Did desmid assemblages in Dutch moorland pools recover from acidification in the past century?

In order to monitor recovery from acidification caused by acid atmospheric deposition, desmids and water chemistry were sampled in three Dutch moorland pools regularly from 1978 to 2014. Reference desmid samples from the early twentieth century were retrieved from old collections. Changes of the desmid assemblages were assessed by analyses of traits, including indicator values for pH and total phosphate, conservation value, cell volume and surface/volume (s/v) ratio. Direct correspondence analysis (DCA) traced relations between desmids and environmental variables. Between 1916 and 2014, species composition altered due to changes in acidifying atmospheric deposition: The change was most pronounced in pools with relatively flat shores exposed to the atmosphere in extremely dry summers. After the dry summer of 1921, changes were slight, but after the dry summer of 1976, changes were dramatic, when the sulphur and nitrogen compounds stored in the water bottom oxidized and acidified the water. The conservation value declined sharply but increased again until the 1990s, partly due to the decrease in acidifying deposition. Although the acid atmospheric deposition continued to decline until the early 21th century, the conservation value declined again, as did the stability of the desmid assemblages. It is likely that internal eutrophication (nutrients), presence of toxic substances (such as hydrogen sulphide), the decline of aquatic macrophytes (substrate), shading by afforestation (light) and/or reduced supply of carbon dioxide (due to decreased local seepage) play a role. The chemical dynamics due to the large stock of sulphur and nitrogen compounds will hamper the development of rare desmids, bound to stable environmental conditions.

Long-term vegetation change in species-rich Nardus grasslands of central Germany caused by eutrophication, recovery from acidification and management change

QuestionsThe impact of environmental changes on species-rich Nardus grasslands has been documented from the Atlantic biogeographic region but not from Central Europe. Which patterns and trends of community change in species-rich Nardus grassland of the Continental biogeographic region occurred in past decades? Are patterns and trends similar across areas within the Continental biographic region of Germany? Do they correspond to identified changes in the Atlantic biogeographic region of Europe?LocationEast Hesse Highlands, GermanyMethodsIn 2012/15, we re-surveyed vegetation relevés on quasi-permanent plots originally surveyed between 1971 and 1986/87 and re-measured soil parameters. We tested for differences in species frequency and abundance, mean Ellenberg indicator values, diversity measures and soil variables. Nitrogen and sulphur deposition data were analysed to evaluate effects of atmospheric pollutants. We used regression analyses to identify the contribution of environmental drivers to changes in species composition.ResultsWe found significant increases in soil pH, Ellenberg R and N values, species of agricultural grasslands and grassland fallows. C:N ratio, Nardus grassland specialists and low-nutrient indicators declined, while changes in species composition relate to changes in pH and management. There was a strong decrease in sulphur and a moderate increase in nitrogen deposition. Local patterns in atmospheric depositions did not correlate with local changes in species composition and soil parameters.ConclusionThe findings indicate significant overall eutrophication, a trend towards less acidic conditions, and insufficient management and abandonment. This is widely consistent across study areas and correspond to recent reports on vegetation changes and recovery from acidification in the Atlantic biogeographic region. We strongly assume reduction in sulphur deposition during recent decades to be a major driver of these changes combined with increased nitrogen deposition and reduced management intensity. This suggests a large-scale validity of processes triggering changes in Nardus grasslands across Western and Central Europe.NomenclatureThe nomenclature follows the German taxonomic reference list (GermanSL version 1.3) of Jansen & Dengler (2008).