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.

中文翻译：

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在过去的三十年中，日本琵琶湖北部盆地的细菌产量下降

自1980年代以来，日本最大的湖泊琵琶湖的外部污染物负荷有所减少，从而改善了水质，例如降低了生化需氧量（BOD）和PO ₄ ^3-的浓度。我们研究了琵琶湖北部盆地在2016-2017年期间在这些环境变化下细菌产量（BP）的长期变化。通过测量自2016年6月和2017年12月的稳定同位素¹⁵ N标记的脱氧腺苷（¹⁵ N-dA）的掺入量，并与1986年和1997-1998年的测量值相比较，来估算BP  。1986年，通过以下³ H标记的胸苷（³H-TdR）并于1997-1998年通过跟踪培养中细菌的丰度并计算比增长率（μ）来确定。为了直接比较¹⁵ N-dA和³ H-TdR掺入率，我们确定了一个转换因子。为了估计2016-2017年的μ，我们确定了将¹⁵ N-dA掺入量转换为细胞数增加的因子。在2016–2017年，¹⁵ N-dA的掺入率为0.13至30.7 pmol l ^-1  h ^-1，μ的范围为0.016至0.70天^-1。1986年³ H-TdR掺入率的BP值和μ1997–1998年的BP值分别是2016–2017年的BP值的4.6和2.1倍，证实了琵琶湖过去3年中BP的下降。水质数据显示，从1980年代开始，BOD和总磷浓度的降低率较低，而PO ₄ ^3-浓度的降低率与BP相当。在琵琶湖中，BP和有机物的分解受到P的强烈限制。我们的结果表明，BP的降低可以通过降低生物利用度PO ₄ ^3-来解释。在PO ₄ ^3-极低的条件下，有机磷也可能是BP的重要P源。 浓度（nM）以及有机磷生物利用度的变化也可能调节了BP动力学。