INDIVIDUAL-BASED MODELING OF THE POPULATION-DYNAMICS OF METRIDIA-LUCENS IN THE NORTH-ATLANTIC

An individual-based population dynamics model (IBM) was used to examin e the effect of different behavioral, bioenergetic, and physiological assumptions on individual growth and development of the copepod, Metri dia lucens, in the North Atlantic Ocean. Both intrinsic (nutritional c ondition, feeding history, size) and extrinsic (temperature, food reso urces) factors that might determine individual growth and development rates were examined. An advantage of an IBM is that it allows for inte r-individual variability, and can thereby provide an indication of the range of responses that might arise from natural variation in environ mental conditions. The model is a refinement of an earlier model that successfully reproduced the observed stage structure and abundance of Metridia pacifica in the sub-arctic Pacific (Batchelder and Miller, 19 89). Consequently, parameters for the ingestion and metabolic function s were set initially to those found appropriate for M. pacifica from p revious model studies. Extrinsic forcing variables used to drive the p opulation model were depth-specific temperature and chlorophyll a conc entration. The model was run in two modes: chronological and individua l. In the former mode, the model used measured temperature and chlorop hyll a data to reproduce the life-history timing (phenology) and seaso nal stage-structure of a M. lucens population measured in the early 19 70s from Ocean Weather Station India in the North Atlantic. The indivi dual mode was used to examine variation in growth caused by inter-indi vidual variability in short-medium term starvation and feeding success . Ingestion, growth, and development were sensitive to variations in f ood resources. Factors that increased consumption rates, such as more effective searching for high chlorophyll layers, or recent starvation resulting in a ''hunger response'' in the functional response relation , led to markedly faster growth and development rates. Model simulatio ns indicate that inter-individual variability in growth dynamics decre ased for copepods capable of plasticity in the physiological hunger re sponse or more effective food-searching behavior. Such searching behav ior accentuated already existing inter-annual and intra-annual differe nces in individual growth dynamics forced by temperature and food. Con versely, hunger acclimation reduced intraannual (seasonal) variability and reduced, although only slightly, inter-annual variability in grow th dynamics. These model results highlight the importance of understan ding how copepods respond to environmental conditions. Two methods beh avioral modification and physiological acclimation-by which copepods m ight ameliorate low food conditions, lead to different responses to te mporal and spatial variability of resources. (C) 1995 International Co uncil for the Exploration of the Sea