Deep-sea animals are most diverse under the surface waters of the subtropical belts.

Here are clear blue waters, where photosynthesis by the microscopic algae of the plankton is possible down to a depth of 100 meters or more.

Apart from the divergent regions of the equatorial current systems, where up welled water brings nutrient salts to the surface and leads to relatively high levels of productivity, these warms central oceanic waters are low in nutrients and standing crops of plankton.

Blue-green algae thrive in such surroundings and a dominant form, Trichodesmium, whose rust-coloured blooms may at times cover wide stretches of the warm ocean, is reported to use fixed molecular nitrogen for its growth.

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In other respects the composition of the phytoplankton of the tropical and subtropical belts is much like that of other oceanic regions. But the size range of the algae extends below that of floras in nutrient-rich waters, such as are found in the antarctic and in upwelling waters off Peru and elsewhere.

For comparable forms the smaller the cell the less are its nutrient requirements, but the more its surface for nutrient absorption compared to its volume. Thus, it somehow seems right that much of the flora of nutrient-poor tropical waters should consist of mobile |i-flagellates, such as crypto monads and chrysophytes.

To be but a few microns in size confers another advantage. In subtropical and tropical regions in particular, it seems that the multiplication of the phytoplankton barely keeps pace with its consumption by all manner of herbivores in the zooplankton.

One might then expect selection pressure to be high for means of countering the grazing inroads of zooplankters. To be small enough to pass through the meshes of a filtering system, particularly of copepods, is advantageous, though there is less escape from the mucus nets of the pelagic tunicates.

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But in overall terms of nutrition (i-flagellates may form no more than a small part of the grazing of herbivores. For instance, during an expedition to the eastern tropical Pacific, counts of phytoplankton constituents were made (under an inverted microscope) from 200 samples covering 48 stations.

When each count was expressed in terms of its carbon content, the main conclusion was that though flagellates (2-5|i in diameter) formed about 80 per cent of the catches in numbers of individuals, they made only a minor contribution to the total content of phytoplankton carbon.

In equatorial waters most of this carbon was contained in the diatoms, whereas din flagellates were the main carbon contributors north and south of the equatorial region. No doubt the relative art founts of the major groups of algae change from time to time, and we must always remember that the fragile and elusive -flagellates are likely to be underestimated.

There is no off-season in the production of phytoplankton in warm oceanic waters: throughout the year diverse zooplankters live on thinly dispersed crops particularly in the subtropical regions bounded by the gyres. In temperate and cold regions there is rich grazing in the favorable season, during which animals form stores of fats and oils to help tide them over the winter months.

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Whatever the place, there are times when the phytoplankton crops are so meagre and thinly spread that one may wonder how the herbivores manage to exist. Indeed, such thoughts, stimulated by the quantitative studies of Hensen, Lohmann and Brandt towards measuring the productivity of the seas, led Putter to investigate the nutritional requirements of marine animals (as measured by their rates of carbon dioxide production).

After comparing the metabolic needs of diverse species with ambient amounts of algal food Putter concluded that marine animals must depend largely on dissolved organic matter, which they absorb through their integument.

Krogh, who was drawn to this problem through his experiments to determine whether aquatic animals are able to absorb organic substances, first concentrated on an accurate method of measuring dissolved organic matter (DOM) in sea water.

Though he found that there is much more DOM than particulate forms of organic matter in the ocean, he also observed that DOM is very largely resistant to bacterial decomposition. Krogh concluded that the food of aquatic animals is generally in the form of organisms and organic detritus.

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Since the 1950s numerous experiments on diverse aquatic invertebrates have shown that most marine, but not freshwater, forms can take up glucose and amino-acids through their integument. But Korgh’s conclusion still stands. Even the Pogonophora, which are confined to sediments rich in organic matter, have a restricted rate of absorption when in their tubes.

Members of the micro-zooplankton, certain copepods and euphausids, shelled periods and pelagic tunicates from the main groups of oceanic herbivores.

More than we realize the lives of these diverse forms (and their predators depend on the natural fact that the phytoplankton is not randomly dispersed.

Statistical analyses of well-planned series of samples show that the constituents of phytoplankton are ‘over dispersed’, with implies the agregation of ‘clumping’ of individuals but not necessarily the forming of physically observable clusters.

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Studies of a water column down to 150 metres off Bermuda, which was so homogeneous that measures of temperature and salinity varied by less than 0.05°C and 0.05 per cent respectively, showed that the concentrations of phytoplankton species differed by several orders of magnitude at different depths.

There are discrete layers of phytoplankton and at times such concentrations lie near the surface, at others near or even below the depth of the euphotic layer. Members of the micro-zooplankton aggregate in these layers. In one way or another enough individuals of any herbivorous species seem to refute Putter’s conjecture, which is also true of the largest marine animals.

For instance, fin whales were estimated to need zooplankton concentrations in excess of 1.5 gm per cubic metre, whereas the average standing stock of zooplankton in the Subarctic Pacific is rather less than one-tenth of this amount.

During crossings of this region Barraclough et al used a 200 kHz echo-sounder and recorded shallow scattering layers that net hauls showed were composed almost entirely of the copepod. Calanus cristatus. Such concentrations were considered to be more or less sufficient to satisfy the appetites of fin whales.

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While there is no off-season in the production of life in subtropical and tropical waters, there are seasonal changes. Indeed, monsoon comes from the Arabian word mausium, meaning a time or season. During the northeast monsoon in the Indian Ocean measurements of carbon assimilated by the phytoplankton yielded a mean figure of 0.15 gmC per square meter per day, which is less than one-third of the production during the southwest monsoon.

The invigorating effect of the southwest monsoon is widespread, but in the warm Pacific Ocean seasonal changes are most pronounced in equatorial regions.

Here from July to December there is greater productivity than during the rest of the year, when there are northeasterly or variable winds. In the great subtropical gyres, where the ‘seasonal’ thermo cline is more or less permanent nearly everywhere there can be little seasonal change in the standing stocks of plankton.

Marked seasonal changes in primary production no doubt lead to changes in the biomass and composition of the zooplankton. Concerning the productive zones of the equatorial Atlantic in the summer Bernikov et al. were impressed by the high biomass of plankton the bulk of which consisted mainly of jelly-like animals, notably of pelagic tunicates. In the autumn most of the zooplankton consisted of crustaceans.

Pelagic tunicates with their efficient plant-gathering nets of mucus and relatively little living tissue to maintain seem well organized to respond quickly to an increase in primary productivity. Indeed, one salp with an instantaneous rate of population increase of 0.4 to 0.91 per day, can double its members in a single day.

Swarms of this salp, which may be very widespread, are generated immediately after phytoplankton blooms, and thus are transient resources exploited to the full. Their small relatives, the appendicularians most of which live is subtropical and tropical regions, are even livelier opportunists. In the Black Sea the turnover time of Oikopleura is about three days.

Appendicularians are among the commonest group of zooplankton (often second only to the copepods) and, like their relatives, are able to feed on the smallest kinds of phytoplankton. The mesh size of the filtering windows in their houses varies from species to species and when the filter becomes clogged with food; another house is quickly secreted (which may be several times in one day).

In each discarbed house is a concentrated package of phytoplankton, detritus and mucus, food for other planktonic animals and bacteria. Appendicularians themselves are prey to such animals as jelly-fishes, siphonophores, arrow-worms and lantern-fishes.

By-products of zooplankters, particularly of appendicularians, salps and pteropods, form many of the flakes and larger organic aggregates of the marine snow’ so often seen in the open ocean by divers and observers in submersibles. Alice Alldredge concludes, ‘These large amorphous bits of mucus and detritus serve as surfaces on which many planktonic organisms can rest or feed.

Bacteria and protozoans use them as a permanent habitat. Appendicularian houses and other particles of marine snow provide tiny solid substratas and introduce heterogeneity into an environment that is generally considered to be relatively homogeneous physically. Although these miniature habitats have not been completely explored, it is certain that they have influenced the adaptation of feeding strategies of many planktonic organisms’.

Pelagic tunicates are what theoretical ecologists call r-strategists, opportunistic animals that grow and reproduce quickly when conditions become favourable. Indeed, nearly all gelatinous forms in the zooplankton, both herbivorous and carnivorous, seem to be r-strategists. Concerning the latter, medusae, siphonophores and crenophores evidently have’.. .a versatile adaptation system, which facilitates flexible synchronisation of the population booms with abundance of planktonic forms which they use for food.

This synchronisation includes the capacity to vary the number of eggs; timing, rates and types of reproduction; to withstand long periods of starvation; to produce resting stages, and to form aggregations.

These properties provide passive pelagic predators with the capacity of “waiting” for a favourable biotic situation to occur’. At first sight, these gelatinous herbivores and carnivores seem to break the rule that r-strategists are small, but if large in bulk, as most species are, they are small in protoplasmic content. To find small kinds of r-strategists, one needs look no further than the protozoans of the micro-zooplankton.

Compared to those of cold regions, zooplankton communities of the warm ocean have a low biomass, a high diversity with no very dominant species, a higher proportion of carnivorous species, more species with a short life span and numerous herbivorous species with a reduced body size (but there is no such diminution in carnivorous species). As Motoda and his colleagues say, one advantage of greater size, in herbivorous zooplankters of cold waters is that they have extra space to store fats etc., to help them over the winter season.

The smaller herbivorous species of the warm ocean live faster and breed faster but store little in the way of food reserves, for phytoplankton is always available, even if the supply may be meagre, as it most often in the deep-blue gyral deserts of the subtropical belts. In the North Pacific gyre productivity is generally limited by the quantity of nitrogenous nutrient present in the water.

Indeed, nitrates are insignificant in quantity; the main source of nitrogen for plant growth seems to be ammonia and urea produced as waste products by the zooplankton and nekton.

The impoverished nature of subtropical waters, compared to equatorial and more northerly waters of the western North Pacific, may be seen in Fig. which shows the geographical variations in phytoplankton crops and the biomass of zooplankton in the surface waters.

In subtropical regions, where the upper waters are kept stable but impoverished by a marked thermocline, plant production barely keeps pace with herbivore consumption : there is, as Cushing calls at a quasi- steady state production cycle. In the Indian Ocean, Timonin who analysed zooplankton communities, especially their copepod.

Euphasiid and chaetognath constituents, in specific and trophic structure, contrasts regions of stable water stratification with those, in the equatorial belt where current divergences lead to upwelling and greater productivity.

Stable regions support communities with a low biomass but high species and tropic diversity whereas communities in regions of intensive divergence have a higher biomass with lower species and trophic diversity. Communities in regions of low divergence were more or less intermediate in the values of the above ecological indices.

After one organism becomes the food of another, matter and energy are lost in the processes that transform prey tissue into predator tissue. Any such transformation can be seen to have a certain transfer efficiency, which Ricker defines as ‘…the percentage of the prey’s annual production that is incorporated into the body tissues of the consumer species. This, in turn, is compounded of two processes characterized by separate coefficients:

E, the ecotrophic coefficient – the fraction of a prey species annual production that is consumed by predators (trophic referring to nutritive or food levels):

Ricker suggests that an overall average growth coefficient for herbivores could hardly exceed 15 per cent, though the figure may be around 20 per cent for fish and othe consumers of animal food. Concerning the ecotrophic coefficient of plant consumers a figure of 66 per cent seems reasonable, while for secondary consumers, such as lantern-fishes which feed on zooplankton, 73 per cent is proposed.

Eventually, one may construct a simplified aquatic food pyramid such that the area of each rectangle representing a particular trophic level is proportional to the estimated production of that level. But the four levels shown in this figure, as Ricker points out, are given too low a trophic value.

Successive steps in a food pyramid need to be distinguished from ecological levels (groups of animals of generally similar size and habit). In order to assess average trophic levels one has to consider within-level predation.

The effect is to reduce the net biomass produced by any ecological level. For instance, the production of lantern- fishes, which feed on zooplankton, is reduced by members of the zooplankton, such as jellyfishes and arrow-worms that feed on larval lantern-fishes.

The overall transfer of production from one trophic level to the next is, of course, represented by the product KE, and Ricker estimates average values of about 10 per cent for the primary consumer and stage and 15 per cent for subsequent stages.

Relative productivities are thus: plants 100, herbivorous zooplankton 10, small pelagic fishes that feed on zooplankton about 1.6, and so forth. The more the links in a food chain, the very much less is the terminal productivity. In some oceanic communities, at least, Ryther considers there may be five links in the food chain:

Nanoplankton – micro-zooplankton – carnivorous crustacean zooplankton-larger carnivorous zooplankton-feeders on zooplankton- fish eaters.

But tuna, which feed, inter alia, on lantern-fishes, saury and euphausiids, may be put between trophic levels 5 and 6. Even so, such lengthy food chains are much less productive than those in peristent upwelling areas, as off Peru and in antarctic seas. Peruvian anchovies feed on phytoplankton as well as small forms of zooplankton, while baleen whales in antarctic regions feed on krill, which of all euphausiids comes closest to being a complete herbivore.

Anchovies may thus be given a trophic level of 1.5: whales are near level 2. Ryther, who assumed that short food chains would be the more efficient, assigned KE values of 20 per cent to both upwelling areas, while the oceanic food chain was assessed at 10 per cent. He concluded that the annual primary production of oceanic areas was generally low and gave 50 gm C/m2/ year as as average figure. The corresponding figure for the Peruvian area was estimated at 300 gmC/m2/year.

Such estimates are provisional and may well need revision. For instance, Cushing, after analysing data from the International Indian Ocean Expedition, concluded that the transfer of living material from primary to secondary trophic levels is three times as efficient in the open ocean as in upwelling areas. Perhaps there are less than five links in the oceanic food chain.

If there are four rather than five, then the productivity of planktivorous fishes and piscivorous tuna will be ten times greater than estimated. There is still much uncertainty. As Rothchild argues, one could take a number of plausible alternative values for the trophic coefficient and arrive at very different conclusions.