A century of diatom monitoring in acidified and warmed Dutch moorland pools

Diatoms are well-known indicators for acidification and recovery of lakes, as mainly with paleolimnological methods. However, from the last few decades, such studies are scarce, especially in Europe, and there is a poor insight into the interaction between the impact of the recovery from acidification and drought periods on diatoms. The surface water chemistry of Dutch moorland pools has shown strong signs of chemical recovery as a response to declines in sulphur and nitrogen deposition between 1978 and 2018. We expected that changes in water chemistry were associated with changes in diatom assemblages and that the diatom assemblages at the end of the monitoring period would be dissimilar to pre-acidification diatom assemblages due to the changed biogeochemical processes in the pools. To monitor the recovery from acidification, diatoms from plants and surficial sediments were sampled in 11 Dutch moorland pools from 1978 to 2018. As far as we know, this is the longest series of semi-annual diatom records in stagnant shallow waters recovering from acidification. Early 20th-century samples were retrieved from collections. Changes in the diatom assemblages were assessed by analyses of traits (including pH-preference) and ecological groups. Direct correspondence analysis revealed high correlations between the species composition and environmental variables (pH, sulphate, ammonium, dissolved organic carbon, calcium, and aluminium). Between 1916 and 1978, the greatest species composition changes occurred in pools with relatively flat banks exposed to the air in extremely dry summers. After the dry summer of 1921, the changes were insignificant, but after the extremely dry summer of 1976, the changes were dramatic due to acidification by oxidation of the reduced sulphur and nitrogen compounds stored in the moorland pool bottom. After 1980, the changes were consistent with chemical recovery from acidification. However, there was a significant interaction with climatic factors: drought episodes in the short term and temperature increase in the long term. Some species in the samples from the last decades indicate an acid, but eutrophic environment, due to internal eutrophication. The species composition did not return to the historical composition (1920 s), as the chemical dynamics due to the large stock of sulphur and nitrogen compounds are now entirely different from those a century ago.