Alien aquatic plants in Poland: Temporal and spatial distribution patterns and the effects of climate change

Distribution of alien aquatic plants in European freshwater ecosystems is still not fully recognized. Little is also known about the impact of climatic factors on the expansion of particular neophyte species in freshwater ecosystems. In this article, we present the first comprehensive assessment of the occurrence of non-native aquatic vascular plants in Poland (CE Europe), with a particular focus on: (1) inventory of the species, (2) temporal and spatial changes in their distribution, and (3) the climate impact on species occurrence and diversity. For this purpose, we collected both published and unpublished data on alien macrophytes occurrence in Poland covering past 70 years. In order to investigate the mechanism of alien aquatic plant expansion, climate data from 24 meteorological stations were analyzed. Redundancy analysis (RDA) was used to analyze the relationship between species composition and microclimate variables. In addition, Generalized Additive Model (GAM) was used to examine the response curves of neophyte species to temperature parameters.
In total, we found records of 15 neophytes of alien aquatic plant taxa in Poland, including 14 species and collective taxon of Nymphaea cultivars. The most frequent was Elodea canadensis (over 25,000 records). Excluding this species, we counted more than 300 records of the occurrence of alien aquatic plants, of which three (E. nuttallii, Azolla filiculoides and Lemna turionifera) were the most abundant. Ten species occurred only in isolated sites. In addition, analyses showed a substantial increase in the number of the aquatic neophyte sites in the last 20 years that could be linked with the raised average monthly and yearly air temperatures in Poland.
Our results showed a strong relationship between raised average temperatures in Poland and the spread of some species, e.g. Azolla filiculoides, while other species such as Elodea nuttallii and Lemna turionifera expanded irrespectively of this factor. The occurrence of some species (e.g. Vallisneria spiralis, Hygrophila polysperma) was exclusively associated to high thermally altered or thermally contaminated waters, and thus was limited to only a small part of the country. Our results confirm the major role of elevated temperatures (mean annual temperatures and minimum monthly temperatures) and thermally polluted freshwater ecosystems in the distribution of alien aquatic plants.