Popular Science Monthly/Volume 68/March 1906/How Rooting Aquatic Plants Influence the Nutrition of the Food Fishes of our Great Lakes
By Professor RAYMOND H. POND
A THOROUGH investigation of the biology of our great lakes is in itself a desideratum worthy the expense. From the purely economic standpoint, however, our Bureau of Fisheries has long recognized the necessity of knowing more of those conditions under which products worth millions annually are produced. Until such an investigation has been made the natural factors which determine the quantity of food fishes these lakes can support must remain unknown.
Fish, as all other living forms, reproduce in a geometrical ratio and, other conditions being favorable, will multiply up to the limit of their food supply. Thus it is that the problem of nutrition is very fundamental. To determine the source of nutrition of our fresh-water food fishes is in itself a considerable undertaking, but to ascertain what factors regulate the quantity of this nutrition is a colossal task.
The nutrient relations of aquatic life are perhaps no more complicated than those of terrestrial, but they certainly are more difficult to determine because of the numerous obstacles to observation and collection of data. The higher orders of animal life are, of course, dependent upon the lower. The low free-swimming forms, called collectively zooplankton, are preyed upon by larger animals. The latter in turn are devoured by still larger forms and so on up to the fish. It is evident that all the animals above the zooplankton are dependent upon it, and whatever increases or decreases the quantity of zooplankton causes a fluctuation in the food supply of the fish. Thus it is that a quantitative study of the plankton forms so conspicuous a feature of the extended investigations which have been made both here and abroad of fresh-water biology.
Aquatic animals, just as terrestrial, are dependent upon plants for the organization of the elements of food into food. As there is an animal or zooplankton, just so there is a vegetable or phytoplankton. The latter is the living basis of the food supply of the aquatic fauna.
This phytoplankton lives on substances which it makes for itself out of carbon dioxid and water and the mineral matter in solution in the water. The supply of water and carbon dioxid is, of course, unlimited. The supply of mineral food varies considerably in different bodies of water and in the same lake several factors may operate to cause a fluctuation. The rooting aquatic plants have long been suspected of being one of these factors, but whether they increase or decrease the mineral food has not until recently been known.
The rooting aquatic plants may be considered in two groups according as they are submerged or emergent. The latter vegetation must, of course, take its nourishment from the soil. It has long been believed, and the statement is current in our latest and best text-books, that the submerged and rooting aquatics take their nourishment from the water which bathes them, that their roots are not for absorption, but merely for anchoring. An examination of the literature convinced me that so far as experimental evidence is concerned there is none which can be accepted as demonstrating this conclusion or a contrary one. The necessity of exact knowledge in this particular is apparent when we consider that if such plants, as hitherto supposed, feed from the water, they then during the entire growing season are diminishing the quantity of food for the phytoplankton. On the other hand, if
this food comes from the soil they not only do not reduce the plankton food supply, but actually become important contributors to it in that the mineral food taken by them from the soil finally through their decay becomes available to the phytoplankton. That the latter alternative is the true one may be seen from the following glimpse of an experimental study.
The special organs of absorption in terrestrial plants are the delicate root hairs which occur on the young roots in a narrow zone situated just back of the growing tip. The quantity of such hairs can, in the laboratory, be made to vary greatly by regulating conditions, showing that they are sensitive structures and not likely to be produced unless needed by the plant. Some land plants are known which do not have root hairs, but they are exceptions. On the other hand, aquatic plants have been supposed to be without such hairs. Examination of specimens taken from the lake bottom very carefully revealed their presence, however, and aquarium cultures in the greenhouse show that, like those on land plants, they are sensitive to the conditions of their environment. The presence of such hairs is almost prima facie evidence that the organs bearing them have an absorptive function in addition to an anchoring one.
This conclusion was tested in several ways. It was found that such plants could not make a normal growth unless rooted in favorable soil. Clean washed sand could not be substituted for good rich soil. Likewise a clay soil does not support a normal growth. Thus we find that these aquatics are quite sensitive to the nutritive quality of the substratum. In fact their existence depends upon this, as none of the larger plants growing submerged in western Lake Erie can mature a generation on the nourishment they are able to secure from the water alone. Chemical analysis shows that plants which have been denied soil nourishment are deficient in nitrogen, potash and phosphoric acid, as compared with others allowed to root in the soil.
Some plants are more dependent upon the soil than others, thus Ranunculus aquatilis made 63 per cent. better growth in soil than in clean washed sand. Elodea canadensis made 340 per cent. and Potamogeton perfoliatus 480 per cent. better growth in soil than in sand. Such results show clearly how important a place the rooting aquatics occupy in that chain of nutrient relations which stretches from the fish down through the orders of animal and plant life to the soil.
In the stocking of ponds for fish culture attention should be given to the plants allowed to grow. The species mentioned above are favorable, while Ceratophyllum is not. The latter plant often makes a very abundant growth. Though frequently accidentally anchored to the bottom, it does not bear roots and must, of course, take its nourishment from the water. A small lake in which this plant had made a very abundant growth was found by another investigator to have less plankton than other neighboring lakes in which the bulk of the vegetation was rooting.