Mikkel Rene Andersen:
Small ecosystems are many-fold more abundant than their larger counterparts. Both on regional and global scale small lakes outnumber medium and large lakes and account for a much larger surface area. Small streams are also far more common than rivers. Despite their abundance small ecosystems are grossly understudied. In this thesis I present new insights into the dynamic nature of small aquatic ecosystems. I show that small lakes can stratify and that the resulting gradients are much steeper than in larger lakes. In a 30-40 cm shallow water-column the surface waters can be more than 200 % supersaturated in oxygen while the bottom waters becomes anoxic. Dense charophyte stands influenced the hydrodynamics and created favorable conditions for the apical parts in the surface waters, while the basal parts withstood anoxia for up to 12 hours in the bottom waters. Nocturnal convective mixing oxygenated the bottom waters and replenished the DIC pool in the surface waters every night. Nocturnal mixing and small distances resulted in similar metabolic signals recorded by many oxygen sensors placed across the small lake. Respiration and gross primary production (GPP) were tightly coupled (1:1 ratio) both in the small lakes and in the small ephemeral streams on the Great Alvar. Downstream respiration was decoupled from GPP as respiration rates were much higher due to agricultural impact.