Assessing the impact of seawater blockade on coastal lake degradation using Chironomidae larvae

Coastal ecosystems, such as lakes and lagoons, are unique and valuable water bodies whose proper functioning depends on hydrological connectivity with the sea or ocean. Human interventions, such as the construction of storm surge barriers, that block the periodic and free influx of seawater into lakes induce a permanent freshwater state. This study presents such disturbances, considered as environmental stressors, initiating changes in the assemblages of Chironomidae larvae inhabiting the bottom of Lake Jamno (southern coast of the Baltic Sea). Changes in the structure of this assemblages were assessed during a long-term study (2010-20), which considered two periods: a time of free seawater intrusion (FF) and seven years of blocked influx (BF). The findings indicate that, following the activation of storm surge barriers, the α-diversity of larvae consistently decreased throughout the lake. Concurrently, the density of Chironomidae larvae decreased by over 20%, although their biomass increased. In the last year of the study with functioning gates, the diversity of the studied insects was drastically reduced and was limited to only two genus: Chironomus sp. and Procladius sp., which serve as indicators of disturbances in aquatic ecosystems undergoing changes in line with deterministic chaos theory. The information provided indicates that periodic increases in salinity significantly affect the structure of Chironomidae larvae, though it should be considered as a component of several other parameters (EC, temperature, or nutrients).