Modelling the physiological performance of Daphnia (Crustacea: Cladocera) under stress

Abstract

A physiological model of an individual Daphnia was developed starting from first principles. The model was based on general life-cycle biology from which general allocation rules were developed. A simple feeding behaviour involving sensitivity to the presence of toxicants and no active food selection was implemented. The model addresses growth, survival, and reproduction under different environmental conditions, e.g. differing food levels and/or toxicant concentrations. Toxic effects are described on the basis of simple interactions with feeding and metabolic processes. Model parameterisation was performed using available literature data together with data generated within this thesis. Values for almost all parameters used could determined from experimental data. Furthermore, parameters which could not be directly or indirectly estimated revealed themselves as insensitive. Experimental work using pulsed food deprivation experiments on adolescent and adult instars was carried out to generate precise data on moulting, while generating information to test the allocation rules used in the model. Samples of individuals were collected at critical points during the experiments to judge if the biochemical composition of the samples was a good indicator of individual physiological state. A full sensitivity analysis was carried out with the model which it was validated using data from differing food conditions and from different non-lethal cadmium concentrations. During the validation process, the model revealed itself to be robust and able to simulate life-history performance under a range of environmental conditions. The model is well-posed and robust, capturing the basic and essential properties of an individual and addressing its development from egg to adult. The model behavior is consistent at both low and high food levels, whilst addressing the description of toxic effects in a realistic way. The model can be used as a valuable tool for teaching and experiment designing, by generating results for long term exposures to different environmental conditions within a short period of time.