Predicted effects of behavioural movement and passive transport on individual growth and community size structure in marine ecosystems

We develop a spatially explicit, continuous, time-dependent model of size spectra to predict how the active movement and passive transport of individuals can influence individual growth and size spectra. Active movements are ‘prey-seeking’ behaviour, with individuals moving locally towards areas with high concentrations of favoured prey, and ‘predator-avoiding’ behaviour, with prey moving away from areas of high predator density. Passive transport represents the effects of turbulent mixing on small individuals. The model was used to explore the individual and community effects of these biotic and abiotic processes and their interactions, and to predict how energy from local sources of primary production is propagated through the food web. Prey-seeking and predator-avoiding behaviour led to systematic changes in the relative abundance of different-sized individuals in relation to centres of primary production and associated changes in size-spectra slopes. In areas of high phytoplankton abundance, community size-spectrum slopes were shallower and larger individuals were present, whereas in low production areas, slopes were steeper and size spectra truncated. Variations in size-spectra slopes were much reduced by spatial aggregation across the gradient of phytoplankton abundance, and regional slopes most closely approximated the slopes close to centres of high primary production. Individual growth was faster when closer to centres of production. The extent to which stability is apparent in size spectra depended on the scale of aggregation. This implied that sampling at relatively large space and time scales in relation to those of phytoplankton ‘blooms’ was necessary to compare emergent properties, such as size spectra, among regions or ecosystems. Further, at larger scales, responses to human impacts will be clearer and less likely to be masked by variability induced by smaller scale processes.