Direct and Indirect Secondary Factors Influencing Population Density
We have already described the methods of reproduction characteristic of the plankton algae. These methods, which involve mainly simple cell division, are highly efficient and under optimal nutritional and metabolic conditions result in building up by geometric progression an excessively dense population. Any factor, then, that disturbs this progression has a far-reaching effect on the total numbers that can be produced in a given period of time.
By way of illustration, Braarud shows in a hypothetical example the difference between two identical populations of 500 cells per liter after nine cell divisions, when one population suffers no loss between successive cell divisions, while the other is assumed to experience a net loss of 10 per cent between the successive divisions. The undisturbed population would have increased at the end of the given time to 256,000 cells per liter, whereas the other at the same time would have only 99,159 cells per liter. The total withdrawal in cells would be only 12,343, but these plus their potentiality to produce yet other cells in the given time would amount to 144,498 cells per liter, in other words, the difference in increase between the two initial populations of 500 cells per liter at the end of nine cell divisions.
Just how much time would elapse during nine cell divisions in nature is dependent upon the living conditions and the species involved. The rate of division is dependent, of course, upon the direct primary factors mentioned above. In culture experiments (Gran, 1933) in the sea at Woods Hole it was found that population increases corresponded to seven divisions in three days for Chaetoceros compressus, six divisions in five days for the oceanic species Rhizosolenia alata, and five to six divisions in three days for other species. It has been estimated by Harvey et al (1935) that diatoms divide at a rate of once each 18 to 36 hours.
In nature, the number of cells lost between successive divisions would not remain constant even when resulting from consumption by a uniform population of filter-feeding animals, for there is some evidence that the rate at which these animals filter water remains the same whether or not the water is thinly or thickly populated with plankton food (p. 901). Harvey et al also show that during periods of abundant food the grazing plankton animals consume more than they are able to digest and void pellets of only partially digested diatoms.
Consumption. Since the phytoplankton is the main pasturage of the sea, it is very heavily drawn upon by the countless numbers of herbivorous plankton animals living within or periodically invading the euphotic zone. This has long been recognized as a foregone conclusion (Hensen, 1887, Lohmann, 1908),
The planktonic copepods are doubtless the chief diatom grazers, and due to their vast numbers they must exact an exceedingly heavy toll. In examining the digestive tracts of copepods, Dakin (1908) found that diatoms were the most abundant organisms present, especially species of Thalassiosira and Coscinodiscus. Species of Peridinium were next in importance. Esterly (1916), in direct examination, also found evidence of phytoplankton grazing, there being included in the digestive tract diatoms, dinoflagellates, and coccolithophores. Lebour (1922), Marshall (1924), and others have also shown that diatoms are important in the diet of copepods.
In addition to the copepods there are also large numbers of other small animals, especially the larval stages of benthic invertebrates, that graze directly upon the phytoplankton. For example, larval annelids are voracious feeders and, when abundant, may diminish materially the diatom population. Lohmann, working at Kiel, calculated that during the course of a year the increase in volume of pelagic plants was 30 per cent daily. Hence, that fraction of the plant population could be consumed daily by grazers without endangering the plant stock, if no other loss occurred.
The unevenness of phytoplankton distribution both in space (horizontally and vertically) and in time may reflect relative intensity of grazing rather than control through nutrient or physical conditions of the water, and actually more organic production may have taken place in areas only moderately rich in plants at any given time (p. 901).
Sinking. Even though diatoms are especially adapted to a floating existence, the suspensory organs and analogous structures usually operate only to retard sinking and not as a complete counter to the constant pull of gravity. Therefore, many individuals of the population tend to sink below the euphotic zone and may be considered as being withdrawn from the reproducing population as completely as if consumed by animals in the euphotic zone, unless before death they are fortuitously returned to the lighted waters by ascending water movements. According to the definition of compensation values (pp. 778 and 779), when the depth to which the plants have sunk is characterized by light with intensity below the “compensation point,” theoretically they would be considered as having become, at least temporarily, nonproducers for the twenty-four hours considered. It is probable, however, that the slowly sinking specimens have an adaptability enabling them to carry on photosynthesis sufficiently to reproduce slowly at a lower illumination than is indicated for experimental cultures at the compensation point. Harvey (1939) found a “physiological state” in Ditylum which indicated an adaptability to
In connection with the velocity of sinking, temperature becomes an important indirect factor influencing production owing to its marked effect on viscosity. A marked discontinuity layer, providing a sharp temperature decrease within the euphotia layer, is an important feature in maintaining the plants in the productive layer, since viscosity is inversely proportional to temperature and the rate of sinking is inversely proportional to viscosity.
Vertical Transport. Some of the points discussed in relation to sinking will be equally applicable in the factor of vertical transport. We learn from the discussion of turbulence (p. 471) that water particles within a water mass are not at rest but may be shifted in various directions. Passive microscopic objects, such as diatoms, caught in these shifts are transported along with the water. But as far as transport is concerned, we are interested here mainly in the vertical component of turbulence. If this vertical component is confined to the transportation of particles up or down within the euphotic layer, its effect on the production will be of no great significance, although it is of importance in maintaining a “seed” supply in the euphotic layer. If, however, the movement involves a transport of water from the productive layer downward into darker layers, it will have a deleterious effect on production in so far as it carries with it a certain portion of the phytoplankton population. This deleterious effect may be partly compensated for by a return of normally sinking plants to the lighted zone by ascending currents, but the time spent below optimum light conditions must constitute a loss.
Indirect Factors. The indirect factors of productivity also belong to the secondary group and operate simultaneously with them, in most instances also with the direct primary factors. Temperature, for instance, modifies the rate of metabolism, while at the same time it regulates the viscosity, and the vertical temperature gradient tends to reduce active turbulence. On the other hand, turbulence, upwelling, or convection currents are essential to the return of nutrients to the euphotic layer; thus the effect on the population is very complicated.
Among the more obvious of indirect factors of productivity may be listed water movements of various kinds, stability of waters within the euphotic layer, discharge of rivers, meteorological conditions, bathymetric conditions, and geographic position. The separate indirect factors exert their influence on productivity through the effect on the direct factors, and also on the other indirect factors. As a result, a complex of forces is constantly at play to increase or to decrease the total production during shorter or longer periods of time.