What is the extent of water brownification in Lake Onego, Russia?

In the past 26 years the transport of allochthonous matter into Lake Onego has increased. Water brownification in Petrozavodsk Bay, a northwestern bay in Lake Onego, and in the Shuya River, a tributary of the bay, was shown earlier to begin in spring. The goal of this paper was to find out whether brownification spreads from Petrozavodsk Bay to the central part of the lake in summer. Spearman’s correlation coefficient and linear correlation coefficient confirm a significant (p < 0.05) increase in water color in Petrozavodsk Bay in the summers of 1992–2018. A tendency for increasing water color was found in the Central Onego’s epilimnion. Total iron and CO2 concentrations in the epilimnion have increased and pH values have decreased both in Petrozavodsk Bay and in the Central Onego. In the Central Onego’s hypolimnion, only total iron and CO2 concentrations have considerably increased. However, the present lake ecosystem is resistant to the increased effect of allochthonous matter due to long water renewal time and self-purification. In spite of a significant increase in CO2 concentration in the lake water, the bicarbonate buffer system maintains the acid-base equilibrium and the changes revealed are not considered critical for the ecosystem.