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.

Impacts of an extreme flood on the ecosystem of a headwater stream

Headwater streams are the smallest parts of rivers but make up the majority of river miles. The chemistry and macroinvertebrate composition of such streams are among the most important indicators of their environmental health. Macroinvertebrates are affected namely by runoff genesis and, in many regions of the world, also by acid atmospheric deposition and its consequences. The aim of this paper is to evaluate the impacts of an extreme summer flash flood on the physical environment, chemistry and macroinvertebrates in a small headwater stream located in the Beech-woods National Nature Reserve of the Jizera Mts. (Northern Bohemia, Czech Republic). The studied stream is characterized by a pluvial hydrologic regime with perennial streamflow uniformly distributed within the year, with peak-flows originating mainly from summer rainstorms, and moderate current anthropogenic acidification. During the observed summer flash flood of the return period near 1,000 years, high currents (1-2.5 m s-1) flushed out 2.7 m3 of sand and gravel from the streambed, resulting in a devastating effect on macroinvertebrates. Both number of species/taxa and diversity were reduced by about 50% while the abundance of surviving taxa was reduced to about 10% compared with before the flood. The following spring after the event, both number of species/taxa, diversity and abundance increased, partially due to the temporary unsuccessful colonization of the site by several alien species creating a peak of biological diversity, but complete recovery of the original macroinvertebrate assemblages was not observed even during the subsequent two years. On the other hand, a significant drop in sulphate contents and rising alkalinity observed in stream waters during base flow conditions after the flood indicate positive effects on recovery of the aquatic environment by depleting the catchment sulphur pool. Thus, the flood did not significantly alter the long-term recovery of the studied headwater stream from acidification.

Indication of environmental changes in mountain catchments by dendroclimatology

In the Czech Republic, mountain watersheds are mostly forested with dominant Norway spruce (Picea abies) plantations. The aim of this paper is to analyse changes in radial growth and xylem anatomy of Norway spruce trees in the upper plain of the Jizera Mountains, related to changes in climate (air temperature, precipitation), air pollution and acid atmospheric deposition. Data of two neighbouring climate stations were used to detect trends in air temperatures and precipitation. At elevations of 745–1060 m a.s.l., the ring-width growth was significantly affected by mean annual temperature, while impacts of elevation and precipitation were not significant. In the period 1975–1995, the detected drop in tree radial growth (ca 60% of the normal period, prior to the peak of acid atmospheric deposition) corresponded to the increase in atmospheric SO₂ concentrations and acid atmospheric deposition. The number of cells in tree rings decreased by ca 30–40% in comparison with the normal period, but the mean size of cells did not change significantly. In the last 20 years, increasing radial growth has been detected simultaneously with rising air temperature, and density of cells decreased by 30% in early wood, and by 10% in late wood, increasing the total number of cells in tree rings by ca 10% in comparison with the normal period. Integrated effects of climate and non-climate variables on the variation of tree radial growth in the Jizera Mountains reflected the legacy of acid atmospheric deposition in the forest ecosystem.

Differences in benthic macroinvertebrate structure of headwater streams with extreme hydrochemistry

A synoptic study of acidified mountain streams covering six Czech sites was performed. The aim was to provide biological data from small mountain streams in catchments with historically high acid atmospheric deposition, which have so far been subject of intensive long-term monitoring of hydrology and hydrochemistry only, in order to follow the development of the structure of benthic macroinvertebrates in the course of recovery from acidification. We focused on small headwater streams with minimum human influence in the catchment and relatively low concentrations of dissolved organic matter. The sites were classified according to their water pH status: three of them were strongly acidified (pH 4.07–4.57, concentration of reactive aluminium R-Al 448–1913 μg L−1), two were moderately acidified (pH 5.08–6.38, R-Al 52–261 μg L−1) and one non-acidified (pH 7.63–7.89, R-Al 12–59 μg L−1). The largest biotic difference detected by PCA was in the presence of the caddisflies Drusus, Rhyacophila, and Chaetopteryx villosa and stoneflies Leuctra pseudocingulata and Diura bicaudata. The results indicate that at the most acidified sites (the Lysina and the right branch of the Litavka), the process of biological recovery has not started yet due to an insufficient increase in stream pH. Indeed, MAGIC modelling published earlier shows that significant increases cannot be expected in the following decades. An average pH of at least 4.5 is needed for return of less acid-tolerant taxa such as Diura bicaudata, Leuctra major, L. pseudocingulata, L. pseudosignifera, Drusus or Rhyacophila. However, at the Sklářský potok — Jizerka site,^both the clear-cut of mature spruce plantations in 1984–1990 and the regional drop in SO2 emissions in the 1990s resulted in a decline of acid deposition and rising streamwater pH. Mean annual pH at the Sklářský potok — Jizerka outlet increased from 4.0 (1982–1985) to 5.3 (1990–1994), but episodic acidification has still resulted in a delay in recovery of the biota, particularly acid-sensitive species, which may be expected within a couple decades.