The grazing rate

Although the interactions between plant and animal populations are difficult to elucidate, the grazing rate of the herbivorous zooplankton is certainly one of the factors which regulates the size of the standing stock of phytoplankton, and therefore influences the production rate. The quantity of epipelagic zooplankton generally correlates more closely with the quantity of plant nutrients in the surface layer than with the size of stock of phytoplankton, indicating how greatly grazing reduces the number of plants in fertile water. In the long term, the primary productivity of an area must determine the size of the animal population it supports, but in the short term there are often wide, and sometimes rapid, changes in both numbers and composition of populations due to a variety of causes. Interactions between species often involve a time lag, and there is consequently a tendency for numbers to fluctuate about mean levels. Although some natural populations show homoeostatic mechanisms which control reproduction within limits which do not exhaust the food resources of their environment, there is obviously a general trend for animal numbers to increase as long as there is sufficient food until consumption diminishes the food supply. Food shortage may then cause a decline in the feeding population, and eventually the reduction in food consumption may allow the food supply again to increase, and these oscillations may involve many links of the food web. If the inorganic environment were to remain uniform the system might in due course settle to a steady state, but in nature the physical conditions fluctuate and the equilibrium is forever being disturbed.

A dominant cause of short-term fluctuations in the plankton of middle and high latitudes is seasonal variation of climate which influences both the production rate and the sequence of species which predominate. These changes are discussed later (see Section 5.4) but in the present connection we should note that the sharp reduction in numbers of diatoms which follows their period of rapid multiplication in the spring occurs before nitrate and phosphate are fully exhausted, but coincides with the growth in quantity of zooplankton. There can be little doubt that the increasing rate of grazing is one cause of the decline of the standing stock of diatoms.

A striking feature of the distribution of marine plankton is its unevenness, with localized patches in nearby areas differing in both quantity and composition. One aspect of this patchiness is the inverse relationship of quantities of phytoplankton and zooplankton which has often been reported. Where phytoplankton is especially plentiful, herbivorous zooplanktonts are sometimes few in number; and where herbivores abound, the phytoplankton may be sparse.

This appearance of an inverse relationship may be due simply to the different reproductive rates of phytoplankton and zooplankton, and the effects of grazing on the size of the standing stock. In favourable conditions phytoplankton can multiply rapidly and produce a dense stock. Zooplankton populations increase more slowly, but as the numbers of herbivores rise the phytoplankton will be increasingly grazed and the stock correspondingly diminished. It may therefore be impossible for any abundance of phytoplankton and zooplankton to coexist for any length of time in natural conditions because of the rapidity with which plant cells can be removed from the water by herbivorous animals.

Measurements of filtering rates and food intake in various herbivorous zooplanktonts indicate that, in high concentrations of phytoplankton, large numbers of plant cells are rapidly ingested, sometimes apparently in excess of the animals' needs. Some of these cells pass through the gut virtually undigested, suggesting a wasteful destruction of plant cells which has been termed 'superfluous feeding'. However, although superfluous feeding can be demonstrated for a time in laboratory conditions when food is exceptionally abundant, the filtering rate later reduces. It seems unlikely that in natural conditions much food is egested unassimilated. Normally, high rates of intake result in increased growth and egg production.

Another explanation for an inverse phytoplankton-zooplankton relationship involves the concept of 'animal exclusion'. According to this hypothesis, animals avoid water rich in phytoplankton because the plants have some effect on the quality of the water which animals find unpleasant. The nature of the excluding influence is uncertain, but it might perhaps be due to secretion of external metabolites by the plants. Small zooplanktonts could avoid this water by controlling their depth so as to remain at deeper levels until the relative movement of the different layers of water carries them to areas where the surface water is less objectionable. The exclusion effect is also observed with some pelagic fish. For instance, the occurrence from time to time of dense patches of Rhizosolenia, Biddulphia or Phaeocystis on the North Sea herring fishing grounds is usually associated with poor catches, the shoals seeming to avoid phytoplankton-laden water. In the mackerel fishing area in the southwest (see page 350), poor catches are obtained in areas which the fishermen recognize as 'stinking water', and this

Figure 5.9 The Perspex horizontal circular apparatus used by Bainbridge to study the

interrelations of Zooplankton and phytoplankton, showing the three sliding doors.

Openings at these three points allow for filling, and introduction of animals. (From Bainbridge (1953), published by Cambridge University Press.)

has also been shown to contain a large amount of phytoplankton. The absence of fish from this water may be due, however, merely to the absence of the Zooplankton on which they feed.

To discover if any evidence of exclusion could be demonstrated experimentally, Bainbridge (1953) devised laboratory apparatus in which the behaviour of Zooplankton could be observed in the presence of high concentrations of phytoplankton. He constructed a horizontal, circular Perspex tube, 4 ft in diameter, divided into three equal compartments by sliding watertight doors (Figure 5.9). One compartment was filled with seawater enriched with cultures of phytoplankton. The other compartments were filled with filtered seawater. The doors were opened for a period to allow the phytoplankton to spread around the tube until a distinct gradient of phytoplankton concentration was seen to exist. At this stage, equal numbers of small planktonic animals were introduced into each of the three compartments, and their distribution around the tube was counted at intervals. Pure and mixed cultures of phytoplankton were tested, including the diatoms Skeletonema, Thalassiosira, Biddulphia, Coscinodiscus, Lauderia, Eucampia and Nitzschia, the flagellates Chlamydomonas, Dicrateria, Rhodomonas, Syracosphaera, Oxyrrhis, Exuviella, Peridinium and Gymno-dinium, and some bacterial cultures. The animals studied included the mysids Hemimysis lamornae, Praunus neglectus, P. flexuosus, Neomysis integer and Mesopodopsis slabberi, also Artemia salina, Calanus finmarchicus and various other small copepods, and some decapod larvae.

The results showed definite migrations by the mysids towards concentrations of certain diatoms, notably Skeletonema, Thalassiosira, Biddulphia, Nitzschia and mixed cultures, and towards the flagellates Chlamydomonas, Peridinium, Dicrateria and Oxyrrhis. Artemia salina and some small copepods moved into cultures of Nitzschia, Biddulphia and Thalassiosira. Cultures of Lauderia, Coscinodiscus, Eucampia, Syracosphaera and Exuviella showed no attractive effect. The mysids showed a definite movement away from the flagellates Rhodomonas and Gymnodinium II, and the bacterial cultures. No repellent effect was demonstrated for any diatoms except Nitzschia at highest concentration.

In this circular tube the results with Calanus finmarchicus were inconsistent. Observations on this animal were also made in another apparatus consisting of a pair of straight, parallel, vertical tubes. One tube was filled with normal seawater, the other with seawater enriched with phytoplankton, and equal numbers of animals were inserted in each. The tubes were suspended in an aquarium tank under lighting of moderate intensity, and the number of animals at different levels of the tubes was counted at intervals. The experiment demonstrated significantly greater numbers of Calanus swimming upwards in cultures of Coscinodiscus, Skeletonema, Ditylium, Chlamydomonas, Gymno-dinium, Oxyrrhis and mixed phytoplankton. Chlorella appeared to depress the numbers swimming up.

Bainbridge's experiments were not designed to elucidate the nature of attractive or repellent substances, but these did not appear to be associated with changes in the concentration of carbon dioxide or oxygen or pH. Positive migrations into concentrations of ammonia were observed. As this is the usual excretory product of marine invertebrates, its attractive effect might be partly accountable for the tendency of many small pelagic organisms to collect in swarms.

From his observations, Bainbridge concluded that 'exclusion' is of quite restricted occurrence in natural conditions, although it may operate during intense blooms of some toxic flagellates. It seems that, in general, natural concentrations of diatoms are likely to be attractive to grazing animals, and Bainbridge suggests that the inverse phytoplankton/zooplankton relationship may be explained in terms of a dynamic cycle of growth, grazing and migration as follows (Figure 5.10):

1 Where conditions are favourable, rapid growth produces a dense patch of phytoplankton.

2 Herbivorous animals are attracted horizontally and vertically into the phytoplankton patch. The grazing rate increases but the concentration of plants will not decline appreciably until the rate of removal by grazing exceeds the rate of increase by cell division.

3 As more animals are attracted to the area, heavy grazing rapidly diminishes the number of plants until the phytoplankton patch is virtually eliminated.

4 Meanwhile, in adjacent water, rapid growth of phytoplankton can now occur because these areas have become denuded of their grazing population by migration into the original phytoplankton patch, and conditions are set for a repetition of the cycle.

There are three species of herbivore commonly present in dense phytoplankton in the Antarctic, namely, the copepods Calanus simillimus and Drepanopus pectinatus and the mysid Antarctomysis maxima. These three forms are strong swimmers, and Bainbridge suggests that their presence in high concentrations of

Figure 5.10 Scheme of grazing and migration cycle. (a) Initial state with inverse relationship; (b) start of migration with some grazing; (c) completion of migration and heavy grazing; (d) reversal of initial state and return to inverse relationship. Oblique hatching represents concentrations of phytoplankton. (From Bainbridge (1953), published by Cambridge University Press.)

Figure 5.10 Scheme of grazing and migration cycle. (a) Initial state with inverse relationship; (b) start of migration with some grazing; (c) completion of migration and heavy grazing; (d) reversal of initial state and return to inverse relationship. Oblique hatching represents concentrations of phytoplankton. (From Bainbridge (1953), published by Cambridge University Press.)

phytoplankton is due to the rapidity with which they move. They arrive before the other, slower-moving herbivores become numerous, but their grazing effect is insufficient to make much reduction in the number of plants.

Bainbridge's experiments were conducted using fresh cultures of phytoplankton. Others have pointed out that there is evidence that the production of antibiotic substances by algae occurs mainly in ageing cells, and this may invalidate some of Bainbridge's conclusions. His experiments at least demonstrate that certain species of phytoplankton exert an influence, either attractive or repellent, to which some zooplanktonts react.

Evidently the appearances of an inverse phytoplankton-zooplankton relationship and the patchiness of plankton distribution are both due to a variety of interconnected causes (Bainbridge, 1957). They can, for example, result from water movements. Positively buoyant organisms will tend to become aggregated along lines of convergence or in the centre of swirls, while negatively buoyant forms must be brought together in upwelling zones beneath divergences. Upward-swimming organisms may form patches above the course of cascade currents (see Section 5.6.3). Wind action sometimes sets the uppermost few metres of water in lines of spiral motion, forming so-called Langmuir vortices, with zones of convergence and divergence between adjacent vortices. This pattern of water movement must cause quite localized patches of organisms which differ in buoyancy or speed and direction of swimming. Any localized turbulence or vertical mixing which affects temperature, salinity or fertility must obviously influence distribution, and may sometimes result in small-scale blooms of phytoplankton.

Differences in multiplication rates of phytoplankton and zooplankton may have see-saw effects on the relative abundance of grazers and their food. Behavioural differences of species making vertical migrations must result in different organisms concentrating at various levels at various times. Attractive or repellent effects of one species on another, influencing direction of movement or the extent of vertical migrations, may cause the appearance or disappearance of certain species in particular places. Swarming behaviour, mainly associated with breeding, may account for localized patches of adults, and subsequently of eggs and larvae. The distribution of meroplankton must obviously reflect any patchiness of distribution of the benthos, which relates to differences in the nature of the sea bottom.

Whatever its various causes (Fasham, 1978; Fasham et al., 1974) there is no doubt that the tendency of many planktonts to occur in patches rather than an even distribution has important implications with regard to feeding. Animals feed more economically where food is abundant than where it is scarce. By concentrating in and around patches of best food supply they have the double advantage of efficient feeding while allowing the recovery of depleted food stocks in other areas. If food were more evenly distributed, its concentration might sometimes be below the starvation threshold.

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