ABSTRACT
Cyanobacterial blooms are an increasing threat worldwide. Invasions of certain cyanobacterial species, mainly towards higher latitudes, add to this concern as they enrich the pool of potential bloom-formers in the invaded region. Among the numerous causes of this escalating process, climate warming is commonly considered the most crucial factor, but empirical studies of this issue are lacking. The aim of our study was to identify physical, chemical and biological factors related to the occurrence of an invasive cyanobacterium at the northern border of its putative current range, and thus enabling its expansion. This study focuses on the relatively little studied species Sphaerospermopsis aphanizomenoides (Nostocales, Cyanobacteria; synonyms: Aphanizomenon aphanizomenoides, Anabaena aphanizomenoides), which is predicted to become one of the main nuisance species of the future. Forty-nine freshwater lakes located between latitudes 51° and 55°N were examined for the presence of S. aphanizomenoides, and environmental factors that could drive its occurrence were studied simultaneously. To identify factors correlated with the presence of the species, principal component analysis (PCA) and Mann-Whitney U test were performed. Water temperature did not differentiate lakes with or without S. aphanizomenoides, however the study was conducted in a particularly hot summer. Total phosphorus concentration was identified as the primary driving factor of the occurrence of S. aphanizomenoides. The species grew in poor light conditions and high phytoplankton biomass, mainly in shallow lakes. As shown by detrended correspondence analysis (DCA), the species accompanied shade tolerant, eutrophic species of native and invasive cyanobacteria as well as eukaryotic algae. Our results indicate that eutrophication may be the primary factor enabling the increasing occurrence of S. aphanizomenoides in temperate environments, and suggest that this process may stimulate expansion of cyanobacterial species towards high latitudes.
