Abstract
We investigated 34 sediment cores to reconstruct spatiotemporal variations in hypolimnetic hypoxia for the past 200 years in Lehmilampi, a small lake in Eastern Finland. As hypoxia is essential for varve preservation, spatiotemporal changes in varve distribution were used as an indicator for hypolimnetic hypoxia oscillations. The hypoxic water volume was used as a variable reflecting hypolimnetic hypoxia and determined for each year by estimating the water volume beneath the water depth where shallowest varves were preserved. As a result, seven hypoxia periods, highlighting the variations in hypolimnetic hypoxia, are established. These periods may be influenced by bioturbation, lake infill, and lake level changes. Furthermore, we evaluated the relationship between hypolimnetic hypoxia oscillations and climatic factors. Diatom assemblage changes were also analyzed to estimate whether the hypoxia periods could be related to anthropogenic eutrophication. The diatom analyses suggest relatively stable nutrient conditions for the past 200 years in Lake Lehmilampi. Climate, on the other hand, seems to be an important driver of hypoxia oscillations based on correlation analysis. The role of individual forcing factors and their interaction with hypolimnetic hypoxia would benefit from further investigations. Understanding climatic and anthropogenic forcing behind hypolimnetic hypoxia oscillations is essential when assessing the fate of boreal lakes in a multi-stressor world.
Highlights
Eutrophication and hypoxia have become environmental challenges worldwide, causing loss of biodiversity, fish kills, and algal blooms [1,2,3]
Eutrophication in lakes is triggered by excess nutrients [5] leading to hypolimnetic hypoxia through enhanced autochthonous production and increased oxygen consumption caused by the degradation of biogenic material [6,7]
As varve preservation is closely linked with oxygen-depleted conditions in the bottom waters, we use spatiotemporal variations in varve preservation as a proxy for hypolimnetic hypoxia oscillations
Summary
Eutrophication and hypoxia have become environmental challenges worldwide, causing loss of biodiversity, fish kills, and algal blooms [1,2,3]. They are an economic problem [4]. Eutrophication in lakes is triggered by excess nutrients [5] leading to hypolimnetic hypoxia through enhanced autochthonous production and increased oxygen consumption caused by the degradation of biogenic material [6,7]. The solubility of oxygen in water corresponds negatively to increasing water temperatures [9]. Increasing water temperature may reduce the mixing of oxygen-rich surface waters with deeper waters [9]
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