Organisms making up the plankton are usually short-lived, particularly the photosynthetic organisms since these are notably sensitive to changes in the physical-chemical living conditions. Seasonal and sometimes interseasonal periods prevail, therefore, when but little phytoplankton can be found; but, at least in temperate regions, a vernal production followed by subsidiary increases of phytoplankton can be depended upon each year. This vernal production of phytoplankton must be considered as an event of great significance, for it comes at a time coinciding with abundant production of pelagic larvae, especially of invertebrates, that feed upon it (see below). Any factor, whether it be in reference to phytoplankton food or to inorganic living conditions, that hampers the success of these swarms of larvae will function immediately to the disadvantage of higher plankton feeders that utilize the larvae directly or the adult population resulting therefrom. Studies have indicated that among these feeders may be placed many commercial fishes, especially the young, but also the adults. Any degree of failure of the spawning or development of larvae of the permanent plankton (of which copepods may be considered typical) must also be reflected only a few weeks later in the adult stock available in any area, for the adults are short-lived and apparently die after the spring and summer propagative periods and therefore depend upon the success of these broods to maintain the adult stock at a high numerical level. Copepods, we know, feed upon the phytoplankton and in turn constitute an important item in the diet of many fishes. Assuming that phytoplankton production may be correlated with abundance of light, an example of the far-reaching effect of phytoplankton production on the mackerel is offered in a study by Allen (1909), which shows a direct correlation between the abundance of fish caught during May and the total hours of sunshine for the preceding February and March, over a period of seven years (fig. 243). Official figures for the mackerel fisheries as a whole show a marked drop in 1906, and Bullen (1909) has shown a
Correlation between sunshine and the abundance of mackerel. (From Allen).
Many investigators have observed that during the growing season when diatoms occur in abundance there frequently appears to be a scarcity of zooplankton. A combination of causes doubtless operates to produce this effect. Two main hypotheses have been advanced in explanation of the phenomenon of inverse relations. The least complicated explanation is found in the hypothesis of grazing (Harvey, 1934), which holds that diatom numbers are controlled through consumption by animals. The alternate hypothesis is that of animal exclusion, advanced by Hardy (Hardy and Gunther, 1935; Hardy 1936).
Animal Exclusion. According to this view, which involves a consideration of the regular vertical diurnal migrations of the zooplankton, certain of the animals during a part of the day swim upward into the layer of water where diatoms are being produced. The duration of their sojourn in the upper layer is inversely related to the concentration of the phytoplankton. Thus they are excluded vertically for a considerable period of time when diatom production has resulted in a dense swarm of diatoms. It should be noted that this exclusion is primarily a vertical one; nevertheless it may result in lateral exclusion when there is a difference in the speed or direction of flow of the upper euphotic layer and of the deeper layer into which the animals have descended and in which, when the diatoms are very abundant, they may spend a disproportionately long time. A lateral displacement may therefore be more or less complete and the greatest concentration of animals will finally occur in areas where diatoms are relatively scarce. The implications of this hypothesis on the habit of diurnal migrations should be noted (p. 836).
As evidence in support of this hypothesis, it was found during the Discovery investigations in the Antarctic that the greatest concentration of zooplankton occurs in areas low in phytoplankton content and relatively high in phosphate content, which indicated that phytoplankton production had been low for some time. Correlated with this disparity was the fact that blue and fin whales, which are known to seek out and feed upon zooplankton, were present in greatest numbers in the areas rich in phosphates. Some support is also given through experiments which indicate that some animals are more strongly negatively phototropic in the presence of many diatoms (Lucas, 1936). Plankton animals exhibiting the most pronounced activity in diurnal vertical migrations are the ones most likely to be excluded according to the hypothesis. Young stages are believed to show less tendency to exclusion. In the hypothesis of animal exclusion, which is considered as a tentative one, the emphasis is placed on the inimical effect (presumably chemical) of plants on the animals.
Grazing. The concept in this hypothesis shifts the emphasis to the effect of the grazing zooplankton as a control of phytoplankton production. That grazing is a highly important factor in the control of phytoplankton is given much support by various investigators (Harvey et al, 1935, Fuller, 1937).
According to this point of view, large phytoplankton and zooplankton populations cannot exist simultaneously in the same area for long, since the extensive grazing by the zooplankton prevents the phytoplankton from building up or maintaining dense growth. As a result of investigations of Harvey et al (1935) it is indicated that in general “a change in diatom population is brought about by a change in one or both of two opposing factors—the rate of growth of the diatoms (depending upon illumination and probably on concentration of nutrient salts) and the rate at which the diatoms are eaten (depending upon the number and kind of herbivorous animals).” Therefore, in any area a dense phytoplankton population is the result of optimum growing conditions combined with a relative scarcity of grazing animals, the yield of plant cells having been greater per unit time than the consumption by animals plus the loss that may result through other causes. Since dense plant growth is favored in the absence of grazers it appears as if the animals have been excluded because they avoid a dense diatom population. Evidence of grazing is found in the fact that phytoplankton may “disappear” when nutrients and other growing conditions are good.
In this connection it is significant to note that computations indicate that during maximum grazing plankton animals may consume somewhat less than half their own weight per day. The copepod Eurytemora hirundoides is said to eat as many as 120,000 small diatoms (Nitzschia closterium) in a day (Harvey et al, 1935). Lohmann (1908), in an exhaustive study of plankton at Kiel, assumed that each metazoan animal requires a daily ration of one tenth its own volume. A 30 per cent daily increase in plants was assumed to take place, and only this much could then be removed without reducing the initial plant stock by overgrazing. As stated elsewhere (p. 885), calculations on this basis showed a plant deficiency during the seasons of low plant production. As a matter of fact, the rate of increase in numbers of plants cannot be so simply stated. It varies greatly and so also does the rate of feeding in grazing animals, and widely different combinations of plants and grazers must occur.
Evidence has been brought to indicate that some plankton filter-feeding animals, that is, copepods and mysids, filter the water at a rather constant rate regardless of the concentration of the microscopic particulate food that is present (Lucas, 1936, Fuller and Clarke, 1936, Fuller, 1937, Fleming, 1939). Other factors, such as light, temperature, and size of particles, function to vary the filtering rate (Fuller, 1937). This
|Time in days||Population (cells per liter)|
|Grazing element doubled||Grazing element increased fivefold|
When plants have been reduced through grazing or when conditions for growth of the plants become less favorable and the division rate diminishes to a point where production of new cells is less than the number consumed by the grazers, animals again dominate the field. The progressively diminishing efficiency in catching plants as they become scarce is important to the survival of the diatoms since as a result there is likely always to be some “seed” left in the water. It is important to note that observations taken at fixed places, in areas where an exchange of water masses occurs, may show an apparent succession of phyto- and zooplankton populations. However, under such conditions no true alternation has occurred but only an apparent one owing to the exchange of water within the area under observation. Investigations of food relations in the plankton can, of course, be most reliably carried on in bodies of water such as bays or other closed systems that are sufficiently isolated to experience little or no influence from inflow or outflow from adjacent waters and therefore support an endemic self-contained population. In open coastal areas where much exchange of water occurs, the details
Some investigations do, nevertheless, show a clear relation between phytoplankton increases and the dependent plankton animals. For example, in Loch Striven, a semienclosed area, Marshall, Nicholls, and Orr (1934) were able to correlate directly three main successful broods of Calanus finmarchicus with diatom increases in (1) March-April, (2) May, and (3) July and August-September, the main spawning having occurred in February-March, May, and July. A fourth period of spawning occurred but was abortive owing to scarcity of food for the early stages of development. There were also indications that a storage of fat can take place during periods of plentiful phytoplankton.
Nielsen (1937) found that in the open coastal waters of Iceland the phytoplankton maximum occurred in May. In these waters the zooplankton was poor and was represented mainly by juvenile individuals, while at the same time in the protected fjords, where diatom maximum came a month earlier, there was an abundance of animal plankton with numerous full-grown individuals.
Mention should here be made of the mutually beneficial relationship derived by the phytoplankton and the zooplankton through an exchange of oxygen and carbon dioxide in solution. We know that during photosynthesis by the plants much oxygen is produced in the waters of the euphotic zone. It is not clear, however, to what extent this means of aeration is a necessary supplement to that which results from diffusion at the contact zone with the atmosphere. In isolated quiet waters it must be an important item. Waters of great depths have a sufficient supply of dissolved oxygen to support their characteristic types of animal life and this must have been transported directly from the surface or euphotic zone through diffusion and the action of water currents. It is probable, however, that a greater rate of metabolism in the more abundant animals of shallower depths, where temperatures are higher and food more abundant, sets up a requirement which could not readily be met by diffusion of oxygen from the surface alone. Plankton animals do occur in the oxygen minimum layer below the euphotic zone of the Pacific, and in the Gulf of California they are found at mid-depth layers where oxygen is near zero. The numbers of animals in the oxygen minimum layer are nevertheless small, and this may have resulted in part from the low oxygen content in the absence of photosynthesis or rapid diffusion from better aerated waters.
Plants in the lighted zone must derive some benefit through the carbon dioxide produced by animals living in the same waters; but here also the significance of this source, though certainly not negligible, has not been established.
The interrelations of the plankton animals have not been studied in such detail as those between the plants and animals. That many of them are carnivorous or omnivorous has been mentioned, and among these the medusae and ctenophores are notably influential in sweeping the waters clean of other planktonic animals upon which they feed (p. 890).
Correlation of the catches of whales with abundance of euphausiids in Davis Strait. (After Hjort and Ruud.)