. Fresh-water biology. Freshwater biology. INTRODUCTION 7. those that (Fig. i) pass through the meshes of the finest silk gauze, Swiss bolting cloth No. 25,* having meshes that measure to mm. square. The nannoplankton is composed chiefly of flagellates and algas; although bacteria are constantly present they apparently form but a minor con- stituent in bulk and weight. The number and variety of these or- ganisms is truly astonishing even in the clear waters of Alpine lakes where according to Ruttner they stand to the organisms of the net plankton numerically in the ratio of 160 :3 an
. Fresh-water biology. Freshwater biology. INTRODUCTION 7. those that (Fig. i) pass through the meshes of the finest silk gauze, Swiss bolting cloth No. 25,* having meshes that measure to mm. square. The nannoplankton is composed chiefly of flagellates and algas; although bacteria are constantly present they apparently form but a minor con- stituent in bulk and weight. The number and variety of these or- ganisms is truly astonishing even in the clear waters of Alpine lakes where according to Ruttner they stand to the organisms of the net plankton numerically in the ratio of 160 :3 and at least two-thirds of them are still un- described and difficult to include in known genera. The maximum number of nannoplanktonts thus far recorded is from Lake Men- dota, Wis., where Cyclotella has been found to the number of over 30,000,000 per liter of water. Ruttner also calcvdates the vol- ume of the nannoplankton in the Lunzer lakes as three times that of net plankton. According to Birge and Juday the weight of its dry organic matter varies in three Wisconsin lakes from slightly less (rarely) to 15 or 20 times more than that of the net plankton and is ordinarily 5 to 6 times as great. This amount is unquestionably of marked importance both scientifically and practically, and the character of the or- ganisms indicates even more clearly their fundamental impor- tance in the problems of aquatic biology. Plankton organisms are characterized by transparency, delicate colors, and above all by their power of floating due *^.o buoyancy and * New No. 25 is identical with No. 20 of older authors (Lohmann). Fig. I. A piece of bolting cloth No. 20 with plankton organisms drawn between the meshes to show relative size. Above, Rhizosotenia alata. Upper row, left mesh: Gymnodinium, beneath Ampltidmium rotundalum and ExuviaeUa baltica, right Pouchctia parva; middle mesh: Prorocen- trum micans and Rhynchomonas marina; right mesh: Nitschia sigmatella. Achradina pulchra, Halteria rubra,
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectfreshwa, bookyear1918
