The movement of water will have a great influence on organisms that largely drift. However, some directional control can be obtained by varying their vertical position in water columns that have vertical stratification of current speed and direction. Currents can influence the trajectories of plankton and can influence concentration. Some eddies are called 'phase eddies' that only occur at one phase of the tide (e.g. on ebb tides behind a reef) while other eddies (and larger gyres) persist over long time periods in mainstream currents. In addition to eddies, currents can interact with abrupt topography such as reefs and seamounts to alter the supply of plankton through upwelling.
Plankton often concentrate in convergences. In tropical waters it is common to see blue green algae (Box 14.2), coral spawn, jellyfishes, spawning appen-dicularians and other plankters in convergence zones generated by physical phenomena that include: tidal and thermal fronts, tidal jets, the edge of eddies, windrows and internal waves. As a result, therefore, convergences are often sites of intense biological activity and nekton (e.g. planktivorous fishes, piscivores) and birds are often attracted to the abundant food in these areas.
Oceanography can also influence the distribution of plankton with depth. Although the water column on the shelf of the GBR is often well mixed (see Chapter 4), thermoclines and haloclines can stratify the water column into different water masses. For example, at the edge of the continental shelf upwelling events are common and 10 m to 20 m deep 'wedges' of cool water move along the bottom over the shelf and under lighter warmer shelf waters. Different planktonic assemblages may be found above and below the thermocline in these water masses. In addition, some plankton will concentrate in and around the thermo-cline. Freshwater plumes will also cause vertical stratification and variation in abundance of plankton with depth.
Interactions between plankton and oceanography are studied intensively, but the behaviour of plankton also has a role. Some plankton will meet in convergences, but will maintain their position for the purposes of reproduction (e.g. Appendicularia and jellyfishes). The larvae of prawns, crabs and fishes regulate depth at different stages of their development and this has major consequences for transport. For example west Australian lobster larvae (Palinurus ornatis) are transported into the Indian Ocean at the surface and migrate to deeper water later in development for transport back toward the Australian coastline.
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