Modelling ecological impacts of the acidification of Welsh streams: temporal changes in the occurrence of macroflora and macroinvertebrates

Models are developed which predict changes in macrofloral and macroinvertebrate assemblages in response to surface water acidification. Empirical relationships between assemblage type and water chemistry are used to predict the probabilities of species occurrences during acidification, as recreated by the hydrochemical model, MAGIC. The water chemistry of two streams is simulated between 1844 and 2124. From 1958, alternative scenarios involved either moorland or conifer forest. From 1984, sulphate deposition was either constant or reduced by 50%. Alternative ecological models driven by pH or total hardness are compared. The floral model showed minor differences between scenarios, probabilities of species occurrence changing gradually and reaching stable values by around 1964. For certain invertebrate species the occurrence probabilities changed rapidly over relatively short periods, for example in the late 20 h century under moorland with constant deposition. Reduced sulphate deposition prevented decrease of acid sensitive species in moorland scenarios but not under forest, which greatly accelerated faunal changes irrespective of deposition pattern. Differences between the pH and hardness models indicated that the effects of these parameters should be separated in future studies. Where an earlier model showed only step changes in invertebrate assemblage type, this new approach can model more precise taxonomic shifts occurring with acidification. Such changes could be important to conservation, or as early indicators of response to pollution. The tentative simulations here suggest that large taxonomic changes may occur over relatively short periods during acidification.