Impact of acidification and eutrophication on macrophyte communities in soft waters. II. Experimental studies

In The Netherlands, there has been a dramatic decline during the last 30 years in the number of stands belonging to the phytosociological alliance Littorellion. Generally, the communities classified within this alliance occur in poorly buffered, oligotrophic waters, with very low phosphate, nitrogen and carbon dioxide levels in the water layer and considerably higher nutrient levels in the sediment. The plant species dominating these communities are isoetids such as Litoorella uniflora (L.) Aschers., Lobelia dortmanna L. and Isoetes lacustris L., which show various adaptations to make successful growth possible under these conditions.Field observations showed that the water where Littorella uniflora had disappeared or strongly decreased could be divided into two groups. A major group (77%) was characterized by the presence of submerged Juncus bulbosus L. and/or Sphagnum species. These water appeared to be strongly acidified (pH < 4.5) and had increased nitrogen levels with ammonium as the dominant N-source. Within this group, the waters with luxuriant growth of Juncus bulbosus and/or Sphagnum spp. had strongly increased carbon dioxide levels in both sediment and water.Different types of experiments proved causal relationships between the observed changes in macrophytes and the changed physico-chemical parameters. Ecophysiological experiments showed that Juncus bulbosus lacks the typical adaptations of the isoetid plant species, i.e. it uses very low amounts of sediment-CO₂ and releases only a little oxygen from the roots. However, Juncus bulbosus is more able than Littorella uniflora to use CO₂ from the water layer. From the nutrient-uptake experiments, the decreased nitrate and increased ammonium levels seem to be favourable to Juncus bulbosus. The culture experiments clearly demonstrated that the biomass of Juncus bulbosus only increased strongly when the sediment was poorly buffered and the pH of water was low. When combining factors like CO₂ enrichment of the sediment, with and without phosphate, and/or ammonium enrichment of the water in the culture experiments, it is clearly shown that phosphate and/or ammonium enrichment without CO₂ enrichment do not lead to an increase in biomass of Juncus bulbosus. Therefore, it is obvious that the changes in the macrophyte community can be ascribed primarily to changes in the carbon budget as a result of acidification.A minor group of waters (23%) was characterized by the absence of submerged Juncus bulbosus and/or Sphagnum spp. In most of these waters, submerged plant species occurred, such as Myriophyllum alterniflorum DC or non-rooted species such as Riccia fluitans L. These waters were not acidified, and generally had an increased alkalinity and higher nitrogen and phosphate levels of sediment and/or water. Culture experiments showed that phosphate enrichment of the sediment alone leads to luxuriant growth of submerged macrophyte species such as Myriophyllum alterniflorum, whereas phosphate enrichment of both sediment and water leads to mass development of non-rooted plant species such as Riccia fluitans.