Plant Nutrients and Vertical Circulation of Water
The inorganic substances required by plants are commonly spoken of as “plant nutrients.” These constitute the raw materials or building blocks out of which the plants, with the aid of sunlight, build up organic compounds entering into their cell structure. The growing plants can obtain these raw materials only from the sea water in which they live. This must therefore be looked upon as a dilute nutrient medium containing all of the elements necessary for plant growth. The concentrations of the nutrient salts are remarkably small; yet a concentration of 1 mg of phosphorus to 10001 of water, for example, is sufficient to produce a vigorous growth of diatoms if other factors are favorable. This is facilitated partly by the very minute size of most phytoplankton organisms and may be illustrated by comparing one plant 1 mm3 in size with a
The mineralized nutrient elements are not uniformly distributed in the sea, however, and they undergo cycles during which there are periods of delay between the available mineralized state and the unavailable organically bound state of the nutrients. This leads to fluctuations in the intensity of plant production both in space and in time. The production of marine phytoplankton is nature's greatest demonstration of hydroponics, or water culture. The operation of the whole system in detail in the sea is different in many respects, however, from the operation of experimental hydroponic tanks. For example, there is a difference in the population; as we have learned, the floating plants of the sea are not specialized with aerial portions for utilization of CO2 and elimination of O2; and roots are not provided for intake of mineral nutrients; neither are there elaborate vascular systems to transport fluids and food. All of the metabolic processes of the marine phytoplankton are carried on within the individual cells which are always submerged beneath the water surface. This universality means that the whole plant must receive light, and the specializations, as already discussed, are along lines designed to maintain the plants in the upper lighted portions of the sea. In the sea the mineral nutrients are not added from without as needed; they have accumulated from the land over eons of time and are stored mainly in the deeper dark-water layers where they are not accessible to autotrophic plants and from whence they must be transported by water movements to replenish the supply periodically exhausted in the productive lighted zone. For discussion of the forces causing this transport see chapter XIII.
It is well known that with return of sunshine and increased temperatures during spring in the temperate and higher latitudes there is a phenomenal outburst of phytoplankton growth in the surface waters. Even the most causal observer is impressed with the brown-green color that is imparted to the waters of coastal areas and over banks during these periods of diatom “bloom.” The production may go on at varying intensities during the summer and frequently becomes augmented again in the autumn. But during the winter months, and sometimes at other seasons as well, there is a dearth of plants and the waters again become deeper blue in color. Similar conspicuous changes in water color occur when one leaves the rich coastal waters and proceeds to the barren open
Brandt's theory that phosphates and nitrates may constitute limiting factors in phytoplankton production has found proof in investigations of subsequent workers (Marshall and Orr, 1927, Schreiber, 1927, Gran, 1930, Hentschel and Wattenberg, 1930, and others). At the initiation of vernal phytoplankton production these salts are relatively abundant in the euphotic layer, but as production proceeds the quantity gradually falls off until it can no longer maintain a large population, and the organisms diminish in number or disappear from the water.
Color of the sea as indicated in per cent of yellow according to the Forel scale (from Schott).
Here Nathansohn's theory forms a complement to Brandt's theory in the explanation of irregularities in distribution and periodic fluctuations in phytoplankton production. (See p. 901 for control through organic factors.) Nathansohn (1906) suggested that the nutrient salts must disappear from the lighted surface layers owing to the fact that they are consumed by the plants and that they must therefore accumulate in the deep sea through sinking of living or dead bodies, both plant and animal, constantly settling towards the bottom. The store of nutrients thus accumulated in the deep water through the dissolution of these bodies is, in time, returned to the euphotic layer through diffusion and vertical circulation of the water.
With improvements of methods in chemical analysis it has, indeed, been shown that usually phosphate concentrations do increase with
The average number of diatoms per liter between the surface and 60 or 70 m, off the California coast. Character of currents is indicated by heavy lines with arrows, and distribution of diatoms is indicated by hatching (modified from Sverdrup and Allen).
Upwelling. The upwelling of subsurface water in which nutrient salts are brought back to the lighted layers is reflected in large diatom production along the California coast (Moberg, 1928, Sverdrup and Allen, 1939; fig. 215), along the western South American coast through the mechanism of the Peruvian Coastal Current (Gunther, 1936), in regions of divergence along the equatorial countercurrents, and along
Not all surface currents show this tendency to unusually rich fertility indicated by the above great coastal populations and the seaward tonguelike extensions. In seeking an explanation we find that the upwelling along the African coast has brought to the surface nutrient-rich water, fertilizing the illuminated euphotic layer. The plants respond with a luxuriant growth, which diminishes in intensity as the nutrients are gradually consumed by the population in the waters as they move to greater and greater distances away from the coast and from the regions of upwelling. Figure 217 shows how closely the distribution of phosphates coincides with the areas shown by direct examination of plankton catches to be fertile in phytoplankton.
The concentration of total plankton (micro- and nannoplankton) in the South Atlantic, surface to 50 m. The numbers on the curves represent thousands of individuals per liter.
The distribution of phosphates as shown in fig. 218, which represents a vertical section from the coast of Africa across the Atlantic to the coast of South America at lat. 9°S, also illustrates how nutrient-rich water is nearer the surface on approaching the African coast. The water with more than 1 μg-atom/L (30 mg/cm3) rises to within about 40 m of the surface, whereas on the South American coast water of nutrient concentration greater than this is not found above about 1000 m. It must be borne in mind that not only phosphates, but also other nutrients needed for plant life that have been regenerated in the deep waters from sinking bodies of organisms produced in the upper layers, are conveyed from the
Regions of Divergence and Convergence. Regions of divergence are defined as those regions where surface water masses of the ocean flow away from each other or away from the coast so that water from the deep must rise, as a feature of upwelling, to replace them. It appears that such areas, whether at the coast or in the open sea, are productive of phytoplankton; this condition is exemplified in the South Atlantic Congo tongue of cold water which extends far out to sea from the region of the Congo River (Hentschel, 1928). Conversely, regions of convergence are areas where surface waters meet and sink. Such areas are relatively barren of plants, the waters having been impoverished by earlier exhaustion of the nutrients.
The hydrographic features of these two types of water movements are discussed on p. 140, and graphically shown in fig. 198, p. 710, for a Carnegie section across the equatorial currents of the Pacific. Here are two parallel regions of divergence between which lies a region of convergence associated with the Equatorial Countercurrent. When the Carnegie plankton volumes from H. W. Graham's analysis (1941) are compared with the hydrography of this section, we find that the relative volumes of general plankton, which must reflect phytoplankton production, show greater concentrations at stations 157 and 151 which lie within the regions of divergence. This is brought out in fig. 219, where the upper diagram shows the relative plankton volumes (precipitated in special bottles) in centimeters per mile of haul, and the lower diagram shows the vertical water movements and selected isolines for phosphate. For greater detail, see fig. 198.
Horizontal distribution of phosphates in the South Atlantic: mg P2O5/m3 (from Wattenberg).
Turbulence. The effect of renewal of plant nutrients from the deeper water as the result of turbulence and accompanying diffusion is demonstrated especially in such areas as the Bay of Fundy and Puget Sound, where strong tidal currents flowing over uneven bottom and through tortuous passages maintain a relatively homogeneous water
Vertical distribution of phosphates in mg P2O5/m3 at lat. 9°S across the South Atlantic.
Upper: concentration of plankton in a section across the Pacific Equatorial Countercurrent. Lower: phosphate concentration for the same section.
As far as renewal of plant nutrients is concerned, upwelling and turbulence are similar in that they are more or less continuous throughout the year with some variation resulting from periodic changes in the direction or intensity of the winds (cf. p. 725) or from stabilization resulting
Where turbulence and upwelling are sufficiently vigorous to involve the euphotic layer, there may always be present a ready supply of nutrients to support at least a moderate plankton production. Under such conditions the cessation of production must result from other factors than depletion of nutrients. During the winter season the controlling factors in some latitudes may be the reduction of light or temperature. The cessation of reproduction during a portion of the year may well represent a period of rest induced by a change in conditions, at least for neritic species.
Stabilization. During recent years there has been a realization that for maximum production a certain degree of vertical stability is required within the euphotic layer. In areas of excessive turbulence this stability is wanting, with the result that, although nutrients and other factors may be optimal, only a moderate production of phytoplankton takes place because the descending currents withdraw a portion of the diatom stock into the deeper water where there is not sufficient light for photosynthesis. Ascending currents may bring some of these back to depths of adequate light. The length of time spent below the euphotic layer would then determine the extent of loss, if any, to total production. The complex problem introduced by turbulence may be stated as mainly one of balancing the beneficial factor against the deleterious factor: ascending currents constantly conveying nutrients from deeper water against descending currents withdrawing a portion of the population to suboptimal lighting conditions. The depth of turbulence and the thickness of the euphotic layer are important in this connection. An example of the beneficial effect is seen when comparing areas in the Gulf of Maine and the Bay of Fundy (Bigelow, 1926, Gran and Braarud, 1935). In the open Gulf, marked stratification due to summer heating leads to the exhaustion of nutrients in the euphotic zone with a subsequent drop in phytoplankton production. Over Georges Bank, however, a fairly rich diatom plankton is maintained during the summer owing to the turbulence of water flowing over the bank.
The deleterious effect is illustrated in parts of the Bay of Fundy where, in June, surface areas rich in nutrients, 0.3 μg-atom/L PO4-P (10 mg PO4-P per m3) and up to 4 μg-atoms/L NO3-N (60 mg NO3-N per m3) at a depth of 1 m, resulting from turbulence, were nevertheless poor in plankton because of the combination of turbulence and low transparency. Diatoms were fewer than 500 cells per liter in the most affected areas. The largest populations were found in areas of moderate stratification. Two months later some stabilization had occurred with the advance of summer, but the most turbulent areas still showed high nutrients and a small population of about 7000 or fewer diatoms per
Convection. In regions where there are marked seasonal changes in temperatures between winter and summer, the changes are of great importance not only because of the effect of temperature directly upon the rate of metabolism but also because of its indirect effect in the renewal of nutrients to the surface layers. This method of renewal is characteristic of fresh-water lakes, but is also very important in the sea, especially in the higher latitudes. The phenomenon is, of course, dependent upon the surface waters cooling to a point where their density becomes sufficient to cause them to sink and be replaced by upward movement of lighter and incidentally nutrient-rich waters from below. Similar effects result from pronounced evaporation and must contribute essentially to the maintenance of phytoplankton in the open tropical seas and in isolated seas like the Mediterranean. In areas where vertical circulation results from convection currents, the biological implications are somewhat different than those resulting when nutrient renewal is brought about through the agency of upwelling or turbulence. Except as discussed later, renewal of surface waters by convection is a seasonal rather than a continuous process. The replenishment of elements essential to plants occurs during autumn and winter, with the result that the euphotic layer is richly fertilized in readiness for diatom growth as soon as sufficient sunshine becomes available in spring and as soon as other favorable living conditions are attained. A period of intense growth then ensues, resulting in depletion of nutrients, and since no general renewal can take place until autumn such areas typically experience a marked summer minimum in diatom production following the spring maximum. There is usually a secondary autumnal maximum before the gradually diminishing light reaches a point too low for effective photosynthesis. The regularity of this system may be disturbed by wind-induced turbulence sufficient to cause an upward transport of nutrientladen water from deeper layers, or by fertilization of the surface layers with nutrients brought down from land by rivers. Thus there may be alternating maxima and minima of greater or lesser duration and intensity throughout the whole growing season.
In boreal waters, when the spring production of diatoms has come to a low ebb owing to marked stratification and near exhaustion of nutrients, the role of organic production may be taken over in a diminished degree by the dinoflagellates, especially members of the genus Ceratium. These organisms, owing to their low nutrient requirements and slow rate of growth (daily increase under summer conditions of only 30 to 50 per cent as opposed to 360 per cent in the diatom Chaetoceros curvisetum) can develop at lower nutrient salt concentrations than can diatoms, and they may therefore continue to propagate in impoverished waters.