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Local adaptations in body size under global change: zooplankton community more important than temperature or food abundance in model of a zooplanktivorous fish

Gabriella Ljungström*, Marion Claireaux, Øyvind Fiksen, Christian Jørgensen

*Corresponding author:

ABSTRACT: One of the most well-studied biogeographic patterns is increasing body size with latitude, and recent body size declines in marine and terrestrial organisms have received growing attention. Spatial and temporal variation in temperature is the generally invoked driver but food abundance and quality are also emphasized. However, the underlying mechanisms are not clear and the actual cause is likely to differ both within and among species. Here we focus our attention on drivers of body size in planktivorous fish that forage through vision. This group of organisms plays a central role in marine ecosystems by linking the energy flow from lower to higher trophic levels. Using a model that incorporates explicit mechanisms for vision-based feeding and physiology, we investigate the influence on optimal body size from several biotic (prey size, prey energy content, and prey biomass concentration) and abiotic factors (temperature, latitude, and water clarity) known to affect foraging rates and bioenergetics. We found prey accessibility to be the most influential factor for body size, determined primarily by prey size but also by water clarity, imposing visual constraints on prey encounters and thereby limiting feeding rates. Hence, for planktivores that forage through vision, an altered composition of the prey field could have important implications for body size, and for the energy available for reproduction and other fitness-related tasks. Understanding the complicated effects of global change on zooplankton communities is thus crucial for predicting impacts on planktivorous fish, as well as consequences for energy flows and body sizes in marine systems.