Spatial and temporal dynamics of phytoplankton biomass in wetlands located in mineral and peat soil in southern Sweden

Sammanfattning

Rewetting wetlands for biodiversity and climate change mitigation is on the Swedish agenda, however, temporal and spatial patterns in relation to wetland soil type are yet not resolved. This thesis studied the temporal dynamics of phytoplankton biomass, measured as chlorophyll-a concentration, over the course of one year, as well as how wetland characteristics (eg size, catchment area, land uses, water physicochemical parameters and nutrients) influence phytoplankton biomass and water quality, with a specific focus on wetlands located in different soil-types, mineral soil vs peat soil. The study included 10 wetlands in southern Sweden, five on mineral soil, and five on peat soil. Sampling occurred 14 times from February to November 2025. The total amount of phytoplankton biomass was estimated by quantifying spectrophotometrically chlorophyll a (Chl-a) concentration after pigment extraction. This resulted in 417 observations of Chl-a. Results showed the seasonal dynamics of Chl-a concentrations being higher during the warmer seasons (spring, summer and autumn) than the cold season (winter). Regarding the spatial variability, Chl-a concentrations were higher in wetlands on mineral soil (9.83 ± 16.6) than on peat soil (4.59± 9.61). Wetlands on mineral soil are characterized for having higher concentrations of phosphorus and total nitrogen in water, and both parameters correlate positively with Chl-a. Higher levels of nutrients in wetlands located on mineral soil relate to agricultural land uses in the catchment, suggesting that for the studied wetlands, the main source of nutrients enrichments in the wetlands is coming from the runoff from the catchment and not leaking from the soil. The results also showed that wetlands with smaller size, smaller catchment and higher cropland percentage in the catchment had a positive effect to Chl-a. Altogether, this study shows a temporal dynamic of phytoplankton biomass and that wetlands on mineral soils tend to have a higher phytoplankton biomass and nutrient levels, which can be seen as poor water quality, than those on peat soil. Therefore, rewetting on peat soil should be prioritized.