Rapid unpredicted changes in the stratification of marine lake Mogilnoe (Kildin Island, the Barents Sea) through the early 21st century

Abstract

Lake Mogilnoe is a rare example of an anchialine lake (with subterranean connection to the ocean) in the Arctic, a refuge for landlocked populations of marine organisms. The lake has been the subject of intensive studies since the end of the 19th century. Here we demonstrate that between the 2003–07 and 2015–18 observation periods this permanently stratified lake experienced significant changes. The surface salinity increased and exceeded the tolerance limits of many freshwater organisms. The bottom anoxia expanded from one-fifth to one-third of the lake volume. Such a turn in stratification affected both composition and distribution of biota: freshwater zooplanktonic species virtually disappeared, while benthic communities shifted to shallower depths. Although recent changes in the lake stratification are consistent with the long-term trend, their scale is much larger than has been observed during the past 120 years. It was earlier considered that the lake dynamics were mainly affected by human activity in the vicinity of the lake. However, lack of human activity around Mogilnoe during last decades persuades us to search for the natural causes of the recorded changes.