Phytoplankton response to the massive expansion of Elodea nuttallii (Planch.) H.St.John, 1920 in a floodplain lake of the Vistula River (Poland)

Abstract

The spread of invasive non-native species, one of the greatest threats to biodiversity and the economy, affects the structure and functions of ecosystems at all levels. At the beginning of the 21 century a rapid expansion of the submerged macrophyte Elodea nuttallii was observed in southern and eastern Europe. However, this plant, native to North America, was already recorded in Europe in the first half of the 20 century. Our study aimed to evaluate changes in phytoplankton communities caused by the presence of this new invader. The research was conducted in a large floodplain lake in the Vistula valley (north-central Poland) and involved regular monitoring of the lake’s ecological status using phytoplankton-based methods. Long-term observations enabled us to track the impact of this invasive plant on phytoplankton and to compare the conditions of the phytoplankton community before and after Elodea nuttallii invasion. In the first stage of the research (2007–09) massive phytoplankton growth (max. biomass over 90 mg/L) and Cyanobacteria blooms (mainly of Aphanizomenon flos-aquae) were noted. Submerged vegetation along the shore was sparse and its development was inhibited by phytoplankton shading. Elodea nuttallii was first reported in this lake in 2009, together with native macrophyte species. The second stage of the research was carried out in the years 2013–15, when E. nuttallii had already colonised the lake, in some parts occupying the entire bottom surface and almost completely outcompeting native species of submerged macrophytes. As a consequence, the abundance and biomass of phytoplankton decreased. The highest biomass recorded in summer 2013–15 was approximately 4 mg/L. Secchi depth increased from 0.5 m in 2007–08 to 1.6 m in 2015. Elodea nuttallii expansion caused a shift from the turbid-water to clear-water state with higher water transparency. Phytoplankton blooms did not develop. The massive growth of E. nuttallii seems to have caused positive changes in the lake ecological status. However, E. nuttallii is considered to be a highly invasive species, threatening native hydrobionts at various levels of organisation.