A health check of Swedish mountain lakes: greenhouse gases, nanoplastics, enzymes and water chemistry

Mountain lakes are vulnerable to global change; particularly the dual threat of climatic warming and atmospheric deposition. These changes can increase greenhouse gas (GHG) emissions, and lead to the accumulation of emerging contaminants such as toxic nanoplastics. In Sweden, the majority of lake GHG research has been on lakes within forest and mire environments, and nanoplastics have only been measured in one lowland catchment. Thus, the biogeochemistry of mountain lakes remains largely an unknown. Here, we report the results of a summer sampling campaign from two mountain regions in Central Sweden: Fulufjället and Jämtland. The regions face different pressures; in Jämtland, reindeer grazing is widespread whilst Fulufjället is ungrazed but closer to central European urban areas, which are a plausible source of long-range dispersal of nanoplastics. Within each region we sampled 16 mountain lakes and 4 lower altitude forest lakes as comparators. We measured dissolved GHGs (CH₄, CO₂, N₂O), carbon isotopes (δ¹³C-CH₄ δ¹³C-CO₂), nanoplastics, DOM composition (via fluorescence) and reactivity, extracellular enzymes and an array of water chemistry (including organic and inorganic C, N, P).

Preliminary findings show the presence of nanoplastics (polymers PE, PP, and PET), low concentrations of inorganic N and P, low DOM reactivity, and relatively low concentrations of CH₄ and N₂O in mountain lakes. Here, we present more detailed analyses, including comparisons between mountain and forest lakes, and between the two regions. Together, our data provide the first integrated assessment of GHGs, nanoplastics, and biogeochemistry in Swedish mountain lakes; and, to our knowledge, the first such study globally. This “health check” highlights the vulnerability of mountain lakes to ongoing environmental change and provides a baseline by which to monitor future anthropogenic changes.