VERTICAL DISTRIBUTIONS OF BACTERIA AND ALGAE IN A STEEPLY STRATIFIED HUMIC LAKE UNDER HIGH GRAZING PRESSURE FROM DAPHNIA-LONGISPINA

The vertical distributions of bacteria and algae in a steeply stratifi ed, highly humic lake were studied during three 24 h periods in summer . The highest bacterial and algal densities and biomasses were recorde d in the anoxic hypolimnion. The bacterial biomass in the hypolimnion was composed mainly of photosynthetic green sulphur bacteria (Chlorobi um) which occurred at very low light intensity (< 1.5-mu-mol m-2 s-1). The numbers and biomasses of bacteria, both in the epilimnion and hyp olimnion, were mostly higher at night than during the day, indicating possible asynchrony between the production and loss of bacteria. Becau se of vertical migration, the diurnal vertical distribution of algae w as more variable than that of bacteria. Particularly in July and Augus t, when cryptomonads were abundant, the biomass of algae was much high er in the epilimnion during the day than at night. The flagellated chl orophytes, Chlamydomonas spp. and Scourfieldia cordiformis, stayed mai nly in the upper hypolimnion close to the oxic-anoxic boundary zone wh ere only a small proportion of Daphnia longispina was continuously pre sent. Unpalatable Mallomonas chrysophytes with silicified plates and b ristles, and small, presumably heterotrophic, flagellates stayed in th e oxic epilimnion together with a dense (up to 300 ind l-1) population of D. longispina. The results indicated that, besides the physical an d chemical properties of the water column, grazing pressure by Daphnia longispina strongly affected the vertical distribution of microorgani sms in this polyhumic lake.