Individual foraging behaviors and population distributions of a planktonic predator aggregating to phytoplankton thin layers

Resource distributions in the ocean are heterogeneous in time and space. Theory predicts planktonic predators may exploit these resource patches by modifying their movements in response to mechanical or chemical stimuli. In the laboratory, we used the protistan predator Oxyrrhis marina to simultaneously quantify changes in predator population distributions on scales of centimeters and hours and predators' individual three-dimensional swimming behaviors on scales of micrometers and seconds. Movements of O. marina in a 0.3-m column were monitored for several hours before and after introducing a 5-mm layer of either Isochrysis galbana prey cells or cell-free L galbana filtrate. Within both types of layers, significant increases in turning rates and decreases in vertical velocities were observed. O. marina swimming speed increased significantly in response to intact I. galbana cells, but not in response to L galbana exudates. Changes in predators' microscopic movements were concurrent with rapid (minutes) and sustained (hours) increases in relative predator abundance within layers. After 4 h, predator abundance inside the thin layers was up to 20 times higher than before introduction of prey. Estimates of realized growth rates for predator populations with aggregate behaviors were an order of magnitude faster than estimates for hypothetical, nonaggregating predators. The observed foraging behaviors of O. marina, and by implication other planktonic predators, increase effective prey availability to the predators. Modulation of individual-level behaviors can result in significant changes in community-level characteristics, including population distributions, growth, and ingestion rates.