Effects of climate change on growth and development of Berula erecta as model species for freshwater macrophytes

Freshwater ecosystems are one of the most diverse, but also one of the most threatened ecosystems in the world. Aquatic macrophytes are highly affected by consequences of climate change like increased concentrations of dissolved organic carbon (DOC) and carbon dioxide (CO2), but also changes in flow dynamics and eutrophication. Knowledge on the effects of DOC and CO2 on macrophytes, and especially their interaction effects with other effects of climate change, is relatively limited. Therefore, the aim of this thesis was to study effects of climate change, like increases in carbon concentrations, using a holistic approach that also focused on their interaction effects with other environmental variables, rather than only studying effects separately. The main macrophyte species studied in this thesis is Berula erecta (lesser water parsnip). Under natural conditions in a temperate lowland stream, this species was found to be highly variable in its biomass, morphology and nutrient content throughout the growing season, and there were interactions between plant growth (biomass and morphology) and environmental parameters like flow velocity and fine sediment depth. This implies that if flow velocity increases due to climate change, it can be expected that macrophyte morphology changes. The effects of climate change were tested in a greenhouse experiment by exposing two macrophytes species, B. erecta and Myriophyllum spicatum to different concentrations of CO2 and DOC in a wide range. The macrophytes responded to both treatments, with the strongest effects in the highest doses. There were large differences between the two species, with regard to growth and morphological responses. In order to study interaction effects among CO2, DOC, flow velocity and nutrients, a series of experiments was done in racetrack flumes. First, B. erecta was exposed a combination of CO2, eutrophication and increased flow velocity. Those stressors sometimes had opposing effects: CO2 stimulated growth, eutrophication indirectly limited growth due to shading by epiphytic algae and increased flow velocity led to a more compact growth form. Due to the combination of CO2 and flow velocity macrophytes developed in a more horizontal way. Plants exposed to CO2 also had a higher C:N ratio, which decreases the quality of the biomass, which can cause a problem when it serves as a food source to other organisms. Combined effects of the three stressors may lead to a decrease in macrophyte abundance. Additionally, B. erecta was exposed to a combination of CO2, DOC and increased flow velocity. Again, stressors had opposing effects where CO2 stimulated growth, DOC limited growth due to shading and increased flow velocity led to a more compact growth form. DOC also had a negative effect on vegetative reproduction (the number of stolons). From studying the interactions between CO2 and DOC effects, it can be concluded that especially elevated DOC concentrations can form a major threat to macrophyte growth. From this thesis it can be concluded that climate change can have a large effect on macrophytes. Different aspects of climate change often have opposing effects, with many interaction effects occurring among them. Taking all aspects of climate change together, the results from this thesis indicate that submerged macrophytes in temperate lowland streams and rivers will decrease in biomass quantity and quality under continuing climate change. This will in turn have negative consequences for ecosystem processes and organisms that depend on the macrophytes.

Publication year:2021

Keywords:Doctoral thesis

Accessibility:Open