Diatom and biogeochemical changes during recent centuries in a small boreal lake: deciphering the influence of large volcanic eruptions

Volcanic eruptions can have severe societal and environmental impacts as shown by ash dispersal and sulphur emissions from recent Icelandic eruptions. However, ice-core sulphur records demonstrate that these events were minor as compared to eruptions of much larger magnitude in recent centuries, with severe implications for past society documented in historical records. Biological and chemical evidence of these events can be found in highly resolved varved sediment records. Such records can provide insight into the responses and resilience of lake ecosystems and surrounding catchments following widespread atmospheric perturbations brought about by large volcanic eruptions. In this study we analysed varved sediments spanning the period 1641–1931 CE from Lake Kassjön, a small boreal lake in northern Sweden. We aimed at assessing the potential impacts of the Icelandic eruption of Laki 1783–1784 and the Indonesian eruption of Tambora (1815), based on diatom analysis in combination with organic and inorganic geochemical analyses at sub-decadal resolution. To provide site-specific process understanding, we also assessed the impacts of an intensive ditching operation, which is known to have occurred in 1900–1902 CE. While no significant responses to the eruption of Tambora were identified, our findings indicate enhanced weathering of minerals in the catchment following the eruption of Laki and changes in nutrient dynamics reflected by multiple decades of succession in the diatom assemblage. In timing with Laki, we found an immediate and sustained increase in the concentrations of Aulacoseira tenella. Increased delivery of terrestrial organic matter to the lake and altered nutrient dynamics persisted for around half a century following the eruption of Laki, coinciding with elevated diatom productivity. With consideration of available land-use records, we identify a series of mechanisms as potentially responsible for the immediate responses of the diatom assemblage as well as the more long-lasting effects on the aquatic environment mediated by catchment processes.