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.

Direct O₂ control on the partitioning between denitrification and dissimilatory nitrate reduction to ammonium in lake sediments

Lacustrine sediments are important sites of fixed-nitrogen (N) elimination through the reduction of nitrate to N2 by denitrifying bacteria, and they are thus critical for the mitigation of anthropogenic loading of fixed N in lakes. In contrast, dissimilatory nitrate reduction to ammonium (DNRA) retains bioavailable N within the system, promoting internal eutrophication. Both processes are thought to occur under oxygen-depleted conditions, but the exact O2 concentration thresholds particularly of DNRA inhibition are uncertain. In O2 manipulation laboratory experiments with dilute sediment slurries and 15NO3- additions at low- to sub-micromolar O2 levels, we investigated how, and to what extent, oxygen controls the balance between DNRA and denitrification in lake sediments. In all O2-amended treatments, oxygen significantly inhibited both denitrification and DNRA compared to anoxic controls, but even at relatively high O2 concentrations (≥70 µmol L−1), nitrate reduction by both denitrification and DNRA was observed, suggesting a relatively high O2 tolerance. Nevertheless, differential O2 control and inhibition effects were observed for denitrification versus DNRA in the sediment slurries. Below 1 µmol L−1 O2, denitrification was favoured over DNRA, while DNRA was systematically more important than denitrification at higher O2 levels. Our results thus demonstrate that O2 is an important regulator of the partitioning between N loss and N recycling in sediments. In natural environments, where O2 concentrations change in near-bottom waters on an annual scale (e.g., overturning lakes with seasonal anoxia), a marked seasonality with regards to internal N eutrophication versus efficient benthic fixed-N elimination can be expected.

Direct O₂ control on the partitioning between denitrification and dissimilatory nitrate reduction to ammonium in lake sediments

Lacustrine sediments are important sites of fixed nitrogen (N) elimination through the reduction of nitrate to N2 by denitrifying bacteria, and are thus critical for the mitigation of anthropogenic loading of fixed N in lakes. In contrast, dissimilatory nitrate reduction to ammonium (DNRA) retains bioavailable N within the system, promoting internal eutrophication. Both processes are thought to occur under oxygen-depleted conditions, but the exact O2 thresholds particularly of DNRA inhibition are uncertain. In O2-manipulation laboratory experiments with dilute sediment slurries and 15NO3− additions at low- to sub-micromolar O2 levels, we investigated how, and to what extent, oxygen controls the balance between DNRA and denitrification in lake sediments. In all O2-amended treatments, oxygen significantly inhibited both denitrification and DNRA compared to anoxic controls, but even at relatively high O2 concentrations (≥ 70 µmol L−1), nitrate reduction by both denitrification and DNRA was observed, suggesting a relatively high O2 tolerance. Nevertheless, differential O2 control and inhibition effects were observed for denitrification versus DNRA in the sediment slurries. Below 1 µmol L−1 O2, denitrification was favored over DNRA, while DNRA was systematically more important than denitrification at higher O2 levels. Our results thus demonstrate that O2 is an important regulator of the partitioning between N-loss and N-recycling in sediments. In natural environments, where O2 concentrations change in near bottom waters on an annual scale (e.g., overturning lakes with seasonal anoxia), a marked seasonality with regards to internal N eutrophication versus efficient benthic fixed N elimination can be expected.

Long-term changes of water physicochemical conditions and benthic microbial processes in a small lake associated with land use in the catchment

Changes in land use in the catchments and areas near the shorelines of lakes may have undesirable consequences for the functioning of lake ecosystems. We studied temporal changes in physicochemical parameters and benthic microbial processes within the small Lake Gulbinas (Lithuania) in relation to the type of land use in the catchment. We compared the period when agriculture activity decreased and increased urban development commenced (2001–2002, transition period) with periods of intense urban land use (2007, 2014–2015). The results were compared to reference data from earlier agricultural periods (1962, 1987–1989). The highest nutrient concentrations in the water were observed during the period of agriculture activity, while increased phosphate concentrations in the near-bottom water and increased organic carbon content and microbial activity in the lake sediments were observed during the period of intense urban land use. Throughout the latter period, anaerobic mineralization of organic carbon via sulfate reduction in bottom sediments was significantly higher than that during the transition period. The intensification of benthic sulfate reduction led to sulfide increase and, thus, to a higher phosphate mobility re-fertilizing the water. Our study suggests that, with a shift of land usage in catchment areas from agricultural to urban, increasing sedimentary organic carbon and its intensive anaerobic mineralization may stimulate internal eutrophication of small lakes.