The importance of thermal stratification and oxygen conditions for internal cycling of nitrogen and release of phosphorus in Lake Årungen.

Abstract

Eutrophication together with its subsequent effects, increased levels of phytoplankton, algae, decreased water transparency and oxygen depletion poses a significant threat to our limnic systems. Cultural eutrophication, in particular, significantly shortens the lifespan of freshwater bodies compared to natural processes. Lake Årungen, a dimictic lake, undergoes seasonal mixing, impacting its oxygen levels and nutrient distribution. During stratification, the lake experiences anoxic conditions in the hypolimnion, affecting the nitrogen and phosphorus cycles. The presence of nitrate in the hypolimnion can buffer against phosphorus release from sediments, mitigating eutrophication. The degree of mixing in Lake Årungen, influenced by wind and weather conditions, is crucial for maintaining water quality. Increased air temperatures due to climate change are contributing to the earlier onset and greater stability of thermal stratification in lakes such as Lake Årungen. This prolonged stratification period can lead to extended anoxic conditions in the hypolimnion and increases chances of internal phosphorus loading. Effective management of nutrient inputs, particularly from agriculture, is essential to counteract eutrophication and preserve freshwater ecosystems. Increased thermal stratification stability from increased air temperatures early in the season reduces oxygen distribution, increasing anoxia duration and necessitating higher nitrate levels to prevent reducing conditions. Comparative data from previous years (2017, 2013, and 2009) indicated that sufficient nitrate levels are critical in buffering phosphorus release from sediments. In 2023, nitrate levels at the onset of anoxia were 2.6 mg/l, maintaining a high redox potential and preventing significant internal phosphorus loading. In contrast, lower nitrate levels in earlier years led to substantial phosphorus release. Threshold levels for nitrate concentration, crucial in preventing internal phosphorus loading, are influenced by various biogeochemical and environmental factors, including sediment composition, pH, and microbial activity. Effective management strategies should focus on reducing external nitrate inputs and maintaining higher nitrate levels to mitigate the impacts of prolonged thermal stratification and prevent eutrophication in Lake Årungen.