Exploring climate and anthropogenic impacts on marine ecosystems requires an understanding of how... more Exploring climate and anthropogenic impacts on marine ecosystems requires an understanding of how trophic components interact. However, integrative end-to-end ecosystem studies (experimental and/or modelling) are rare. Experimental investigations often concentrate on a particular group or individual species within a trophic level, while tropho-dynamic field studies typically employ either a bottom-up approach concentrating on the phytoplankton community or a top-down approach concentrating on the fish community. Likewise the emphasis within modelling studies is usually placed upon phytoplankton-dominated biogeochemistry or on aspects of fisheries regulation. In consequence the roles of zooplankton communities (protists and metazoans) linking phytoplankton and fish communities are typically under-represented if not (especially in fisheries models) ignored. Where represented in ecosystem models, zooplankton are usually incorporated in an extremely simplistic fashion, using empirical descriptions merging various interacting physiological functions governing zooplankton growth and development, and thence ignoring physiological feedback mechanisms. Here we demonstrate, within a modelled plankton food-web system, how trophic dynamics are sensitive to small changes in parameter values describing zooplankton vital rates and thus the importance of using appropriate zooplankton descriptors. Through a comprehensive review, we reveal the mismatch between empirical understanding and modelling activities identifying important issues that warrant further experimental and modelling investigation. These include: food selectivity, kinetics of prey consumption and interactions with assimilation and growth, form of voided material, mortality rates at different age-stages relative to prior nutrient history. In particular there is a need for dynamic data series in which predator and prey of known nutrient history are studied interacting under varied pH and temperature regimes.
Exploring climate and anthropogenic impacts on marine ecosystems requires an understanding of how... more Exploring climate and anthropogenic impacts on marine ecosystems requires an understanding of how trophic components interact. However, integrative end-to-end ecosystem studies (experimental and/or modelling) are rare. Experimental investigations often concentrate on a particular group or individual species within a trophic level, while tropho-dynamic field studies typically employ either a bottom-up approach concentrating on the phytoplankton community or a top-down approach concentrating on the fish community. Likewise the emphasis within modelling studies is usually placed upon phytoplankton-dominated biogeochemistry or on aspects of fisheries regulation. In consequence the roles of zooplankton communities (protists and metazoans) linking phytoplankton and fish communities are typically under-represented if not (especially in fisheries models) ignored. Where represented in ecosystem models, zooplankton are usually incorporated in an extremely simplistic fashion, using empirical descriptions merging various interacting physiological functions governing zooplankton growth and development, and thence ignoring physiological feedback mechanisms. Here we demonstrate, within a modelled plankton food-web system, how trophic dynamics are sensitive to small changes in parameter values describing zooplankton vital rates and thus the importance of using appropriate zooplankton descriptors. Through a comprehensive review, we reveal the mismatch between empirical understanding and modelling activities identifying important issues that warrant further experimental and modelling investigation. These include: food selectivity, kinetics of prey consumption and interactions with assimilation and growth, form of voided material, mortality rates at different age-stages relative to prior nutrient history. In particular there is a need for dynamic data series in which predator and prey of known nutrient history are studied interacting under varied pH and temperature regimes.
The classic portrayal of plankton is dominated by phytoplanktonic primary producers and zooplankt... more The classic portrayal of plankton is dominated by phytoplanktonic primary producers and zooplanktonic secondary producers. In reality, many if not most plankton traditionally labelled as phytoplankton or microzooplankton should be identified as mixotrophs, contributing to both primary and secondary production. Mixotrophic protists (i.e. single-celled eukaryotes that perform photosynthesis and graze on particles) do not represent a minor component of the plankton, as some form of inferior representatives of the past evolution of protists; they represent a major component of the extant protist plankton, and one which could become more dominant with climate change. The implications for this mistaken identification, of Chlorophyll a fluorescence in phytoplankton: relationship to photosynthesis and biomass. J. Plankton Res., 7, 715-731.
The traditional view of the planktonic food web describes consumption of inorganic nutrients by p... more The traditional view of the planktonic food web describes consumption of inorganic nutrients by photoautotrophic phytoplankton, which in turn supports zooplankton and ultimately higher trophic levels. Pathways centred on bacteria provide mechanisms for nutrient recycling. This structure lies at the foundation of most models used to explore biogeochemical cycling, functioning of the biological pump, and the impact of climate change on these processes. We suggest an alternative new paradigm, which sees the bulk of the base of this food web supported by protist plankton communities that are mixotrophic -combining phototrophy and phagotrophy within a single cell. The photoautotrophic eukaryotic plankton and their heterotrophic microzooplankton grazers dominate only during the developmental phases of ecosystems (e.g. spring bloom in temperate systems). With their flexible nutrition, mixotrophic protists dominate in more-mature systems (e.g. temperate summer, established eutrophic systems and oligotrophic systems); the more-stable water columns suggested under climate change may also be expected to favour these mixotrophs. We explore how such a predominantly mixotrophic structure af-fects microbial trophic dynamics and the biological pump. The mixotroph-dominated structure differs fundamentally in its flow of energy and nutrients, with a shortened and potentially more efficient chain from nutrient regeneration to primary production. Furthermore, mixotrophy enables a direct conduit for the support of primary production from bacterial production. We show how the exclusion of an explicit mixotrophic component in studies of the pelagic microbial communities leads to a failure to capture the true dynamics of the carbon flow. In order to prevent a misinterpretation of the full implications of climate change upon biogeochemical cycling and the functioning of the biological pump, we recommend inclusion of multi-nutrient mixotroph models within ecosystem studies.
The relationship between algae and their zooplanktonic predators typically involves consumption o... more The relationship between algae and their zooplanktonic predators typically involves consumption of nutrients by algae, grazing of the algae by zooplankton which in turn enhances predator biomass, controls algal growth and regenerates nutrients. Eutrophication raises nutrient levels, but does not simply increase normal predator-prey activity; rather, harmful algal bloom (HAB) events develop often with serious ecological and aesthetic implications. Generally, HAB species are outwardly poor competitors for nutrients, while their development of grazing deterrents during nutrient stress ostensibly occurs too late, after the nutrients have largely been consumed already by fast-growing non-HAB species. A new mechanism is presented to explain HAB dynamics under these circumstances. Using a multi-nutrient predator-prey model, it is demonstrated that these blooms can develop through the selfpropagating failure of normal predator-prey activity, resulting in the transfer of nutrients into HAB growth at the expense of competing algal species. Rate limitation of this transfer provides a continual level of nutrient stress that results in HAB species exhibiting grazing deterrents protecting them from top-down control. This process is self-stabilizing as long as nutrient demand exceeds supply, maintaining the unpalatable status of HABs; such events are most likely under eutrophic conditions with skewed nutrient ratios.
An ingestion-based prey selectivity function (IS) is described specifically to aid the simulation... more An ingestion-based prey selectivity function (IS) is described specifically to aid the simulation of zooplankton activity in circumstances in which selectivity varies, for example, as functions of prey palatability or abundance. In IS, the ultimate control of the rate of predation is not ...
... For example, mixotrophic experimentation typically focuses on either impacts of light or nutr... more ... For example, mixotrophic experimentation typically focuses on either impacts of light or nutrient limitation on grazing ( [Nygaard and Tobiesen, 1993] , [Stoecker et al., 1997] , [Li et al., 2000] , [Smalley et al., 2003] and [Adolf et al., 2006] ). ...
A mathematical model was developed to represent the dynamics of predation and assimilation of ing... more A mathematical model was developed to represent the dynamics of predation and assimilation of ingested material by heterotrophic marine micro-zooplankton. Predation rate was made a rectangular hyperbolic function of prey carbon (C) concentration modified to simulate the prey selectivity that these organisms have been observed experimentally to exhibit in response to prey nutritional quality. We chose prey N:C ratio as an index of selectivity and related both the maximum predation rate (PM) and the assimilation efficiency (AE) to changes in prey nitrogen:carbon (N:C). Changes in PM simulated the phenomena of "surge feeding" and "prey rejection". Changes in AE simulated sub-optimal assimilation of material from ingested prey, which in turn, served to decrease the ingestion rate on this material. The model was parameterised using laboratory data sets that followed ingestion of non-growing phytoplankton prey of different N:C ratio by a microflagellate predator. Investigative simulations in transient conditions, incorporating growing prey and hence with changing N:C ratio, indicated both PM and AE influenced the quantitative and temporal dynamics of C transfer to the higher trophic level. In particular, we noted that although various model formulations predicted similar trophic transfer of C, this was achieved on very different time scales. The inclusion of heterotrophic micro-zooplankton within food web models is a necessary step in accurate prediction of pelagic nutrient flux but requires physiological models carefully parameterised and tested in both steady state and transient conditions to ensure accurate simulation.
Exploring climate and anthropogenic impacts on marine ecosystems requires an understanding of how... more Exploring climate and anthropogenic impacts on marine ecosystems requires an understanding of how trophic components interact. However, integrative end-to-end ecosystem studies (experimental and/or modelling) are rare. Experimental investigations often concentrate on a particular group or individual species within a trophic level, while tropho-dynamic field studies typically employ either a bottom-up approach concentrating on the phytoplankton community or a top-down approach concentrating on the fish community. Likewise the emphasis within modelling studies is usually placed upon phytoplankton-dominated biogeochemistry or on aspects of fisheries regulation. In consequence the roles of zooplankton communities (protists and metazoans) linking phytoplankton and fish communities are typically under-represented if not (especially in fisheries models) ignored. Where represented in ecosystem models, zooplankton are usually incorporated in an extremely simplistic fashion, using empirical descriptions merging various interacting physiological functions governing zooplankton growth and development, and thence ignoring physiological feedback mechanisms. Here we demonstrate, within a modelled plankton food-web system, how trophic dynamics are sensitive to small changes in parameter values describing zooplankton vital rates and thus the importance of using appropriate zooplankton descriptors. Through a comprehensive review, we reveal the mismatch between empirical understanding and modelling activities identifying important issues that warrant further experimental and modelling investigation. These include: food selectivity, kinetics of prey consumption and interactions with assimilation and growth, form of voided material, mortality rates at different age-stages relative to prior nutrient history. In particular there is a need for dynamic data series in which predator and prey of known nutrient history are studied interacting under varied pH and temperature regimes.
Exploring climate and anthropogenic impacts on marine ecosystems requires an understanding of how... more Exploring climate and anthropogenic impacts on marine ecosystems requires an understanding of how trophic components interact. However, integrative end-to-end ecosystem studies (experimental and/or modelling) are rare. Experimental investigations often concentrate on a particular group or individual species within a trophic level, while tropho-dynamic field studies typically employ either a bottom-up approach concentrating on the phytoplankton community or a top-down approach concentrating on the fish community. Likewise the emphasis within modelling studies is usually placed upon phytoplankton-dominated biogeochemistry or on aspects of fisheries regulation. In consequence the roles of zooplankton communities (protists and metazoans) linking phytoplankton and fish communities are typically under-represented if not (especially in fisheries models) ignored. Where represented in ecosystem models, zooplankton are usually incorporated in an extremely simplistic fashion, using empirical descriptions merging various interacting physiological functions governing zooplankton growth and development, and thence ignoring physiological feedback mechanisms. Here we demonstrate, within a modelled plankton food-web system, how trophic dynamics are sensitive to small changes in parameter values describing zooplankton vital rates and thus the importance of using appropriate zooplankton descriptors. Through a comprehensive review, we reveal the mismatch between empirical understanding and modelling activities identifying important issues that warrant further experimental and modelling investigation. These include: food selectivity, kinetics of prey consumption and interactions with assimilation and growth, form of voided material, mortality rates at different age-stages relative to prior nutrient history. In particular there is a need for dynamic data series in which predator and prey of known nutrient history are studied interacting under varied pH and temperature regimes.
The classic portrayal of plankton is dominated by phytoplanktonic primary producers and zooplankt... more The classic portrayal of plankton is dominated by phytoplanktonic primary producers and zooplanktonic secondary producers. In reality, many if not most plankton traditionally labelled as phytoplankton or microzooplankton should be identified as mixotrophs, contributing to both primary and secondary production. Mixotrophic protists (i.e. single-celled eukaryotes that perform photosynthesis and graze on particles) do not represent a minor component of the plankton, as some form of inferior representatives of the past evolution of protists; they represent a major component of the extant protist plankton, and one which could become more dominant with climate change. The implications for this mistaken identification, of Chlorophyll a fluorescence in phytoplankton: relationship to photosynthesis and biomass. J. Plankton Res., 7, 715-731.
The traditional view of the planktonic food web describes consumption of inorganic nutrients by p... more The traditional view of the planktonic food web describes consumption of inorganic nutrients by photoautotrophic phytoplankton, which in turn supports zooplankton and ultimately higher trophic levels. Pathways centred on bacteria provide mechanisms for nutrient recycling. This structure lies at the foundation of most models used to explore biogeochemical cycling, functioning of the biological pump, and the impact of climate change on these processes. We suggest an alternative new paradigm, which sees the bulk of the base of this food web supported by protist plankton communities that are mixotrophic -combining phototrophy and phagotrophy within a single cell. The photoautotrophic eukaryotic plankton and their heterotrophic microzooplankton grazers dominate only during the developmental phases of ecosystems (e.g. spring bloom in temperate systems). With their flexible nutrition, mixotrophic protists dominate in more-mature systems (e.g. temperate summer, established eutrophic systems and oligotrophic systems); the more-stable water columns suggested under climate change may also be expected to favour these mixotrophs. We explore how such a predominantly mixotrophic structure af-fects microbial trophic dynamics and the biological pump. The mixotroph-dominated structure differs fundamentally in its flow of energy and nutrients, with a shortened and potentially more efficient chain from nutrient regeneration to primary production. Furthermore, mixotrophy enables a direct conduit for the support of primary production from bacterial production. We show how the exclusion of an explicit mixotrophic component in studies of the pelagic microbial communities leads to a failure to capture the true dynamics of the carbon flow. In order to prevent a misinterpretation of the full implications of climate change upon biogeochemical cycling and the functioning of the biological pump, we recommend inclusion of multi-nutrient mixotroph models within ecosystem studies.
The relationship between algae and their zooplanktonic predators typically involves consumption o... more The relationship between algae and their zooplanktonic predators typically involves consumption of nutrients by algae, grazing of the algae by zooplankton which in turn enhances predator biomass, controls algal growth and regenerates nutrients. Eutrophication raises nutrient levels, but does not simply increase normal predator-prey activity; rather, harmful algal bloom (HAB) events develop often with serious ecological and aesthetic implications. Generally, HAB species are outwardly poor competitors for nutrients, while their development of grazing deterrents during nutrient stress ostensibly occurs too late, after the nutrients have largely been consumed already by fast-growing non-HAB species. A new mechanism is presented to explain HAB dynamics under these circumstances. Using a multi-nutrient predator-prey model, it is demonstrated that these blooms can develop through the selfpropagating failure of normal predator-prey activity, resulting in the transfer of nutrients into HAB growth at the expense of competing algal species. Rate limitation of this transfer provides a continual level of nutrient stress that results in HAB species exhibiting grazing deterrents protecting them from top-down control. This process is self-stabilizing as long as nutrient demand exceeds supply, maintaining the unpalatable status of HABs; such events are most likely under eutrophic conditions with skewed nutrient ratios.
An ingestion-based prey selectivity function (IS) is described specifically to aid the simulation... more An ingestion-based prey selectivity function (IS) is described specifically to aid the simulation of zooplankton activity in circumstances in which selectivity varies, for example, as functions of prey palatability or abundance. In IS, the ultimate control of the rate of predation is not ...
... For example, mixotrophic experimentation typically focuses on either impacts of light or nutr... more ... For example, mixotrophic experimentation typically focuses on either impacts of light or nutrient limitation on grazing ( [Nygaard and Tobiesen, 1993] , [Stoecker et al., 1997] , [Li et al., 2000] , [Smalley et al., 2003] and [Adolf et al., 2006] ). ...
A mathematical model was developed to represent the dynamics of predation and assimilation of ing... more A mathematical model was developed to represent the dynamics of predation and assimilation of ingested material by heterotrophic marine micro-zooplankton. Predation rate was made a rectangular hyperbolic function of prey carbon (C) concentration modified to simulate the prey selectivity that these organisms have been observed experimentally to exhibit in response to prey nutritional quality. We chose prey N:C ratio as an index of selectivity and related both the maximum predation rate (PM) and the assimilation efficiency (AE) to changes in prey nitrogen:carbon (N:C). Changes in PM simulated the phenomena of "surge feeding" and "prey rejection". Changes in AE simulated sub-optimal assimilation of material from ingested prey, which in turn, served to decrease the ingestion rate on this material. The model was parameterised using laboratory data sets that followed ingestion of non-growing phytoplankton prey of different N:C ratio by a microflagellate predator. Investigative simulations in transient conditions, incorporating growing prey and hence with changing N:C ratio, indicated both PM and AE influenced the quantitative and temporal dynamics of C transfer to the higher trophic level. In particular, we noted that although various model formulations predicted similar trophic transfer of C, this was achieved on very different time scales. The inclusion of heterotrophic micro-zooplankton within food web models is a necessary step in accurate prediction of pelagic nutrient flux but requires physiological models carefully parameterised and tested in both steady state and transient conditions to ensure accurate simulation.
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