Introduction

The importance of bacterial productivity in the production process of lakes has been

emphasized by many authors (Kusnetsov 1958, 1959, Nauwerck 1963, Sorokin 1964,

1965, Kusnetsov & Romanenko 1966, Overeeck 1969). Bacteria are not only the final

stage of the food chain but also play a considerable role for subsequent producers.

Within the I. B. P. it seemed desirable to study their importance in water bodies being

very different from those investigated before.

The Vorderer Finstertaler See (Kühtai, Tyrol, Austria) is situated in the Central Alps

at an elevation of 2237 m a.s.l., above the timber line. The lake has a maximum depth

of 28.5 m, an area of 15.8 hectares, the volume of the water body is 2,33 • 10 6 m 3

(cf. Pechlaner 1966, Pechlaner et al. 1972). Thermal stability is low due to insignificant

warming-up and intense water renewal. Maximum surface temperatures never exceed

12° C. A distinct metalimnion is absent. As consequence of the high transparency

of the water, photosynthesis reaches the bottom of the lake (cf. Tilzer 1972).

Since the content of dissolved oxygen never decreases to zero even above the sedi¬

ment, only aerobic processes can be assumed for the pelagic zone. The lack of measurable

amounts of H 2 S and CH 4 makes credible that chemosynthesis is of minor importance. At

the surface of the mud, however, deposits of iron oxides suggest the existence of iron bac¬

teria. The investigations presented here are restricted to the pelagic zone.

We can assume that in the free water oligocarbophile aerobic saprophytes are domi¬

nating. As found by Sorokin (1964,1965) 3—5 %, and by Kusnetsov & Romanenko (1966)

6 % of the total productivity of aerobic heterotrophic bacteria is effected by the uptake

of dissolved CO 2 . Besides the measurement of algal photosynthesis by the method of

Steemann-Nielsen (1952) the heterotrophic production rates were estimated by the dark

uptake of 14 C (Kusnetsov & Romanenko 1966, Overbeck 1969). Bacterial biomass was

determined by direct counts on membrane filters (Sartorius, Göttingen, pore size 0,2 fixa)

after staining with erythrosine (Ivanov 1955, Kusnetsov 1958, 1959, Overbeck pers.

comm.).

Results

As shown in Fig. 1 A, bacterial biomass is highest in the ice-free period. Cell

numbers are within a range of 220—800 • 10 3 cells per cubic centimeter. While in

lowland-lakes both algae and bacteria show distinct and most characteristic vertical

stratifications, this does not seem to be the case in high-mountain lakes. Bacterial

biomass is almost unstratified. Contrary to this, phytoplankton shows a charac¬

teristic depth-time distribution (hatched area in Fig. 1A). Phytoplankton pre¬

dominantly consists of flagellate forms which can maintain stratification against the

relatively high turbulence of the water body, whereas bacteria are suspended almost