Phytoplankton Production

Data on phytoplankton production may be obtained by direct census or pigment analysis of the phytoplankton population, or through chemical analysis of the water in which it has grown.

Direct Census. Any sample of the population is only a measure of the standing crop, that is, the density of individuals or total volume or weight present at the moment and place of sampling. The common method of phytoplankton population analysis consists of centrifuging or filtering a known volume of water and counting directly under the microscope the number of cells of species making up the population thus concentrated. The population is then reported in numbers of cells of the various species prevailing per liter or per cubic meter of the water.

This method is essential to detailed analysis of the constituents of the phytoplankton communities and for arriving at an understanding of the factors operative in controlling the production of the different ecological types. To illustrate this it is necessary only to recall the plankton diatom types of Cleve (see p. 791) or to consider the divergence of requirements of the two main synthesizing groups, namely the diatoms and dinoflagellates. When the capacity of diatoms to continue a lively rate of organic production drops with increasing temperatures or diminution of essential nutrients, this drop is often in part compensated for by a flare of production by the warmth-loving dinoflagellates, which by reason of their low nutrient requirements and their motility may be able to utilize the nutrients below the lower threshold for spring diatoms (see p. 385).

It is, however, extremely difficult to arrive at accurate estimates of volumes or weights of organic matter through plankton counts alone, since diatom and dinoflagellate species and even individuals of the same species vary greatly in size and organic content and an appreciable portion of the diatoms is inorganic material forming the siliceous shells. Measurements of volume are sometimes made by centrifuging or allowing the plankton catch to settle in graduate cylinders and noting the volume of the precipitated material in milliliters. This method, however, does not serve to separate plant from animal material and the widely different types of organisms with respect to radiating spines and fluid content prevent comparable compactness in settling, with the result that the volume readings represent relative values which give only the major differences. This objection is partly overcome by measuring the total catches in terms of the number of milliliters of fluid which they displace. This is accomplished by noting the fluid volume before and after filtering out all of the organisms. It is not possible however, completely to drain

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off all of the water, hence the volume obtained includes the water residue and also the inorganic structures of the organisms.

Plant Pigments and other Plankton Equivalents. In view of the fact that the direct plankton measurements are inadequate for arriving at figures of total vegetative organic matter in the standing crop or the total production for a given period of time, attempts are made to use chemical analysis of the essential plant nutrients that have been consumed from the water by the plants in their production.

In order to estimate the organic content of a standing crop of phytoplankton, Harvey (1934) has described an adaptation of the method of Kreps and Verjbinskaya (1930) whereby the yellow-green pigment present in the plants filtered from a known volume of water is extracted by acetone and the pigment thus obtained in a known volume of acetone solution is determined colorimetrically with an arbitrary standard of 25 mg of potassium chromate and 430 mg of nickel sulphate dissolved in 1 : 1 of water. One milliliter of the standard solution is equivalent to one “pigment unit” of chlorophyll. This pigment unit amounts to 0.88 ± .01 μg (1 μg = 0.001 mg) of chlorophyll (Riley, 1938) or the equivalent of 3.3 × 10⁻³ mg of organic carbon. The total organic carbon content can then be calculated for the volume of water filtered. For more direct comparison with plankton counts it is also computed that, in the English Channel, one pigment unit is equal to 4000 diatoms of “average cell contents.” Additional plankton equivalents are summarized in table 101, compiled by Fleming (1940).

In this table the “wet plankton” is defined by Atkins (1923) according to whom 1520 mg of wet plankton contains one milligram of phosphorus. This definition, however, is not based on analyses of phytoplankton but of seaweed and may be subject to revision. The equivalents of dry plankton, settling volumes, and displacement volumes are based on observations by Moberg and by M. W. Johnson (unpublished material). The equivalents of carbon, nitrogen, and phosphorus are based on the experimental results that the ratios C:N:P = 41:7.2:1 by weight. If other ratios are introduced, the equivalents must be altered correspondingly. The oxygen equivalent is obtained by assuming that the oxidation of one atom of carbon requires two atoms of oxygen, but Gilson (1937) suggests that the oxygen equivalent should be 20 per cent greater because allowance should be made for the oxygen consumed in oxidizing ammonia to nitrate.

So far, we have dealt with population only. To arrive at the total production of diatoms, for example, for a given unit of time it is necessary to have a series of samples over an interval of time so that calculations can be made on the basis of differences between the standing crops at each sampling. The average reproductive rate of diatoms constituting the population, the rate at which they are regularly eaten by the animal plankton (cf. p. 901), and perhaps also the rate at which they sink below the euphotic zone are factors for which corrections must be made in the final calculations. An example from Lohmann's observations (1908) at Kiel will illustrate the procedure. Lohmann calculated that the daily plant increment was 30 per cent of the standing crop, that the protozoan animals consume daily one half their own weight in plants, and that the metazoan animals (copepods and other forms above the protozoa) consume daily one tenth their own weight. The daily phytoplankton production would then amount to the 30 per cent increase assigned to the rate of production, but if this increase be exactly the same as the day's ration required by the volume of animals present per cubic meter of water for the day, the standing crop of diatoms would show no increase for the day.

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As stated elsewhere, Lohmann arrived at figures indicating a plant production of 10 mm³/m³ during February, the month of minimum plant production. But though the animals were few during the same period their food requirement was 18 mm³/m³. This left a daily deficiency of 8 mm³/m³ in food production for the month. The standing crop must therefore have been on a decline due to grazing, provided the animals at that season actually consumed the normal ration calculated for that month. On the other hand, most months showed a considerable surplus in production of plant material per cubic meter of water over that of consumption by animals. In August, the month of highest production, this surplus amounted to 290 mm³/m³. During that month the animals were also numerous and required a daily supply of food equal to 60 mm³/m³. Therefore the daily total organic material produced in August must have been 290 + 60, or 350 mm³/m³.

It should be noted here, however, that the production and consumption rates are not so simply estimated as assumed in these calculations. The rate of diatom division is not constant as here indicated, but may vary within wide limits between individuals or species, depending upon nutritional and physical factors which vary greatly over the year. Gran (1929) estimated that Ceratium spp. during the summer may have a daily increase of 30 per cent whereas some diatoms, for example Chaetoceros curvisetum, under favorable conditions may increase over 350 per cent daily. Thus during the period of maximum volumes of plants the conditions for rapid multiplication must have been more pronounced than during the periods represented by minimum volumes. With regard to consumption by animals, recent information indicates that the grazing animals apparently do not eat a fixed ration daily in relation to their weight, but instead during periods of high phytoplankton production eat much more than they can digest. This undoubtedly applies to filter feeders, which consume amounts of plants proportional to the concentration of the plants in suspension during the time of filtering. The

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rate of feeding is also controlled by such external factors as temperature. In view of the uncertainties involved in computation of production on the basis of observed changes in population, attention has been directed in recent years toward estimates of production on the basis of changes in the concentrations of essential plant nutrients or on the basis of observations as to oxygen production or consumption.

Plant-nutrient Consumption as an Index of Organic Production. Changes in the concentrations of the essential plant nutrients coincident with plant growth are highly useful as indexes of production. Redfield, Cooper, and others have demonstrated the existence of a rather constant ratio between the elements carbon, nitrogen, and phosphorus in the organic content of mixed plankton (p. 768). The ratios C:N:P = 41:7.2:1 by weight, or 106:16:1 by atoms, are also in close agreement with the proportions of these elements in sea water (p. 236). Therefore a measurement of the drop in any one of these elements in the mineralized state in the sea water could reasonably be supposed to indicate an equivalent incorporation into organic material during synthesis of carbohydrates or proteins. The assimilation of CO₂ and its effect on pH was used as an index by Moore in estimating the organic production in the Irish Sea (p. 768) and it has subsequently been used elsewhere in conjunction with other chemical changes associated with plant growth. When CO₂ utilization is used as an index, the respiration of animals and the tendency for CO₂ of the water to be in equilibrium with that in the air must be taken into consideration.

During winter in the higher latitudes there is a maximum accumulation of phosphate in the water. This, when returned to the euphotic zone, is the available supply for the following spring growth of plants. In chapter XVI we noted that in some areas a vigorous consumption by plants may lead to a depletion of this nutrient during the growing season. In such an event the area has yielded its maximum production until further regeneration has taken place or further renewal has occurred, for instance by currents from subsurface nutrient-rich layers or by influx from elsewhere. There is considerable variation from year to year in the amount of phosphate that has been drawn upon by the plants and this must be a reflection of the variability in the capacity of plants to use the supply available because of other environmental factors either inorganic or organic which promote or retard the use of the nutrient. In a review of phosphate records covering fourteen years of observations in the English Channel, Cooper (1938) found that the percentage consumption of the total inorganic phosphate of the water column fluctuates between 49 per cent and 81 per cent for the spring productive period and between 63 per cent and 93 per cent for the entire period between the winter maximum and the summer minimum of phosphate. He points out that by April, on an average, one half of the phosphate supply

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for the year has been used up. An average annual utilization of about 78 per cent of the available supply, if occurring in such areas as the Antarctic or Puget Sound, where the store of phosphate is high, would lead to an exceedingly high annual organic production. Not only is there variability in the efficiency of plants in the utilization of phosphate, but the supply also varies from year to year owing to the nature of water flowing into the region. For example, Cooper's figures show that the average maximum phosphate supply available for the vernal plant production varies from an average of 0.67 μg-atom/L to only 0.47 μg-atom/L. This has a profound effect on both plant and animal production. Years of low phosphate are correlated with scarcity of plankton animals and poor production of young fish. The differences between the winter maximum and summer minimum of phosphate can, however, give only an approximate minimal measure since in the euphotic zone some regeneration of phosphate is known to occur, which would permit of interseasonal turnover in the supply and thus make possible a greater total production.

In connection with phosphate regeneration it should be pointed out here that when a higher concentration of phosphate occurs in isolated areas below the euphotic zone than occurs in the waters forming the regular source supply of phosphate to the euphotic layer, such phenomena must indicate that the excess phosphate has been regenerated in situ. The high phosphate values reflect a high organic production in the immediate overlying waters or in the surface layers of the region from which the phosphate-rich water spreads out. In these instances the growing organisms have served to entrap the phosphate in their bodies which subsequently sink and decay at intermediate depths, thus leading to high concentrations of this substance in these layers. As illustrative of this cycle, see fig. 45, p. 239. Such areas also show low oxygen content as a result of oxidation of organic material.

As a complementary study to phosphate utilization Harvey et al (1935) made analysis of the phosphorus compounds of the planktonic organisms. They found only a fraction of the phosphate removed from the water to be present in the plankton population, and concluded that much of the organic phosphorus compounds passes through the animals undigested and is then dissolved in the water until it is later regenerated to inorganic state.

The utilization of nitrate by the growing phytoplankton also gives an index of seasonal production. The various steps in the regeneration (see p. 913) are believed to retard the process so that nitrate accumulation during winter is especially pronounced in the northern latitudes, where well-defined growing seasons occur. However, since it is only the nitrogen that plants require from this compound, they can acquire this nitrogen from ammonia, and perhaps also from amino acids. Ammonia

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is an early product of putrefaction of organic material (p. 914), and therefore the nitrogen supply may, like that of the phosphate, experience some interseasonal turnover in its inorganic and organic phases.

Oxygen Production and Consumption as an Index of Organic Production. The plants obtain carbon for carbohydrate synthesis from the CO₂ in the water according to the equation

Above the level of the compensation depth (p. 779), so much oxygen is evolved as a by-product that the surrounding water may become supersaturated. A measure of this oxygen production provides a means of calculating the amount of carbon that has been bound in organic compounds. For each 1 ml of oxygen set free, 0.536 mg of carbon has been assimilated. Experiments on the rate of assimilation at various depths in the Gulf of Maine (table 93, p. 777) showed a maximum oxygen production of 2.33 ml/l at the surface in 9 hours and 10 minutes. The oxygen production decreased with depth and reached zero at a depth of about 44 m. For the whole water column beneath 1 m² the total production of oxygen equaled 29.1 1, corresponding to a fixation of 15.6 g of carbon for the duration of the experiment. This example illustrates part of the technique which can be employed in order to determine the gross production in the sea. In the quoted experiment the submerged water samples were all taken from the surface layers and the plankton associations in the bottles were therefore not identical with those at the depths where the bottles were exposed. In order to obtain correct values of the gross production in any locality, plankton samples should be taken from a series of depths and should be exposed at these depths, but so far no results of such experiments have been published.

In the experiment to which we have referred (table 93, p. 777), the exposed plankton association was a mixture of phyto- and zooplankton. The respiration of both plants and animals is therefore included, and this explains the high observed values of the respiration. Observations on pure cultures of phytoplankton organisms indicate that the ratio of respiration to maximum photosynthesis is usually less than 10 per cent. In a pure culture of Nitzschia closterium (Barker, 1935a) this ratio was 8 per cent; in a pure culture of a species of Peridinium (Barker, 1935b) it varied from 7 per cent to 14 per cent; in another culture of Nitzschia closterium (Clarke, 1936) it was less than 1 per cent; and in an apparently pure culture of Coscinodiscus excentricus (Jenkin, 1937) it varied from 3 per cent to 12 per cent.

In these instances the phytoplankton production would nearly equal the gross production, but the values were obtained under conditions when photosynthesis was rapid. On an average for a whole year the ratio of

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respiration to assimilation must be greater and may in some regions be as high as 50 per cent, in which case the phytoplankton production will be only half of the gross production (Riley, 1941).

The efficiency of the gross production, that is, the fraction of the energy penetrating the sea surface which is utilized in photosynthetic activity, is, in Long Island Sound, 0.55 to 0.82 per cent (Riley, 1941).

In nature, the oxygen accumulates in the layers of organic production and thus its fluctuations in time and space give a measure of the intensity of phytoplankton outbursts. It can provide only minimal values because the exact quantity of oxygen produced is obscured by the respiratory activities of animals and bacteria, and in case of surface supersaturation some oxygen is given off to the atmosphere.

Associated with the above is the problem of estimating the production of organic material by a study of the oxygen consumed in the eventual oxidation of the material below the water layers in which it was produced. This association necessitates a consideration of the oxygen used in the oxidation of the carbon and in converting ammonia to nitrates, but the hydrography of the area must be such that the water layer in which oxygen consumption is being measured can be definitely referred to the euphotic layer in which the production of organic material is supposed to have occurred some time previously. As yet no method has been developed by which the amount of oxygen consumed can be determined directly, but indirectly it can be found if it is assumed that in the subsurface layer the distribution of oxygen is stationary (p. 161). On this assumption the amount of oxygen which in unit time is brought into a given volume by processes of diffusion and advection must equal the amount which in the same time is consumed in the same volume. The replenishment of oxygen by diffusion and advection can be determined if sufficient hydrographic data are available (for example, Seiwell, 1935, Sverdrup and Fleming, 1941), and so far our knowledge of oxygen consumption in the subsurface layers is derived from such computations only.

In closing this brief review of the various indices of organic production in the sea, it may be stated that the most reliable figures are, doubtless, to be obtained by a combination of the various methods, both biological and chemical, in order to provide corroborative evidence.