Stratification in non-tidal shallow coastal lagoons during extreme summer heatwaves 

Due to their shallow depth, coastal lagoons are often considered to be vertically well-mixed. However, past studies have shown that, depending on forcing conditions, vertical density stratification may in fact occur even in lagoons of only a few meters depth. Further, many coastal lagoons are faced with a multitude of anthropogenically caused pressures, including eutrophication as well as a likely increasing occurrence of summer heatwaves and other extreme atmospheric conditions due to climate change. While eutrophication leads to increased biological productivity and a subsequently increased oxygen demand, high water temperatures during heatwaves lead to reduced oxygen solubility, thus aggravating the risk of anoxic conditions within the water body. As vertical stratification acts to suppress vertical mixing, it may facilitate the occurrence of bottom oxygen depletion in such waters. Since coastal lagoons are of great ecological and economical interest due to their multiple ecosystem services, e.g., as spawning grounds for fish, it is of utmost importance to assess the conditions under which vertical stratification may occur. Only with such knowledge it will be possible to estimate the future development of coastal lagoon ecosystems. While many past studies have covered stratification of freshwater lakes during heatwaves, there is a significant gap of research covering coastal lagoons under the same conditions, where an additional forcing is added via the connection to the open sea. In particular, non-tidal, non-choked lagoons are currently understudied with respect to summer heatwaves. In our study, we therefore aim to assess the conditions under which stratification may occur in such lagoons during a mid-latitude summer heatwave. To this end, we have applied a non-dimensional parameter space analysis to a numerical, one-dimensional water column simulation of such a lagoon. Here, we present first results from this analysis.