Small-scale turbulence, marine snow formation, and planktivorous feeding

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Abstract

This paper examines how turbulence influences two very basic properties of planktonic ecosystems, namely trophic interactions and vertical flux of particulate material. It starts with a simple account of classical particle encounter theory which forms the basis of the substance of 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 motility will be presented. The classical (coagulation) theory is then applied to marine snow formation in the ocean: colliding suspended particles may stick together and form mm-cm sized aggregates (marine snow). These aggregates are believed to account for the vertical flux of matter 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 stickiness may be measured, how coagulation determines the sedimentation of particulate matter in the ocean, and how it may control the population dynamics of phytoplankton. Subsequently the collision equations are used to describe how planktivorous predators encounter prey in turbulent environments, and the equations are modified to take predator and prey behaviour into account. Simple equations that describe prey encounter rates for cruising predators, suspension feeders, ambush feeders, and pause-travel predators in calm and turbulent water are derived. The influence of fluid motion on post-encounter prey capture (pursuit success) is examined. Experimental results on various copepod and larval fish predators will be used to illustrate the theory. Finally, the significance of size and behaviour is discussed. It is shown that turbulence is potentially very important for prey encounter in mm-cm sized planktonic predators, while it is unimportant for most larger and smaller ones.
Original languageEnglish
JournalScientia Marina
Volume61
Issue numberSuppl.
Pages (from-to)141-158
ISSN0214-8358
Publication statusPublished - 1997

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