The motion of large bodies of water in can be a very complicated subject.
Currents can proceed in varying directions and with varying speeds and
frequencies, and are largely driven by the action of the wind. One of the more confusing
dynamics occurs when you look at the surface of a lake and observe rows of
foam and
biofilm which are oriented
perpendicularly to the direction of the
waves. These rows, called either
windrows or slicks, are created by Langmuir spirals
* (named after
Irving Langmuir, the physicist who first described the phenomenon).
The spirals are created by the interaction between surface waves and wind-driven drift currents. Each spiral, normally only 1-5 meters in diameter (their diameter is normally equal to the depth of the thermocline), proceeds in the direction of the prevailing wind across the surface of the water, but rotating in a plane perpendicular to their direction (imagine a series of long corkscrews where the long axis is parallel to the wind). Each spiral rotates in a direction opposite that of the two neighbouring spirals, meaning you have a series of clockwise and counter-clockwise rotating spirals. The oppositional rotation of these spirals creates regularly spaced local zones of upwelling and downwelling, which is what creates the peaks and troughs of the windrows.
The downwelling flow is often between 2 and 8 cm s-1, much faster than the swimming capabilities of most zooplankton and phytoplankton. Thus, in the Langmuir spirals the biological organisms find themselves fairly thoroughly mixed. This mixing action creates zones of rich phytoplankton and bacteria concentrations. Many larger zooplankters (such as Daphnia, will migrate up to these zones and feed opportunistically. Following these zooplankters into these zones will be the planktivorous fish (such as the cyprinids -- minnows), and these in turn will be targeted by pelagic piscivores. These zones are, therefore, often good places for anglers to ply their trade.
* These are also called Langmuir cells and the dynamic is called Langmuir circulation.