Temporal and vertical difference in factors limiting growth rate of heterotrophic bacteria in Lake Biwa

Dilution bioassays were performed to examine the seasonal and vertical diff erence in the relative importance of factors limiting growth of heterotroph ic bacteria in Lake Biwa. The lake water diluted by 0.2 mu m lake filtrate (1:6.6) was enriched either with glucose (C), inorganic phosphorus (P), amm onium nitrogen (N), amino acids (AA), or a combination of these, and incuba ted for 2 days at the depths where lake water was collected (2.5, 20 and 30 m depths). Experiments showed that at 2.5 m, P was the most deficient reso urce for bacterial growth, but the magnitude of P limitation depended on wa ter temperature. Among others, amino acids showed a slight but significant stimulation of bacterial growth rates during the fall. At 20 and 30 m, howe ver, growth stimulation by resource addition was rarely detected. Verticall y reciprocal translocation experiments revealed that the growth rate was li mited by low temperature rather than resource supply at the greater depths. The results support a simple view that bacterial growth rate is basically regulated by water temperature, but high growth rate is not realized in sum mer because of resource depletion. The present study suggests that both tem perature and P supply play a crucial role in biogeochemical cycling of orga nic matter in Lake Biwa through the bacterial growth rate.