Long-term fluctuations of Cystoseira forests along the west Istrian Coast (northern Adriatic, Croatia)
The northern Adriatic represents the northernmost and thus the coldest biogeographic sector of the Mediterranean Sea. For this reason, marked changes of the composition of benthic macroalgal communities due to global warming could be expected in this area. The sea bottom along the west Istrian Coast is mainly rocky and therefore particularly suitable for the growth of canopy-forming brown macroalgae of the genus Cystoseira. A series of Cystoseira distribution and demography surveys were conducted during the last 20 years at different depths along the Istrian coast. Comparison with historical data revealed long-term changes of Cystoseira species abundance. A phase of regression, occurring during the last decades of the past century, was followed by a phase of recovery. The composition of Cystoseira forests in the period from 2009-2013 might be considered similar to that assessed during the 1950s. Starting from 2015, a regression of Cystoseira forests was observed at some sites located in the central part of the west Istrian Coast. These sites were not directly threatened by anthropogenic pressure. According to the EU Water Framework Directive, the Ecological Evaluation Index (EEI) and the Cartography of littoral communities (CARLIT), which consider Cystoseira distribution in relation to geomorphological features of the coast, were used for the assessment of the coastal ecological quality. Our recent surveys revealed that along the west Istrian Coast patterns of Cystoseira distribution continuously change likely because of increased seawater temperatures during summer and an intensification unusual benthic microalgal (frequently alien and invasive) and macroalgal blooms (usually filamentous macroalgae of the order Ectocarpales).