Decrease in bacterial production over the past three decades in the north basin of Lake Biwa, Japan

In Lake Biwa, the largest lake in Japan, external pollutant loads have decreased since the 1980s, leading to improved water quality, such as reduction in biochemical oxygen demand (BOD) and PO43− concentrations. We examined long-term variation of bacterial production (BP) under these environmental changes from 2016–2017 in the north basin of Lake Biwa. BP was estimated by measuring the incorporation of stable isotope 15N-labeled deoxyadenosine (15N-dA) from June 2016 and December 2017 and compared with measurements from 1986 and 1997–1998. In 1986, BP was measured by following 3H-labeled thymidine (3H-TdR) incorporation and in 1997–1998 by tracking bacterial abundance in incubations and calculating specific growth rates (μ). To allow direct comparison of 15N-dA and 3H-TdR incorporation rates, we determined a conversion factor. To estimate μ in 2016–2017, we determined a factor for converting 15N-dA incorporation to cell number increase. In 2016–2017, the 15N-dA incorporation rate ranged from 0.13 to 30.7 pmol l−1 h−1 and μ ranged from 0.016 to 0.70 day−1. BP values from 3H-TdR incorporation rates in 1986 and μ in 1997–1998 were 4.6 and 2.1 times BP values in 2016–2017, respectively, confirming the decrease in BP over the past 3 decades in Lake Biwa. Water quality data showed only low decrease rates for BOD and total phosphorus concentration from the 1980s, whereas the rate of decrease for PO43− concentrations was equivalent to that of BP. BP and decomposition of organic matter are known to be strongly P-limited in Lake Biwa. Our results suggest that the decrease in BP can be explained by a reduction in readily bioavailable PO43−. Organic phosphorus can also be an important P-source for BP under conditions with very low PO43− concentrations (nM), and changes in the bioavailability of organic phosphorus might have also regulated BP dynamics.