Plankton Behaviour Sinking Buoyancy And Vertical Migration

Cell size has a significant impact on the ability of phytoplankton cells to maintain their position at depths with adequate light and nutrients to sustain growth. In general, an increase in cell size results in an increase in sinking rate - with dead cells sinking at faster rates than live cells. Large phyto-plankton cells (such as diatoms) are disadvantaged by being highly susceptible to sinking, and may require strong vertical mixing (for example, caused by upwelling or strong winds) to maintain their position in surface waters.

Sinking of cells can be reduced by morphological structures that increase cell, or colony, resistance to sinking. The flagella of many nanoflagellates serve, in part, to overcome sinking. Adaptations of large and heavy cells (large diatoms and dinoflagellates) to reduce sinking, and to maintain near neutral buoyancy and vertical position in the euphotic zone, include chain formation and cell extensions that provide a high surface area: volume ratio. Cell extensions can be highly numerous and include protuberances, spines, horns, wings and hair-like structures. They increase frictional drag and also increase the effective size of phytoplankton cells, which makes them more difficult for zooplankton grazers to capture and ingest. Another advantage of cell extensions - particularly diatom spines - is that they can house large numbers of chloroplasts and thus increase the ability of cells to harvest light for photosynthesis.

Cell density, and thus rate of sinking, is also affected by the composition of cells. Silica-laden diatoms are particularly heavy. Mechanisms to control cell density, and thus location within the water column, may include production of gas vacuoles and the accumulation of fats and oils, which are lighter than water. Cell aging and nutritional state of phytoplankton cells are physiological conditions that affect cell density. Post-bloom nutrient-starved diatoms tend to sink significantly faster than nutrient-rich diatoms (Tilman and Kilham 1976). This effect is frequently demonstrated in temperate and polar waters, where mass sinking of phytoplankton blooms occurs following nutrient exhaustion. A large proportion of bloom material may settle to the bottom as diatom flocs or aggregates (>0.5 mm) composed of algal cells, zooplankton remains, faecal pellets and other forms of detritus. These highly visible settling flocs are commonly referred to as 'marine snow'.

Zooplankton features that increase drag, and thus reduce sinking, include long, thin or flattened body shapes, and projections such as hairs, long spines and wings. Buoyancy may also be assisted by small droplets of oil. Many planktonic animals can swim reasonably well, or are able to control their position by selecting different depths and currents, or by adjusting buoyancy. Many species of crustacean zooplankton - especially the adult forms - are strong swimmers and conduct diel vertical migrations through the water column (Figure 2.5). This involves rising to surface waters at dusk and grazing heavily on phytoplankton cells throughout the night, before descending to deeper waters well before dawn (although some interesting cases of reverse migrations are known: that is, rising up in the day, and dropping back down at night). The distance travelled during diel vertical migration can range from a very short distance (less than 2 metres in coastal lagoons) to hundreds of metres up and down in 24 hours in oceanic waters).

Diel migratory behaviour is triggered by changes in light intensity, and is largely an adaptation to avoid visually feeding predators, particularly fish. Migratory patterns can be variable, and are known to differ with the sex and age of the species, habitat type and season (van Gool and Ringelberg 1998). Many gelatinous plankton (such as jellyfish) and larval crustaceans (such as prawns) exhibit tidal-driven vertical migrations into estuaries. They move

Figure 2.5 Representative catches of zooplankton during the day (two on the left, with <1 mm displacement volume), and during the night (two on the right, with 22 and 10 mm displacement volume). In some years there may be no difference between day and night zooplankton abundance.

up into the flood tide waters - especially at night - and are transported into the estuary, and move lower in the water column during ebb tides to avoid being carried out. Such migrations are entrained into the circadian rhythm of many organisms, such that some diel and tidal activities continue to be observed even after the organisms are removed from their natural environment (for example, when maintained in a laboratory).

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