Seasonal dynamics of instream mesohabitats over a long section of a large subarctic river

Identifying, mapping and protecting suitable habitats are essential for species conservation. Most studies have focused on short river reaches or limited discharge scenarios. Such restricted coverage hampers effective decision-making because small-scale, short-term analyses cannot capture full habitat dynamics. This study examined mesohabitat types along a 53-km river section at 1-m xy resolution, quantified seasonal variation in mesohabitat occurrence, extent and spatial distribution across nine representative discharge scenarios derived from a 66-year daily flow record, and characterized mesohabitat class transitions between spring–summer and summer–autumn, to answer: How do typical seasonal flow dynamics during the ice-free period reorganize instream mesohabitats important for Atlantic salmon (Salmo salar)? Using multispectral airborne laser scanning (ALS), 2D hydraulic modelling, the Norwegian Mesohabitat Classification Method (NMCM) and spatial raster analyses, we developed an approach enabling examination of ecological phenomena that previously could not be studied on the Tana River due to limitations in high-resolution data availability with large area coverage, and/or due to discharge variability. Deep glides, pools, walks and deep rapids were the dominant mesohabitats across all discharge scenarios, but mesohabitats were also dynamic and seasonally variable. After the spring flood, the river's wetted area contracted, drying many spring mesohabitats by summer. From summer to autumn, mesohabitats were more stable, although the proportion of walk mesohabitats increased. Such seasonal dynamics have the potential to support Atlantic salmon life stages, especially spawning and juvenile rearing. The approach is broadly applicable to rivers where water clarity, turbulence and bank structure allow green-laser penetration and where reliable discharge data exist.