Working Group III - Structural Aspects of Secondary Production Models

Hofmann (chair), Durbin, Landry, Moloney, Nival, Taylor

The working group identified four issues related to the structural aspects of models that should be part of U.S. GLOBEC modeling activities.

Realistic Model of a Key Species--The development of a detailed model that realistically parameterizes and describes processes for a single zooplankton species should be undertaken. Such a model will require approaches for partitioning the organism into basic biochemical components (e.g., lipids), which would allow differentiation of energy within the animal (e.g., separate reproductive and somatic tissue). Parameterization of an animal in this manner will require improved information on the biochemistry (especially of growth and lipid storage) of marine zooplankton and its effect on behavioral and physiological ecology. Also, measurements that will allow parameterization of environmental control on animal metabolism, through temperature variability for example, are needed. Thus, development of this type of individual based model will require considerable effort from experimentalists as well as modelers. Specific recommendations are to: 1) use common units for measurements so that they can be easily incorporated into models; 2) refine temperature relationships for organism metabolic responses for modeling and experimental studies; and, 3) to allow the model for a specific organism to have a mechanistic basis that incorporates variability within the species.

Population Dynamics Model--The existence of population models would allow comparisons between different species, processes underlying co-existence of species, investigation of trade-offs and balances that different species make, and the role of nonlinear processes in the control of population dynamics. Initially these models should focus on a small set of species and the models should be structured to track animal growth. It is also important for population models to include natural population variability that arises through genetic variations. This introduces stochastic variations into population models, which has not traditionally been done, but may be important.

Type of Model--Lagrangian and Eulerian models provide frameworks for investigating questions relating to the growth and development of marine zooplankton populations. Each approach has strengths that can be exploited to provide insight as to the role of circulation and biological factors in regulating zooplankton population structure and secondary production. At present, most of the models for secondary production provide simulations of secondary production or animal concentration on a fixed grid (i.e., an Eulerian approach). However, circulation models are becoming more generally available and these can be used in a Lagrangian mode to consider basic questions relating to animal distribution and dispersion. This is a promising approach for combining circulation and individual based models.

The use of stochastic modeling, in which a range of outcomes is allowed, e.g., encounter models, should be encouraged. This will allow for uncertainty in model solutions and in sensitivity studies performed on the model parameters. This is in contrast to what can be obtained with deterministic models, which give a single solution for a set of parameters. Additionally, for some applications, it may be possible to recast a detailed model in a simpler form by using stochastic input functions. This will give a range of possible outcomes that may more accurately reflect the possibilities in nature than the single outcome from the deterministic model.

Importance of Lower Trophic Levels--The characteristics of the lower trophic levels (the prey field) are important inputs to models of secondary (animal) production. The structure of the food source (e.g., phytoplankton community composition), the quality of the food (carbon to nitrogen ratios) and the quantity of the food all regulate secondary production. Moreover, microzooplankton may be as important as phytoplankton as prey for mesozooplankton. This has implications for trophic efficiency of the planktonic system, and for response (turnover) times to environmental perturbations. Generally, the lower trophic levels may respond faster to environmental perturbations, thereby providing a filter through which these variations are transferred to predators. However, the longer time response of marine mesozooplankton can make it difficult to include detailed models of primary production (or microzooplankton) in a model of secondary production. Therefore, modeling effort should be directed at developing parameterizations of feeding responses that can incorporate many of these effects and account for flexibility in feeding environments. Moreover, microbial processes, seasonal changes in phytoplankton community composition and changes in environmental conditions all contribute to regulation of the food resources that are available to marine zooplankton. Hence, it is important to distinguish what would be needed to construct true ecosystem-level models as opposed to models in which a food supply is simply specified.

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