Browsing by Subject "freshwater"

Lake ecosystems are shaped by water chemistry processes that affect the lake environment and the species communities within. Changes in the water chemistry thus have far-reaching consequences. Water colour is one variable that affects water chemistry and stems from humic substances in the water. Dark water reduces light availability and also affects nutrient and oxygen availability. A trend of brownification of freshwater systems has been observed in recent years and it is expected to influence species community’s diversity and composition. The aim of this thesis was to study whether brownification is an ongoing issue in the study lakes and whether it has had a negative effect on phytoplankton diversity and resulted in shifts in the phytoplankton composition. A data set including about a 100 lakes in Finland with measurements from 1965 up until now served as the study system which was analysed with statistical methods. The results indicated a brownification trend in the past decades. The brownification so far had a positive impact on species richness but a negative impact on beta diversity. Brownification also affected species composition. Flagellates and autotrophic species increased in darker waters but mixotrophic species that are known to dominate in dark water colour, did not show a clear increase with water colour. Other hydrological variables than water colour could have had a bigger impact on the phytoplankton community than water colour but future monitoring of the phytoplankton community is recommended to see if water colour will have a negative impact on species diversity in the future.