SMALL-SCALE TURBULENCE, MARINE SNOW FORMATION, AND PLANKTIVOROUS FEEDING

This paper examines how turbulence influences two very basic propertie s of planktonic ecosystems, namely trophic interactions and vertical f lux of particulate material. It starts with a simple account of classi cal particle encounter theory which forms the basis of the substance o f both problems. Turbulent fluid motion will bring suspended particles to collide, and the basic equations describing the collision rate as a function of dissipation rate and particle size, concentration and mo tility will be presented. The classical (coagulation) theory is then a pplied to marine snow formation in the ocean: colliding suspended part icles may stick together and form mm-cm sized aggregates (marine snow) . These aggregates are believed to account for the vertical flux of ma tter in the ocean. Aggregation of microscopic phytoplankton cells is a special case. Examples from laboratory and field experiments are used to demonstrate how phytoplankton cells may coagulate, how their stick iness may be measured, how coagulation determines the sedimentation of particulate matter in the ocean, and how it may control the populatio n dynamics of phytoplankton. Subsequently the collision equations are used to describe how planktivorous predators encounter prey in turbule nt environments, and the equations are modified to take predator and p rey behaviour into account. Simple equations that describe prey encoun ter rates for cruising predators, suspension feeders, ambush feeders, and pause-travel predators in calm and turbulent water are derived. Th e influence of fluid motion on post-encounter prey capture (pursuit su ccess) is examined. Experimental results on various copepod and larval fish predators will be used to illustrate the theory. Finally, the si gnificance of size and behaviour is discussed. It is shown that turbul ence is potentially very important for prey encounter in mm-cm sized p lanktonic predators, while it is unimportant for most larger and small er ones.