, in which the rapid deepening of the mixed layer reduces zooplankton grazing rates sufficiently to initiate the bloom. We use Bayesian parameter inference on a simple Nutrient‐Phytoplankton‐Zooplankton (NPZ) model to investigate the DRH and also investigate how well the model can capture the multiyear and spatial dynamics of phytoplankton concentrations and population growth rates. Every parameter in our NPZ model was inferred as a probability distribution given empirical constraints, which provides a more objective method to identify a model parameterization given available empirical evidence, rather than fixing or tuning individual parameter values. Our model explains around 75% of variation in the seasonal dynamics of phytoplankton concentrations, 30% of variation in their population rates of change, and correctly predicts the phases of population growth and decline. Our parameter‐inferred model supports the DRH, revealing the sustained reduction of grazing due to mixed‐layer deepening as the driving mechanism behind bloom initiation, with the relaxation of nutrient limitation being another contributory mechanism. Our results also show that the continuation of the bloom is caused in part by the maintenance of phytoplankton concentrations below a level that can support positive zooplankton population growth. Our approach could be employed to formally assess alternative hypotheses for bloom formation.

Identification of key factors influencing primary productivity in two river-type reservoirs by using principal component regression analysis.

To understand the factors controlling algal production in two lakes located on the Han River in South Korea, Lake Cheongpyeong and Lake Paldang, a principal component regression model study was conducted using environmental monitoring and primary productivity data. Although the two lakes were geographically close and located along the same river system, the main factors controlling primary productivity in each lake were different: hydraulic retention time and light conditions predominantly influenced algal productivity in Lake Cheongpyeong, while hydraulic retention time, chlorophyll a-specific productivity, and zooplankton grazing rate were most important in Lake Paldang. This investigation confirmed the utility of principal component regression analysis using environmental monitoring data for predicting complex biological processes such as primary productivity; in addition, the study also increased the understanding of explicit interactions between environmental variables. The findings obtained in this research will be useful for the adaptive management of water reservoirs. The results will also aid in the development of management strategies for water resources, thereby improving total environmental conservation.

Biology and air–sea gas exchange controls on the distribution of carbon isotope ratios (δ&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C) in the ocean

Abstract. Analysis of observations and sensitivity experiments with a new three-dimensional global model of stable carbon isotope cycling elucidate processes that control the distribution of δ13C of dissolved inorganic carbon (DIC) in the contemporary and preindustrial ocean. Biological fractionation and the sinking of isotopically light δ13C organic matter from the surface into the interior ocean leads to low δ13CDIC values at depths and in high latitude surface waters and high values in the upper ocean at low latitudes with maxima in the subtropics. Air–sea gas exchange has two effects. First, it acts to reduce the spatial gradients created by biology. Second, the associated temperature-dependent fractionation tends to increase (decrease) δ13CDIC values of colder (warmer) water, which generates gradients that oppose those arising from biology. Our model results suggest that both effects are similarly important in influencing surface and interior δ13CDIC distributions. However, since air–sea gas exchange is slow in the modern ocean, the biological effect dominates spatial δ13CDIC gradients both in the interior and at the surface, in contrast to conclusions from some previous studies. Calcium carbonate cycling, pH dependency of fractionation during air–sea gas exchange, and kinetic fractionation have minor effects on δ13CDIC. Accumulation of isotopically light carbon from anthropogenic fossil fuel burning has decreased the spatial variability of surface and deep δ13CDIC since the industrial revolution in our model simulations. Analysis of a new synthesis of δ13CDIC measurements from years 1990 to 2005 is used to quantify preformed and remineralized contributions as well as the effects of biology and air–sea gas exchange. The model reproduces major features of the observed large-scale distribution of δ13CDIC as well as the individual contributions and effects. Residual misfits are documented and analyzed. Simulated surface and subsurface δ13CDIC are influenced by details of the ecosystem model formulation. For example, inclusion of a simple parameterization of iron limitation of phytoplankton growth rates and temperature-dependent zooplankton grazing rates improves the agreement with δ13CDIC observations and satellite estimates of phytoplankton growth rates and biomass, suggesting that δ13C can also be a useful test of ecosystem models.

Factors Controlling Phytoplankton Biomass in a Subtropical Coastal Lagoon: Relative Scales of Influence

Patterns in phytoplankton biomass are essential to understanding estuarine ecosystem structure and function and are the net result of various gain and loss processes. In this study, patterns in phytoplankton biomass were explored in relation to a suite of potentially regulating factors in a well-flushed, subtropical lagoon, the Matanzas River Estuary (MRE) in northeast Florida. We examined temporal variability in water temperature, light availability, nutrient concentrations, phytoplankton productivity, and phytoplankton standing stock over 8 years (2003–2010) and explored relationships among variables through correlation analysis. Laboratory experiments in the spring and summer of 2009 quantified phytoplankton growth rates, nutrient limitation potential, and zooplankton grazing rates. The potential influence of oyster grazing was also examined by scaling up population metrics and filtration rate estimates. Results indicated that phytoplankton biomass in the study area was relatively low mainly due to a combination of low temperature and light availability in the winter and consistent tidal water exchange and bivalve grazing throughout the year. Relatively low levels of phytoplankton standing stock and small inter-annual variability within the MRE reflect a balance between gain and loss processes which provide a degree of resilience of the system to natural and anthropogenic influences.

Decomposing the effects of ocean warming on chlorophyll a concentrations into physically and biologically driven contributions

Recently compiled observational data suggest a substantial decline in the global median chlorophyll a concentration over the 20th century, a trend that appears to be linked to ocean warming. Several modelling studies have considered changes in the ocean’s physical structure as a possible cause, while experimental work supports a biological mechanism, namely an observed increase in zooplankton grazing rate that outpaces phytoplankton production at higher temperatures. Here, we present transient simulations derived from a coupled ocean general circulation and carbon cycle model forced by atmospheric fields under unabated anthropogenic global warming (IPCC SRES A1FI scenario). The simulations account for both physical and biological mechanisms, and can reproduce about one quarter of the observed chlorophyll a decline during the 20th century, when using realistically parameterized temperature sensitivity of zooplankton metabolism (Q10 between 2 and 4) and phytoplankton growth (Q10 ∼ 1.9). Therefore, we have employed and re-calibrated the standard ecosystem model which assumes a lower temperature sensitivity of zooplankton grazing (Q10 = 1.1049) by re-scaling phytoplankton growth rates and zooplankton grazing rates. Our model projects a global chlorophyll a decline of >50% by the end of the 21st century. While phytoplankton abundance and chlorophyll a experience pronounced negative effects, primary production and zooplankton concentrations are less sensitive to ocean warming. Although changes in physical structure play an important role, much of the simulated change in chlorophyll a and productivity is related to the uneven temperature sensitivity of the marine ecosystem.

Grazing rates of crustacean zooplankton communities on intact phytoplankton communities in Canadian Subarctic lakes and ponds

Zooplankton grazing can potentially affect the biomass and composition of phytoplankton communities directly and indirectly. Low chlorophyll a concentration for a given TP concentration and simplified fishless food webs lead to the expectation that zooplankton community grazing rates are high in Subarctic regions; however, zooplankton community grazing rates have not been determined for Subarctic lakes/ponds. We estimated zooplankton community grazing rates on phytoplankton in 12 lakes and ponds in Wapusk National Park, Canada using a microcosm grazing experiment. Lakes and ponds differed in zooplankton taxonomic composition, Chl-a concentration, and zooplankton biomass. We found that the grazing rates on the total chlorophyll a (GRTotal) ranged 0–13.7% grazed per day and the grazing rates on the edible (<30 μm, GR<30) chlorophyll a was 0 to 16.7% per day. GRTotal increased with lake Daphnia and cladoceran biomass, as did GR<30, which also had a negative relationship with the total in-lake Chl-a. The calculated zooplankton grazing rates were within the range found for larger, temperate lakes.