Responses of Mytilus edulis L. to varying food concentrations: testing EMMY, an ecophysiological model

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Abstract

In this paper a complex ecophysiological model is presented, which aims to simulate individual growth and reproduction of Mytilus edulis L. The model includes feedback mechanisms in the acquisition and metabolism of natural food sources and partitioning of carbon and nitrogen to the internal state variables: somatic tissue, storage, organic shell matrix, blood and gametes before and after spawning. The model was calibrated with statistical distributions for 38 parameters. The resulting a posteriori parameter sets were applied in a validation procedure. First inputs of one system were used to produce model outcomes with uncertainty bands in order to compare these with system observations not used for calibration. In a second validation step, the model was run with inputs of two different ecosystems. The results of this step were promising, but no acceptable growth could be predicted for the system with low seston and food concentrations.

Introduction

Owing to their high abundance and their huge impact on ecosystems, bivalve filter feeders play a dominant role in many estuarine and coastal waters (Gosling, 1992, Dame, 1996). Bivalves are used in many areas for water quality monitoring (Goldberg, 1975, Smaal and Widdows, 1994). Their commercial exploitation poses questions on carrying capacity and ecosystem management (Korringa, 1956, Smaal, 1991, Héral, 1993), resulting in a large body of knowledge on the physiological ecology of bivalve filter feeders. Part of this knowledge is incorporated in models, ranging from theoretically oriented models of individual filter feeders to submodels of ecosystem models. The main part of these models deals with mussels, oysters or a functional group `filter feeders'.

In the ecosystem model of the Oosterschelde estuary (SMOES) the activity of mussels was explicitly modeled (Klepper, 1989Klepper et al., 1994Scholten and Van der Tol, 1994), based on the energy budget models of Bayne (1976a)and Verhagen (1983). In these models, the scope for growth is calculated as the net result of energy gain by feeding, and energy loss by maintenance (respiration and excretion) and reproduction. Brylinsky and Sephton (1991)have developed a mussel model in which a distinction is made between somatic, storage and gonad tissue. Also Ross and Nisbet (1990)and Van Haren and Kooijman (1993), based on Kooijman (1986), have presented dynamic budget models of the mussel with various compartments (state variables) within the animal. This approach is more appropriate to model growth and reproduction in a dynamically varying environment, but in these models the physiological response to a variable food concentration is oversimplified. Ross and Nisbet (1990)assume that the energy uptake and partitioning to the various tissues depend mainly on body size, ingestion is not selective, absorption efficiency is constant, and respiration has no relation to the animal's activity. Van Haren and Kooijman (1993)lumped the rates of ingestion and respiration. In contrast to Ross and Nisbet (1990), Van Haren and Kooijman (1993)worked with temperature-dependent physiological functions in their model. They also assumed 100% ingestion of filtered organic material and pseudofaeces without organic content. These assumptions ignore the large body of knowledge with respect to physiological energetics, as reviewed by Bayne and Newell (1983), Jørgensen (1990)and Hawkins and Bayne (1992).

In this paper, a dynamic carbon and nitrogen budget model is presented of a single subtidal mussel, based on experimental data, on physiological rates and on expert knowledge. The temporal resolution is 1 day, so no dynamics within a tidal cycle is relevant. The model EMMY (Ecophysiological Model of Mytilus edulis) is validated for a wide range of environmental realistic conditions. The model has feedback loops for food acquisition and absorption, maintenance requirements and gonad development and resorption.

The model aims (i) to integrate ecophysiological knowledge (especially the animal's response to significantly different environmental conditions including food availability), (ii) to identify knowledge gaps, and (iii) to be a management tool in eutrophication and carrying capacity studies. The model attempts to simulate growth and reproduction over the lifetime of a mussel under seasonal variation in temperatures, food availability and metabolic demands. The model translates insufficient or imprecise knowledge as parameter uncertainty and choices of alternative process descriptions. EMMY is restricted to mussel activity and all population dynamic aspects are disregarded.

Section snippets

Modeling procedure

EMMY (Ecophysiological Model of Mytilus edulis) was developed according to the approach proposed by Scholten and Udink ten Cate, 1995, Scholten and Udink ten Cate, 1996. In this paper only the major steps will be summarized. In the first step the available knowledge (experimental results, hypotheses and theories from literature and existing models) is used to construct a conceptual model (Fig. 1). The conceptual model corresponds with the structure of EMMY. Those entities are chosen as state

Model concept

The state variables of EMMY are listed in Table 1. All compartments have state variables for C and N, except the storage compartment, which consists of glycogen C (StorC). The compartment `blood' (BloodC, BloodN) is used for maintenance, mucus production and partitioning to the other tissues. If the blood compartment is exhausted, more energy is withdrawn from the storage (StorC), gonads/gametes (GametC, GametN) or somatic tissue (SomC, SomN), in that order. Organic shell weight (OSW) is used

Validation procedure

The model concept and its implementation was verified and validated in many updating loops. EMMY was calibrated according to the method of Scholten and Van der Tol (1994), using Oosterschelde inputs (see Table 2) and expert knowledge in the model outputs. Based on our experience during a sensitivity analysis, we have chosen 38 parameters (Table 3) which were used in calibration, while the other parameters (Table 4) were kept constant. Of these calibration parameters a priori uniform

Validation Oosterschelde

The results of the validation of EMMY using Oosterschelde inputs for a 5-year period and comparing the model outcomes in terms of uncertainty bands (minimum and maximum values of the simulations, run with the a posteriori parameter sets after calibration) are acceptable. Measured values match rather well with the model outcomes for total dry weight (Wtot, Fig. 2), shell length (LShell, not shown) and total wet weight (TWW, not shown). More detailed results are discussed in Smaal and Scholten

Discussion

Integration of existing knowledge of the mussel's ecophysiology, identification of knowledge gaps, and prediction of growth and reproduction under various ambient food conditions requires a detailed mechanistic model, which includes information on the accuracy of the knowledge, i.e. uncertainties. The EMMY model is based on existing knowledge on ecophysiological processes, including feed-back mechanisms. Some elements of this knowledge are used in EMMY, which is discussed in detail in Smaal and

Acknowledgements

This study was partially funded by EU Concerted Action AIR3-CT94-2219 `Trophic capacity of coastal zones for rearing oysters, mussels and cockles'.

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    Present address: Netherlands Institute for Fisheries Research, P.O. Box 77, 4400 AB Yerseke, The Netherlands.

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