Abstract <p>Phytoplankton response to weather fluctuations is a major focus in aquatic ecology. Will the lakes unaffected by serious human disturbances experience an annual increase in water chlorophyll, and what are the relative roles of temperature and precipitation? We conducted a multi-year (2015–2023) study of chlorophyll <i>a</i> in the littoral of Lake Hupujarvi, a small, shallow lake in European Russia, chosen as a case study because of its autochthonous regime (drainage ratio 7.71) and slow water exchange (5.31 years). Temperature was identified as a positive driver of eutrophication, determining the interannual central tendency of chlorophyll, while precipitation had a negative effect primarily at the intra-seasonal level. A lake can attain a higher trophic status only if the mean seasonal temperature exceeds the multi-year average by more than 1°C. The effect of temperature and precipitation was greater than that of nutrients, at least for the nitrate nitrogen studied. Thus, the temperature conditions alone can trigger eutrophication in mesotrophic lakes during a single year, even with minimal phosphate concentrations in the water (in our study, under high nitrate concentrations). This study is the first to explain the conflicting data on the relative roles of temperature and precipitation in chlorophyll dynamics.</p>