Abstract
It is expected that climate change will cause more frequent extreme events of heavy precipitation and drought, changing hydrological conditions in riverine ecosystems, such as flow velocity, evapotranspiration (drought) or runoff (heavy precipitation). This can lead to an increased input of terrestrial organic matter and elevated levels of dissolved organic carbon (DOC) and CO2 due to degradational processes in water. Consequences for submerged macrophytes, as essential organism group, are still poorly understood. The combined effects of changing flow velocity, DOC and CO2 have not been studied before, so this was tested in a racetrack flume experiment on the macrophyte Berula erecta using a trait-based approach. The plants were exposed to two different flow velocities, two DOC concentrations and two CO2 concentrations in a full factorial design. Apart from individual dose-response tests, two climate change scenarios were tested: a wet scenario simulating heavy precipitation and runoff with high flow velocity, high DOC and CO2 concentrations and a dry scenario simulating evapotranspiration with low flow velocity, high DOC and high CO2 concentrations. Growth rate, biomass, morphology, chlorophyll and nutrient content (C, N and P) were measured. B. erecta responded strongly to both scenarios. Biomass and the relative growth rate increased and stems were shorter, especially in the wet scenario, and vegetative reproduction (the number of stolons) decreased. In both scenarios, the N content was lower and P content higher than in conditions without climate change. It can be concluded that climate change effects, especially shading by DOC, strongly influence macrophytes: macrophyte abundance will probably be negatively affected by climate change, depending on the macrophyte species and abundance of epiphytic algae. This may have consequences for other components of the aquatic ecosystem.
Highlights
As a result of human-induced climate change, worldwide precipitation patterns are altering
Many macrophyte species can take up two forms of inorganic carbon [bicarbonate (HCO3−) and CO2], B. erecta can only take up CO2 (Sand-Jensen et al, 1992), so we expected that this species would respond strongly to changes in CO2 availability
In this study CO2, dissolved organic carbon (DOC) and, to a smaller extent, flow velocity had strong effects on the growth and development of B. erecta, which is consistent with what was found in literature (Steinberg et al, 2006; McElarney et al, 2010; Cao and Ruan, 2015; Reitsema et al, 2020)
Summary
As a result of human-induced climate change, worldwide precipitation patterns are altering. If temperatures increase by 1.5◦C, it has been predicted that heavy precipitation intensity (annual maximum 5-day precipitation) increases by at least 5–10% in many parts of Europe, whereas precipitation may decrease by 5–15% in some periods, especially in the Mediterranean area (Jacob et al, 2018). Because precipitation is an important driver of changes in river discharge (Dai et al, 2009), more extremes in discharge can be expected in the future, which can profoundly affect water quality and riverine ecosystems (van Vliet et al, 2013). When discharge and flow velocity are high, macrophytes can break or uproot due to increased pulling forces acting on the plants (Schutten et al, 2005). Hydrodynamic stress caused by increased flow velocity can affect plant physiology: photosynthesis can decrease by 30–60% (Madsen et al, 1993). Altered plant biomass and nutrient stoichiometry can indirectly affect other organisms that depend on macrophytes
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