Diel changes in bacterial ingestion by a mixotrophic flagellate, Cryptomonas sp., and heterotrophic nanoflagellates (HNF) were examined in situ at 4-h intervals for 2 d in the epilimnion and metalimnion of Lake Biwa using bacteria-sized fluorescent microspheres as a tracer food. Clearance rates of HNF for the microspheres ranged between 1.3 and 4.5 nl cell Ϫ1 h Ϫ1 , but the average rate did not differ between day and night. In contrast, clear diel changes were observed in the clearance rate of Cryptomonas sp. in the epilimnion from Ͻ0.5 nl cell Ϫ1 h Ϫ1 at midnight to Ͼ3 nl cell Ϫ1 h Ϫ1 at noon. In the metalimnion where light intensity was lower, however, the clearance rate of Cryptomonas sp. was always Ͻ0.5 nl cell Ϫ1 h Ϫ1 through the study period. Thus, bacterial ingestion of Cryptomonas sp. is not to acquire supplementary energy or carbon at low phototrophic activities. During the study period, both inorganic phosphorus and nitrogen concentrations were less than or close to the detection limits (10 nM P and 1 M N) in the epilimnion, but much higher in the metalimnion. The results strongly support the idea that Cryptomonas sp. utilizes N and P from bacteria as substitutable nutrients when photosynthesis takes place under conditions of nutrient depletion. To assess the grazing effect of mixotrophic algae on bacterial populations, it is essential to consider diel changes in their phagotrophic mode of nutrition that are induced by light regime and nutrient concentrations in ambient water.

No abstract

No abstract

Lake Hovsgol is a large tectonic lake located in northern Mongolia, which has extremely transparent lake water. In our survey, the dissolved organic carbon of the lake water was 80-100 µM-C, and the fluorescence intensity in an excitation and emission matrix was very low. The brown color and high content of humic substances in river water flowing from a watershed consisting of grassland and forests rapidly declined in the coastal area of the lake. The decrease in humic content may be due not only to dilution by the lake water but also to flocculation and photobleaching. Among tectonic lakes in Asia, Lake Hovsgol would appear to have unique biological and hydrological features that reduce humic content and help to maintain water transparency.

No abstract