By G. W. PRESCOTT
ALBION COLLEGE, ALBION, MICH.
THE history of limnology and the growth of the science of lake biology have been, from the very start, so completely interwoven that the two cannot even be thought of separately. There is no intention in this paper to deal with the history of our knowledge of lake phytoplankton, but to do so would be very interesting and illuminating. Such an examination would, for example, serve to establish an understanding of the present complete dependence of phytoplankton ecology on limnology, and at the same time would point out the many ways in which limnology has come to take count of phytoplankton and other aquatic plants.
Although biologists in the last part of the nineteenth century appreciated the determining influence of lake chemistry on the composition of aquatic populations, it was not until the early part of the twentieth century that correlations began to be understood and critically discussed. Limnological studies, especially those made during the past two decades by Birge and Juday, G. M. Smith, Pearsall, Thienemann, Naumann, Wesenberg-Lund, Welch, Ström, Hutchinson, Wiebe, Huber-Pestalozzi, and many others, have resulted in the accumulation of an enormous amount of data.
In this paper the term phytoplankton will refer only to algal floras, although we are well aware that certain phanerogamic plants may constitute important plankton components. The presence of a plankton flora in a lake suggests biological problems as intriguing as, or even more so than, those of a land flora. These problems, both of purely scientific interest and also of well- recognized practical importance, are many indeed, and only a few can be considered here.
It is obvious that these problems usually resolve about some phase of plant ecology, although taxonomy must of necessity serve as a systematizing basis for our knowledge.
When a plant known only from lakes of the Alps, or from Brazil, is next found in western North America or New England, or, when an association of phytoplankton species is found to have a world-wide latitudinal distribution, certain questions naturally come to mind. One of these is, of course: What do the lakes in Brazil have in common with those of New England which permit a certain species to thrive in them and not in others? Or, what are the factors which provide for a geographical distribution of a phytoplankton association?
Certainly the cosmopolitan distribution of many phytoplankters and the wide distribution of little-known or apparently rare species is good evidence that there has been ample time and opportunity for a much more equal distribution of algal species than we now find. In other words, if time and means of translocation were the only factors involved, we might well expect to find nearly the same species of plants in all lakes throughout the world. We do not, of course; and the reasons why we do not are to be found in the combinations of limnological factors which determine aquatic flora and fauna. In fact, we learn from limnology that lakes are even more exacting of their biota components than terrestrial habitats.
While we can follow Thienemann in his classification of lakes, we realize that each lake is an individual entity, a point which cannot be overemphasized. The many interacting factors of an aquatic environment, many of which are based upon or are related to the underlying or surrounding physiographic features as well, provide for innumerable types of habitats. Each lake represents a special and particular combination