Lake Peipsi, one of the world’s largest lakes, is shared between Estonia and Russia. The water quality in different parts of the lake has so far been assessed independently. Here we explore opportunities for combining data of Estonian and Russian monitoring. For that, we 1) analysed the compatibility of data for some water quality variables; 2) estimated the potential effects of the differences in sampling frequency; 3) provided a few regression models to calculate the missing data for months not sampled by the Russian side. Data of the concurrent Estonian and Russian sampling indicated a good compatibility. Estonian data analysis suggested that water quality assessment results are sensitive to sampling frequency. For example, total phosphorus (TP) in the largest basin showed a long-term decreasing trend in three month data that disappeared when data for other months were added. Disregarding some months may lead to under- or overestimation of certain factors with no consistency in the response of different basins. Hence, data of the whole ice-free period are recommended for an adequate water quality assessment. Furthermore, we demonstrated that monthly values of the water quality variables of the same year are autocorrelated. Based on this, we filled the gaps in the long-term data and compiled a dataset for the whole lake that enables its most comprehensive use in water quality assessment and management. Long-term data revealed no water quality improvement of Lake Peipsi. Further reduction of the external nutrient load is needed. Eutrophication is sustained by high internal phosphorus load.

佩普西湖是世界上最大的湖泊之一，由爱沙尼亚和俄罗斯共享。迄今为止，湖泊不同部分的水质已得到独立评估。在这里，我们探索结合爱沙尼亚和俄罗斯监测数据的机会。为此，我们1）分析了一些水质变量的数据兼容性； 2）估计采样频率差异的潜在影响； 3）提供了一些回归模型来计算俄罗斯方面未采样的月份的缺失数据。爱沙尼亚和俄罗斯同时采样的数据显示出良好的兼容性。爱沙尼亚的数据分析表明，水质评估结果对采样频率敏感。例如，最大流域的总磷（TP）在三个月的数据中呈现出长期下降的趋势，当添加其他月份的数据时，这种趋势就消失了。忽略某些月份可能会导致对某些因素的低估或高估，并且不同流域的响应不一致。因此，建议使用整个无冰期的数据来进行充分的水质评估。此外，我们证明了同年水质变量的月值是自相关的。在此基础上，我们填补了长期数据的空白，编制了全湖数据集，使其能够最全面地应用于水质评估和管理。长期数据显示，佩西湖的水质没有改善。需要进一步减少外部养分负荷。富营养化是由高内部磷负荷维持的。