Weather conditions and chlorophyll concentrations determine long-term macrophyte community dynamics of Lake Bohinj (Slovenia)

Abstract

Lake Bohinj is a Long-term Ecological Research (LTER) site. This contribution discusses the environmental factors that define the macrophyte community of Lake Bohinj and explains the observed long-term trends in the species present and their abundance and depth distributions from 1996 to 2016. These macrophyte parameters are related to water quality parameters (i.e. total nitrogen and phosphorus, chlorophyll a, water transparency) and meteorological parameters (i.e. precipitation rate, monthly temperature, hours of solar radiation and frequency of storm events per month). We determined 10 submerged macrophyte species, with the stonewort Chara aspera and Eurasian watermilfoil Myriophyllum spicatum prevailing. The variability in the chemical parameters of the water in Lake Bohinj over the study period was low, while the regional air temperatures in the peak season were increasing (R2 = 0.56). Cumulative macrophyte abundance revealed a decreasing trend in the studied period, while the maximal depth distribution was less affected. Cumulative macrophyte abundance was positively related to total nitrogen (r = 0.67) and maximal chlorophyll a concentrations (r = 0.59), while negatively to total phosphorus (r = − 0.56) and minimal transparency (r = 0.63). Redundancy correspondence analysis (RDA) run with the water quality parameters revealed the importance of the maximal chlorophyll a concentrations in the lake water, which explained 54% of variability in the species composition and abundance, followed by total nitrogen that explained additional 10%. Regarding their maximum depth distribution, only 22% of its variability was explained by maximal chlorophyll a concentrations. RDA run with the meteorological data showed the importance of temperature in July in explaining the variability of the species present and their abundance and depth distribution. The study revealed differences in macrophyte community structure during the experimental period, even though during the monitoring period, annual averages of water quality parameters had never exceeded the values, indicating oligotrophic conditions according to the criteria of Organisation for Economic Co-operation and Development. This study points out the importance of macrophytes for assessing the ecological status of lakes and a need for detailed survey of direct disturbances into the lake littoral.