MICROPATCHINESS, TURBULENCE AND RECRUITMENT IN PLANKTON

A series of models are presented which examine the relative importance of microscale patchiness and turbulence to growth and recruitment in planktonic consumers. The analyses apply over scales from centimeters to meters (e.g. from copepods to fish larvae), and we assume food-limited conditions, since, otherwise, patchiness would not affect growth. A model of individual growth response to fluctuating food is developed which shows that growth is approximately exponential and is linearly related to food concentration. A random walk model reveals that the swimming process can be approximated as a simple diffusion term which, when included in the exponential growth model, leads to accumulation of consumers in high growth ( = prey) areas. This diffusive migration of consumers up the prey gradient is rapid; for example, half- maximum growth is reached in < 2 hours for fish larvae swimming in a 10 m patch of copepod nauplii. Enhancement of the net growth by this process is substantial; larval fish growth rates increase by 25% when 10 m prey patches appear at 5 hour intervals and by > 100% for steady patches. Physical turbulence, at intermediate levels, causes patch dissipation and reduced growth, whereas, at higher levels, it causes growth to be restored to original, low-turbulence, values due to increased encounter velocities. Variations in population growth rate due to turbulence and micropatchiness, even when small (< 10%), can cause large fluctuations in recruitment by affecting duration of pre-recruit life.