The impact of salinization on benthic macro-crustacean assemblages in a Mediterranean shallow lake

The shallow Vrana Lake in Dalmatia, Croatia, is directly connected to the Adriatic Sea by the Prosika canal (0.85 km), constructed in the eighteenth century. The aim of this research was to examine the impacts of salinization on long-term changes in littoral macro-crustacean (Malacostraca) populations and assemblages in Vrana Lake and connecting canals. Benthic macroinvertebrates were sampled at seven sites during the period 2011–2020 (14 months) using a hand net (25 × 25 cm, 500 µm). During the study period, saltwater intrusions (strongest in 2012) through the Prosika Canal and site V4 caused increased salinization, resulting in a shift from normal oligohaline (0.5–5 PSU) to mesohaline (5–18 PSU) salinity. Out of a total of 18 macro-crustacean taxa identified, five widespread species—amphipods Echinogammarus stocki and Gammarus aequicauda, isopods Lekanesphaera hookeri and Proasellus coxalis, and decapod Palaemon antennarius constituted 91.6% of collected specimens (26,986). The first three brackish/marine species were significantly positively correlated with the average salinity 12 and 24 months before sampling. They disappeared or had very low abundance during the second low salinity phase (< 1–2 PSU), when freshwater/oligohaline P. coxalis and P. antennarius were the dominant macro-crustaceans. Two  years of low salinity were needed for a strong population decline or disappearance of three brackish/marine species. Salinization is a major stressor in the Vrana Lake basin, and measures for its active control are urgently needed. The use of tested macro-crustacean metrics is recommended for future ecological monitoring as it could provide fast information about the effects of water management on aquatic biodiversity.