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An overview of alien aquatic flora of Croatia - new insights from national monitoring of macrophytes

Invasions of alien organisms are nowadays considered one of the major threats to biodiversity. Intrusions of alien species in aquatic ecosystems are particularly interesting and heavily studied, due to the increased spreading potential of organisms in systems interconnected with flowing waters. Data on the distribution of alien aquatic plant species were gathered within the monitoring of surface waters, conducted in order to meet the requirements of the Water Framework Directive. Monitoring of macrophyte vegetation was carried out during the vegetation seasons 2013-2017 in over 600 locations, ultimately covering most of the Croatian territory, being the first comprehensive study of macrophytes in Croatia so far. Watercourses were surveyed along 100 m-long transects, while multiple 6 x 100 m-transects were used when surveying the lakes. Cover and abundance of each macrophyte species were estimated according to Braun-Blanquet scale. We have found altogether five alien species during our research. Three species were recorded only once, in continental part of Croatia: Elodea nuttallii (Planch.) H.St.John, Azolla filiculoides Lam. and Lemna minuta Kunth. Noteworthy, L. minuta was not previously recorded in Croatia, therefore this finding (Čarna River) is the first known site. Another newly found species is Egeria densa Planch. recorded on five sites in the Neretva River Basin. Furthermore, we provide updated distribution of Elodea canadensis Michx. with as many as 19 new records. It was mostly found in the continental part of Croatia, but also in the Cetina River Basin, which is the first record in the Mediterranean part of the country. Along with the data on the distribution, we present the data on habitat characteristics of finding sites, as well as our remarks on the observed trend of its spread. Including the literature data, we can account for nine alien aquatic plants in Croatian waters so far, out of which only E. canadensis is considered invasive. Our results show that the number of alien aquatic species in Croatia is still relatively low, and their distribution is rather limited, especially in comparison with Central European countries.

invasive alien species, allochtonous plants, macrophytes, monitoring, Croatia, Water Framework Directive

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