The Production in Different Regions

It is very well known that some regions of the oceans give evidence of being on the whole more productive than others. Locally also, considerable variation exists in different coastal areas. We have already learned what some of the underlying reasons may be. Precise quantitative data on initial organic production are scanty and diverse as far as the methods of estimating and the period of time involved are concerned. In table 102, which is taken mainly from Riley (1941), some estimates are given from separate areas which will serve to illustrate the best available figures, in terms of grams of carbon per square meter or per cubic meter, on the order of magnitude of phytoplankton production.

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Large seasonal variations in the nutrient elements have been observed in coastal areas in temperate latitudes where there are marked variations in the physical, chemical, and biological factors during the year. In the English Channel, which is only about 100 m deep, conditions are such that complete mixing from the surface to the bottom takes place in the winter months. Even during the summer this condition prevails for short periods, consequently the entire water column may be depleted of nutrients. In the Gulf of Maine a similar mixing occurs during the winter months and near Friday Harbor strong tidal currents and irregular topography contribute to intense vertical mixing throughout the year.

For these and other reasons (such as the proximity to land) the annual variations in the plankton population and in the distribution of the nutrient elements characteristic of these individual localities can not be applied to other regions or to the open sea without a careful consideration of the different factors which may be involved. In areas such as the English Channel, where the nutrients are more or less completely removed from the water column during the summer, it is quite probable that the amount of production is related to the store of nutrient elements present at the beginning of the vegetative season. In localities where there are vertical gradients in the nutrient concentrations, diffusion and turbulence will carry a certain amount into the euphotic zone. Such processes are so effective in certain areas, for instance near Friday Harbor, that plant development does not deplete the nutrients to a point where the amounts present can affect the rate of growth. In these areas other factors must limit the organic production (p. 769). In table 103 are given, for a number of localities, the approximate averages for the contents of nitrate, phosphate and silicate in the upper 25 m at the times

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of the winter maxima and the summer minima. As shown previously, virtual depletion of the surface layers takes place in the English Channel and similar conditions occur in the Barents Sea, whereas at Friday Harbor (surface values only), in the Gulf of Maine and particularly around South Georgia, nutrient elements are present in the surface layers at all times.

In order to estimate the rate of production in areas where there is no marked seasonal change in the distribution of nutrients, it is necessary to take into account the supply of these materials which will result from diffusion. Even in localities where there is a marked seasonal change, estimates of production based on the removal of nutrients during the vegetative season must be corrected for this process or a layer of sufficient thickness to eliminate this factor must be used when calculating the total amount of any element removed from the water column. The latter method has been used at a number of localities to estimate the differences in the amounts of the nutrient elements in the water column at the times of winter maxima and summer minima. From these data, the net utilization in grams per square meter column of NO³-N, PO⁴-P and SiO³-Si during the vegetative season have been obtained. The values are given in table 104. The net removal of nitrate and phosphate in the Barents Sea and around South Georgia is about twice as great as the utilization in the English Channel and in the Gulf of Maine. The great difference between the silicate utilization in the English Channel and around South Georgia is of interest in connection with the deposition of siliceous sediments. Estimates of seasonal production based on data of this kind are minimal because they include no consideration of the effects of regeneration, that is, that the elements may pass through the biological cycle more than once during the season. Kreps and Verjbinskaya (1932) estimate that, in the Barents Sea, production, calculated from the phosphate utilization, should be increased by about one third to correct for the concurrent regeneration.

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Many observations indicate a higher rate of organic production in the high latitudes than in the tropics. This possibility is brought out clearly in fig. 250, from the Meteor observations, where the standing population in the Antarctic is about tenfold that in the tropical regions. The greater standing crop in high latitudes may not indicate a greater annual production because in high latitudes the production is concentrated into a few months of vigorous and spectacular growth with return of sunshine or following nutrient replenishment over the winter, whereas in the lower latitudes the growth may proceed at a slower but more continuous rate as nutrients become available.

The marked cycle of meteorological events characteristic of temperate and high latitudes leads to a number of associated phenomena vital in determining the periodicity and extent of organic production in these latitudes. The decrease of sunlight inhibits or diminishes growth during the darkest winter months and the low winter surface temperatures give rise to active convection currents effecting an abundant annual renewal of essential plant nutrients in the euphotic layer. In the Gulf of Maine these currents reach a depth of 100 m, and in shallower areas the whole column of water with its store of nutrients is involved in the mixing. Spring and summer conditions bring about a stabilization which, together with an ample supply of solar energy, provides the conditions essential for a vigorous though short period of production. Reference to the discussion of hydrographic conditions in the Antarctic will show that in that region the ascent of deep water near the Antarctic Continent brings to the surface waters an excessively rich supply of nutrients which, in moving toward the Antarctic Convergence (fig. 164, p. 620), give rise to a great flare of diatom production during the Antarctic spring and summer when sunshine is ample and the upper water layers have become stabilized through melting of northward-drifting ice (Hart, 1934). So great is the

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supply of nutrients in the euphotic zone that depletion of these salts never becomes a limiting factor, since the content never falls below a minimum of about 0.9 μ-atom of PO₄ per liter. This leaves a summer minimum higher than the winter maximum of the English Channel. In the Antarctic the checks on production are not clearly understood, but they are perhaps associated with conditions of light and stability of the surface layers. The excessively dense masses which discolor the water (figs. 214 and 216, pp. 784 and 786) must produce a screening of light that leads to regulation of photosynthesis. The large volumes of zooplankton occurring in this region must indicate that grazing is also an important factor in control.

That plankton production is indeed great in the Antarctic in spite of the short season is proven by the invasion of large numbers of feeding whales which migrate each year to these waters to fatten on the rich supply of available food there. The bottom fauna of these areas is also unusually rich, as was found by the Challenger and other expeditions. That diatom production is perhaps even greater than its animal utilization during the maximum growing period is suggested by the accumulation of the broad band of diatomaceous ooze so characteristic of the bottom deposits encircling the Antarctic Continent (fig. 253, p. 975). Hart has pointed out, however, that the types of diatoms found in these oozes belong largely to species which go unaltered through the digestive tracts of animals.

As to production in lower latitudes we can say that in general the open sea is relatively sterile (figs. 214 and 216, pp. 784 and 786). Examinations of figs. 217 and 218, pp. 787 and 788, will show why this must be so, for the supply of nutrients lies below the euphotic zone and the strong and persistent thermal stratification of the water within this zone precludes any complete or rapid renewal of nutrients in the upper layers despite the greater thickness of the euphotic layer in tropical or subtropical regions. In coastal areas with upwelling or in offshore regions of divergence, the situation is quite different. As an example of the former we have already mentioned the west African coast (p. 786). We may consider also the Peru Current, which in the tropical and subtropical regions brings to the surface an abundant continuous supply of nutrients that provide fertilization for heavy phytoplankton production, near the coast mainly but also in some sections in almost undiminished intensity to a distance of 320 km seaward (Gunther, 1936). The great production of this oceanic system is manifest in the tremendous quantities of marine birds in this area. Some idea of the magnitude of production can be had from Schott's report (1932) that on one small island of the Chinchas group there are estimated to be some five or six million marine birds, such as cormorants, pelicans, and gannets, which daily remove at least 1000 tons of small fish from the surrounding water. The great Peruvian

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guano deposits on the shores of these regions may be as much as 30 m in thickness.

In comparing the great productivity of the Peru Current with that of the Antarctic, it must be borne in mind that the latter system is one of very great geographic extent while the former, like other highly productive areas in lower latitudes, is more localized and to that degree of less general significance in the economy of the region as a whole. The greater fisheries of the north and north temperate regions are in themselves indicative of great organic production and they have profoundly influenced the settlement of coastal areas by man.

It should be mentioned that the question of relative production in high and low latitudes is still an open question. Some evidence points to a greater production in the latter than has been indicated by the various oceanographic expeditions whose data have been concerned mainly with the open sea. An example of subtropical production in a restricted area is offered by the investigations of the Great Barrier Reef Expedition.

In using the zooplankton as an index of productivity, Russell (1934) concluded that the Great Barrier Reef lagoon was somewhat richer in numbers of plankton animals than all but one of five representative stations in the North Sea and the English Channel. Over two years the monthly average at these northern stations was 2815 animals per vertical haul with a 50-cm net, while a one-year investigation with a similar net, used in the same fashion in the Barrier Reef lagoon, showed the number of animals to be 5684 per haul. Usually copepods made up 70 per cent or more of the animals. In tropical regions copepods are usually less than 1 mm in length, while in the north the most abundant species are over 1.5 mm in length. When allowance is made for this small size and for a supposedly more rapid rate of metabolism for the tropical species, the conclusion is that the two areas are about equal in production of zooplankton. The phytoplankton was always scanty in the Barrier Reef lagoon, however, and this appears to be of special interest in indicating that the area was supporting its maximal zooplankton population and that, because of the even rate of phytoplankton production, no surplus of plants was being produced as it is in higher latitudes during the height of the growing season. This equilibrium is of significance also in the study of sedimentation, for it is in the high latitudes that the diatom oozes are found.

Recent studies indicate a need to consider more closely the significance of the vastly deeper euphotic zone of the tropical seas. In one investigation (Riley, 1939) it was found, for example, that in stations of latitude about 40°N plant pigments were most abundant at the surface, the maximum depth of occurrence being 100 m, whereas at tropical and subtropical stations the pigments were most abundant at 100 m and at depths of 300 to 400 m.

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Many investigations have indicated a lower production of phytoplankton and zooplankton in the oceanic waters than in the neritic waters. The blueness of the waters of the open sea, as indicated in fig. 214, p. 784, is correlated with this fact. This difference is what one would expect, since the ease of nutrient renewal both by water circulation and by runoff from land is all in favor of the coastal regions. The offshore regions are favored, however, by a greater transparency of the water owing to the absence of suspended particles of terrigenous origin.

In neritic areas, the diatoms form resting spores that are believed to be important in quickly “seeding” the waters when favorable conditions return. The oceanic flora depends upon survival of sporeless individuals to initiate the crop. This may at times result in a scarcity of suitable species to take advantage of the return of favorable conditions (see p. 770), but a limited nutrient supply is doubtless the main reason for low oceanic production.