Diel patterns

The study of SSLs together with data from deep-level net samples indicate that throughout the oceans, in both deep and shallow water, a numerous and varied assortment of animals perform vertical migrations with a diel rhythm. There are differences in the depths through which different species move, the speeds at which they ascend and descend, and the precise times at which they make their movements, and the same species may behave differently in different areas and at different times, but there is none the less a remarkable consistency in the general behaviour pattern. Most marine zooplanktonts make a single daily ascent and a single descent. During daylight, each species appears to collect near a particular level. Shortly before sunset they commence an ascent which continues throughout the twilight period. During total darkness the population tends to disperse, but shortly before dawn it again congregates near the surface and then makes a fairly rapid descent until, about an hour after sunrise, the daytime level is reached (Figure 4.11). The movements are therefore slightly asymmetrical with respect to midnight.

As well as this common pattern, two other patterns have been recognized. In twilight migration, there are two ascents and descents. The animals rise at sunset to their preferred level, but during the night they sink once again though not necessarily to their original depth. This is called the midnight sink. At sunrise the animals rise again before later decending to their normal daytime depth. A few animals exhibit reverse migration, ascending during the day and descending at night.

Regulating factors

There are many features of the migrations which are not understood, but it seems certain that changes in illumination play a dominant role in regulating the activity. No other factors are known to which the movements can be so exactly related. Although the relative durations of night and day vary with latitude and season,

Noon afternoon Dusk Midnight Dawn morning Noon

Noon afternoon Dusk Midnight Dawn morning Noon

Figure 4.11 Generalized diagram illustrating diurnal changes of vertical distribution of many epipelagic zooplanktonts. The dotted lines show levels of approximately equal illumination indicated in arbitrary units. Logarithmic decrease of illumination accounts for the kite-shaped pattern of distribution in daytime.

Figure 4.11 Generalized diagram illustrating diurnal changes of vertical distribution of many epipelagic zooplanktonts. The dotted lines show levels of approximately equal illumination indicated in arbitrary units. Logarithmic decrease of illumination accounts for the kite-shaped pattern of distribution in daytime.

the migrations are always closely synchronized with dawn and dusk. In high latitudes they occur during spring and autumn when there are daily alternations of light and dark, but the migrations are not observed when darkness or daylight persists throughout the 24 hours.

Many of the organisms which make these diurnal movements must be extremely sensitive to light. During the day their level rises when clouds cross the sun. At night, moonlight has a noticeable effect, some creatures remaining further from the surface in bright moonlight than on dark nights, while others appear to be attracted by moonlight and reach the surface in greatest numbers at full moon on clear nights.

The majority of migrating forms are fairly active swimmers which regulate their depth by the rate and direction of swimming. In some cases, strong light may have an inhibitory effect on swimming, preventing these animals from reaching the surface during daytime even though they make no directional response to light. However, the movements of many zooplanktonts show an orientation to the direction of light, moving away from a source of strong illumination. A creature which is both negatively phototactic (moving away from light) and negatively geotactic (moving against gravity) presumably swims towards the surface in darkness, but in daytime moves down to the level where its responses to light and gravity are in balance, moving upwards or downwards as illumination changes. Some species orientate to the plane of polarization of light, which changes with the angle of the sun. This would cause the population to rise and fall, though the migration would not be a simple ascent and descent. The movements may be partly regulated by an intrinsic 'physiological clock' operating in phase with changes of illumination. For instance, it has been shown that various copepods, if removed from the sea and kept in total darkness in a tank, continue for some days to exhibit periodic changes of activity having a 24-hour rhythm which results in diel alterations of their level (Palmer, 1974).

These mechanisms perhaps explain the movements of some organisms but do not account for the tendency of many species to disperse during total darkness. It seems as if many planktonts move towards a preferred 'optimum' intensity or spectral composition of illumination which differs according to the species. During daytime the animals tend to assemble at whatever level they find their preferred illumination, and move up or down as this level changes with rising and setting of the sun. During darkness, when there is no longer sufficient illumination to cause the population to concentrate at one level, they scatter. Experiments in which planktonic animals have been enclosed in long tubes exposed to an illumination gradient have demonstrated that some species do collect within particular ranges of light intensity, the optimum differing for different species. Blaxter (1973) observed the distribution of herring and plaice larvae in a vertical tube illuminated from above and found that, in strong illumination, changes of light intensity had little effect on the larvae; but vertical migrations took place at critical levels of illumination equivalent to intensities at late dusk or early dawn. However, the idea that organisms are following preferred light intensities is often a better explanation of their ascent than of their descent, which frequently begins before the sky appreciably lightens. There are some mesopelagic species which start their descent at midnight and have reached their daytime level well before sunrise.

To study the effects of illumination on speed and direction of swimming, Hardy and Bainbridge (1954) devised an apparatus which has been termed a 'plankton wheel'. It consisted of a circular transparent tube, 4 ft in diameter, mounted vertically by spokes on a central axle on which it could be rotated. The tube was filled with seawater, and small animals could be inserted. In effect, the animals were enclosed within an endless water column. By rotating the wheel one way or the other as the enclosed animals swam upwards or downwards, they could be kept at a fixed position relative to the observer. On the inside wall of the tube were mounted a series of small valves, automatically operated by floats and weights as the wheel was turned. These valves ensured that the water turned with the wheel, but did not interfere with the animals in the observation position. The apparatus was placed in a small glasshouse provided with movable screens to give variations in illumination. Movements of the wheel were recorded on a rotating smoked drum.

With this instrument it was possible to measure the speed and duration of upward and downward movement of small animals and to record the pattern of their activities under controlled conditions of lighting, although the apparatus did not provide the gradients of illumination, temperature or pressure that organisms encounter in the sea. Among the animals studied were copepods, euphausids, the polychaete Tomopteris, the arrow worm Sagitta elegans, and nauplii of Balanus and Calanus. The experiments confirmed that the swimming capabilities of these animals are more than adequate to account for the extent of the migrations inferred from net hauls. Results were incomplete because of the difficulty of setting up the experiments, and the time needed for recording, but many of the animals tested showed a positive movement downwards in strong light. Their responses varied at different times of day, but during the evening in dim illumination the animals moved upwards towards the light. If kept in total darkness during daytime, they did not consistently move upwards, indicating that geotaxis was not automatically operating in the absence of strong light. The results generally suggested a directional movement towards an optimum level of illumination of low intensity.

Other factors have been suggested as contributing causes of vertical migration; for example, temperature changes in the surface layers of water may have some effect. At night the surface layers undergo slight cooling, and this might permit some of the inhabitants from colder water below to approach the surface, while in daytime the warmer surface temperature would discourage their rising so far.

It has been observed in some animals that increasing temperature reverses the direction of their phototaxes. Apart from physiological and behavioural effects, the physical effects of changes of temperature upon water density and viscosity must have some influence on buoyancy. Surface cooling at night might be expected to cause passively floating forms with neutral buoyancy to float at slightly higher levels than during the day, and the effect might be more marked on small upward-swimming creatures. It has also been suggested that the weight gained by organisms during their nightly feeding in the abundant food supplies of the surface layers may cause them to sink to lower levels until weight is subsequently lost through respiration, excretion and egestion. However, these processes cannot be closely correlated with either the extent or the periodicity of the migrations. The distances covered by most species are too great to be explained simply as passive rising or sinking due to alterations of buoyancy, but must involve active swimming upwards and downwards at considerable speed. Nor does the timing of the migrations correspond closely with the change of temperature of the surface water, which falls gradually throughout the evening and night until shortly before dawn, whereas the majority of vertically-migrating animals reach the summit of their ascent soon after sunset.

In the course of their upward and downward movements, organisms traverse a pressure gradient, and often a temperature gradient, and these must play some part in regulating the extent of the migrations. Increase of pressure causes some zooplanktonts to increase their upward swimming rate, and this presumably limits their depth of descent. Where there is a sharp thermocline between the surface and deeper levels, some migrating forms do not pass through the discontinuity layer. The thermocline then acts as a boundary between two groups of organisms, those of the warmer surface layers which descend during daylight only as far as the thermocline, but not through it, and those of deeper levels to which the thermocline is the limit of ascent during darkness.

It has been suggested that 'exclusion' (see page 189) may have some connection with diurnal migration. If during photosynthesis phytoplankton liberates into the water metabolites which are distasteful or toxic to animals, the zooplankton would be expected to remain below the photosynthetic zone throughout the hours of daylight. Only during darkness, when photosynthesis ceases, would the surface water become suitable for the entry of animals. If the phytoplankton should be very abundant, the exclusion effect might persist throughout the night, upsetting the rhythm of the migration. However, it is uncertain to what extent exclusion operates in natural conditions, nor can this explanation account for the timing of the main movement downwards, which for the majority of animals begins before dawn. Some zooplanktonts become more photonegative when well fed on phytoplankton and so would be expected to move downwards out of phytoplankton patches in daytime without any external metabolites being involved.

There are many anomalies. The migratory behaviour sometimes becomes erratic or ceases completely. Animals which usually seem consistently to move up or down with changes of illumination may occasionally swarm at the surface in bright sunshine. The extent of the movements sometimes varies seasonally; for example, around the British Isles they are of more general occurrence during the summer months, and many planktonts cease this behaviour during winter when they remain at a deep level. Often the migrations are made only by particular stages of the life history. In some cases the migrations are made by only a proportion of the population, resulting in a greater spread of the population through the water column at night rather than a general change of level.

In conclusion, light appears to be the main factor initiating and regulating diel migrations. However, other factors including temperature, pressure and hunger probably also play a part. Different organisms may respond to different stimuli.

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Renewable Energy 101

Renewable Energy 101

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