Dissertation: Environmental changes affect phytoplankton biomass in lakes (Ahonen)
The loading of humic matter to many lakes has been increasing, causing browning of lake water due to a strong light absorption by humic matter. With browning, the penetration of light decreases and water colour shifts towards brown. According to the dissertation by Salla Ahonen from Ģֱ, changes in the underwater light field affect the light availability and biomass of phytoplankton in lakes. With browning, humic matter absorbs a greater fraction of the available light, leaving less light for phytoplankton. Also, the proportion of red light absorbed by phytoplankton increases.
However, despite the lowered light field, phytoplankton biomass was higher in more humic lakes. This can be explained by nutrient availability, which was higher in more humic lakes.
- Phytoplankton requires light and nutrients to produce biomass. Increased nutrient availability in more humic lakes can support higher phytoplankton biomass, as long as the light availability is sufficient, explains Salla Ahonen from the Ģֱ.
Windless heatwave periods can restrict phytoplankton biomass
During summer stratification in lakes, phytoplankton resides in the mixed layer close to surface, the depth of which affects light and nutrient supplies. In a continuous monitoring of a humic lake, Ahonen noticed that the phytoplankton biomass was higher on days when mixed layer was deeper, as the weather was windy. The amount of nutrients often increases with depth, thus limiting the phytoplankton biomass can be limited if the wind does not mix the water and bring nutrients to the upper layers.
- Summertime heatwaves are becoming more frequent with climate change. Longer heatwave periods restrict the mixed layer depth, which can also restrict phytoplankton biomass, according to my study. This can be further intensified by the browning. Understanding the responses of phytoplankton to environmental changes is essential, as changes taking place in phytoplankton can be reflected to the entire food web, explains Ahonen.
The examination of M.Sc. Salla Ahonen’s doctoral thesis “The impact of optical properties on spectral light availability and biomass of phytoplankton in boreal lakes” is held on 31.5.2024 at 12:00 at Physics department room FYS1 at Ylistönrinne. The opponent is Professor Sebastian Diehl (University of Umeå, Sweden) and Custos Senior Lecturer Anssi Vähätalo (Ģֱ). The doctoral dissertation is held in English.
The dissertation “The impact of optical properties on spectral light availability and biomass of phytoplankton in boreal lakes” can be read on the JYX publication archive:
Further information: Salla Ahonen, salla.a.ahonen@jyu.fi, +358504039876.
