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 limnicsystems. Cultural eutrophication, in particular, significantly shortens the lifespan of freshwaterbodies compared to natural processes. Lake Årungen, a dimictic lake, undergoes seasonalmixing, impacting its oxygen levels and nutrient distribution. During stratification, the lakeexperiences anoxic conditions in the hypolimnion, affecting the nitrogen and phosphorus cycles.The presence of nitrate in the hypolimnion can buffer against phosphorus release fromsediments, mitigating eutrophication. The degree of mixing in Lake Årungen, influenced by windand weather conditions, is crucial for maintaining water quality. Increased air temperatures dueto climate change are contributing to the earlier onset and greater stability of thermalstratification in lakes such as Lake Årungen. This prolonged stratification period can lead toextended anoxic conditions in the hypolimnion and increases chances of internal phosphorusloading. Effective management of nutrient inputs, particularly from agriculture, is essential tocounteract eutrophication and preserve freshwater ecosystems. Increased thermal stratificationstability 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 nitratelevels are critical in buffering phosphorus release from sediments. In 2023, nitrate levels at theonset of anoxia were 2.6 mg/l, maintaining a high redox potential and preventing significantinternal phosphorus loading. In contrast, lower nitrate levels in earlier years led to substantialphosphorus release. Threshold levels for nitrate concentration, crucial in preventing internalphosphorus loading, are influenced by various biogeochemical and environmental factors,including sediment composition, pH, and microbial activity. Effective management strategiesshould focus on reducing external nitrate inputs and maintaining higher nitrate levels to mitigatethe impacts of prolonged thermal stratification and prevent eutrophication in Lake Årungen.