P-cycling in sub-Arctic lakes – the importance of sediment release and retention

Abstract: Oligotrophication is generally caused by a decreased nutrient availability, usually due to phosphorous limitation. This is a phenomenon impacting the entire food web and at worst causing cascading alterations within an ecosystem. Lakes of the Northern Hemisphere have lately been documented to be subjected to oligotrophication (e.g., Arvola et al. 2011, Eimers et al. 2009, Huser et al. 2018). The decreasing P concentration trends have previously been coupled with e.g., vegetation and pH shifts within ecosystems as well as changes in biogeochemical processes due to climate change and global warming. In Sweden, the greatest declines have formerly been reported within the northern, alpine lakes that are already oligotrophic (Huser et al. 2018). The aim of this study is to elaborate the hitherto hypothesised pressures causing the changed P-cycling within sub-Arctic lakes, expanding on the possibility of global warming being the fundamental cause. Additionally, potentially increased P retention within sediment metal complexes was investigated. This possibility was evaluated by coupling water chemistry (1996-2021) and sediment subjected to P sequential extraction analysis. Seven sub-Arctic lakes and eight lowland lakes were investigated regarding the pressures of global warming to the P retention and release cycles. The sediment concentrations and water chemical analyse showed no indications of a temporary increased mobile (potentially releasable) sediment P pool, contrary to expectations. The water chemical analysis, on the other hand, adds further evidence to former studies, agreeing with the previously detected oligotrophication. The magnitude of the TP declines, however, seem to have weakened compared to results from prior studies of the same area. Nevertheless, the PO4-P concentrations have in the intermittent years further decreased to less than analytical detection limits, lending additional evidence to the ongoing oligotrophication of lakes within the Northern Hemisphere and alpine area specifically. A relationship between sediment organic P concentrations and TP concentration in lake surface water was detected and hypothesised to be caused by the documented shift in terrestrial ecosystems within these watersheds. The correlations of this phenomenon, however, need further, quantitative study.