Nutrient loading and drainage channel response in a ricefield system

The distribution of photosynthetic bacteria in Lake Kinneret and their contribution to primary productivity was investigated. Both Chlorobium phaeobacteroides and Rhodopseudomonas palustris were detected from mid-June through October in 1987 and reached a maximum of 7 × 10⁶ cells/ml in the metalimnion by mid-August. Chlorobium density was greater towards the end of the bloom period, when the thermocline was exposed to higher sulfide concentrations and lower light intensities. Rhodopseudomonas peaked earlier in the season, when less sulphide was present and light irradiation was higher. This was the first time that R. palustris was observed in Lake Kinneret; in addition to its presence at the thermocline, it was found also at the epilimnion and hypolimmon. The contribution of photosynthetic bacteria to the total primary productivity of the lake was small (∼1%). Low but perceptible rates of N₂ fixation in situ were also demonstrated by photosynthetic bacteria present in the metalimnion.