Deliverable 3.5

Abstract: Manuscripts on effects of temperature change on internal loading in shallow and deep lakes.

Internal loading has a fundamental role in the nutrient recycling and ecosystem functioning of large and shallow lakes, but it can also influence deep lakes This deliverable comprises two manuscripts examining  i) examining temperature-induced changes on internal loading in shallow and deep lakes and ii) and long-term trends and seasonal changes in phosphorus retention in shallow Danish lakes. The manuscripts are reproduced here.

  1. Järvinen et al. (Effects of temperature changes on internal loading in shallow and deep lakes) assess the impacts of temperature changes on the internal loading of nutrients in shallow and deep lakes using output from published literature, and also demonstrated by using the high-frequency data from the eutrophic and shallow Lake Säkylän Pyhäjärvi (Finland). Data from Säkylän Pyhäjärvi support the general findings that shallow lakes react rapidly to changes in thermal regime, and mixing brings nutrients and other suspended material to surface water layers which will increase (for example) the probability of cyanobacterial blooms. Overall, the literature review and analytical results suggest that the reduction of nutrient loads, in particular in eutrophic shallow lakes, may prove even more difficult in future due to the positive effects of increased temperature on internal loading mechanisms. Shallow and small lakes seem to be more sensitive to temperature-induced changes in the retention and release of P than the deeper dimictic lakes.
  2. Søndergaard et al., 2012 (Persistent internal phosphorus loading during summer in shallow eutrophic lakes. Hydrobiologia DOI 10.1007/s10750-012-1091-3). Twenty one years of monthly mass balance and lake chemistry data from six shallow eutrophic and rapidly flushed eutrophic Danish lakes were analysed to investigate long-term trends in yearly and seasonal patterns of phosphorus (P) retention. The external P input has experienced major changes in only one lake during the last >20 years. The lakes showed a distinct seasonal pattern with high P concentrations and typically negative P retention during summer (up to -300% of the external loading from May to August). During winter, P retention was up to 50% of the external loading from December to April. Internal P loading from the sediment delayed recovery by approximately 10 years in the lake with the most recently reduced external loading, but in all the lakes net release of P from the sediment occurred during summer. P release in the lakes has not abated during the past decade, indicating that the sediment of eutrophic and turbid shallow lakes remains a net source of P during summer. The seasonal variations in P retention became more pronounced with increasing P levels, and retention decreased with increasing temperature, but increased if clear water conditions were established.

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