Lake Tåkern

Lake Tåkern is a shallow, moderately eutrophic lake in south-central Sweden (58° 21′ N, 14° 48′ E), covering approximately 44 km², with 70% open water and 30% reedbeds.

In 1974, Lake Tåkern was designated a Ramsar site due to its biodiversity and significance as a waterfowl feeding and nesting area in Northern Europe. It became a nature reserve in 1975 and has been part of the Natura 2000 network since 2016, protecting nine natural habitats. The Lake Tåkern reserve spans 54 km² and hosts around 270 bird species, with 130 species nesting there.

The common reed (Phragmites australis) is a clonal, rhizomatous perennial grass species that forms dense stands, which cover extensive areas in wetlands. These stands often originate from a single root system and spread through rhizomes. The species reproduces through multiple mechanisms: seed dispersal, vegetative spreading, or fragmentation of rhizomes, which can form floating reed mats. The primary mode of reed reproduction is vegetative, as seed germination and establishment require specific conditions with lower water levels sustained over time. In the late 1800s and 1930s, during dry spells and deliberate water-level reductions at Lake Tåkern, the reed expanded significantly due to seed regeneration.

Reed has long been harvested for livestock fodder and roofing material. Today, reed mowing at Lake Tåkern is only allowed in specific areas within the western part of the reserve, and only in winter or spring, with approval, to avoid damaging reedbeds and bird habitats.

Greylag goose (Anser anser) (Fig. 3) is the most numerous goose species at Lake Tåkern. As all migratory birds, geese exhibit specific habitat-use patterns during phases of their annual lifecycle. During the breeding stage, geese tend to stay in wetlands with nourishing vegetation and close access to water for nesting, incubation, and raising their young. During the moulting stage, a period of replacement of flight feathers, geese prefer large open water bodies with ample food resources and minimal disturbances. In post-moulting period, the birds prepare for autumn migration to winter quarters, they spend the night in the lake and in the morning fly out to surrounding farmlands for foraging. These habitat-use patterns are observable at Lake Tåkern, where greylag geese are present in substantial numbers throughout all periods.

To study long-term changes in reed coverage at Lake Tåkern, a reed coverage analysis was performed using high-resolution satellite images from the the available years 1960, 2004, 2013, 2017, and 2022. Four larger reedbed areas were selected (Fig. 4), and TIFF files were exported for further assessment.

TIFF files were converted to 8-bit and underwent minor adjustments in Lightroom Classic to mask areas misidentified as reed (e.g., waves, spots, and other vegetation). Finally, in ImageJ, manual thresholding was applied to determine the percentage of reed in each area.