Regime shifts in a euxinic marine lake

Numerous limnological studies have been investigating lake overturning regimes in the frame of possible future climate scenarios, mainly regarding the predicted surface air temperature change. Overturning regime shifts are expected to manifest in the stratification onset and offset timing, as well as in stratification duration. In addition, the lakes' vertical water column is expected to become more stable, and thus less inclined to overturn. Since aquatic life in the lake depends thoroughly on its phenology, the predicted regime shifts are going to influence the whole lake ecosystem, along with the microclimate of its surrounding area.

An interesting example of regime shifting can be found in the central part of the Adriatic coast (43° 32' N, 15° 58' E). Lake Rogoznica (also known as "Dragon's Eye" lake) is a marine lake that is connected to the nearby Adriatic Sea through the porous karstic landscape. Although this connection is confirmed by high salinity values that reach those found in the surrounding sea (~38 PSU) and the visible tides, the newest research suggests that the complete overturning of the lake is governed by the delicate balance between the freshwater input (precipitation and runoff) and seawater input (Adriatic Sea), together with substantial surface air temperature fluctuations. Namely, Rogoznica Lake seems to exhibit complete overturns with irregular periodicity: sudden and abrupt holomixis occurred in the Autumn of 1997, 2011, 2016, 2020, and 2021. Another peculiarity of the lake is the complete anoxia with high levels of free sulfides that accompanies every such event, and the consequent mass mortality of the lake's aerobic life - the complete opposite of the oxygenation effect that seasonal overturning usually has in lakes.

It is still unclear whether sudden anoxic holomixis and their frequency are a consequence of local tourist activities, regional climate variability, or climate change affecting the Mediterranean, as the effects of all mentioned could have an intertwined effect on the small, complex system of Rogoznica Lake. In this work, we present the latest findings regarding the Rogoznica Lake water column stratification with a two-sided approach: a comprehensive sea-atmosphere measurements analysis and numerical modelling attempts.