235-239 Halvorsen.p65

Chemical recovery of acidified waters due to reductions in emissions has been reported from Europe and North America (1). The western Norwegian river Nausta is an example of a river where chemical recovery was observed during 1989–1998. Continuous monitoring of the zoobenthos using acidification indices indicated that biological recovery has followed chemical recovery (2). The Nausta was, therefore, used in this study to test a multivariate technique of detecting recovery. The previously established acidification indices are based on data for the acid sensitivity of selected benthic animals from western and southernmost Norway, and were developed as means for early warnings of acidification (3). Briefly, the method is as follows: The different species/taxa are assigned to 4 classes based on their acid tolerance, i.e. very sensitive, moderately sensitive, slightly sensitive, and tolerant taxa, and are then given the acidity scores 1, 0.5, 0.25, and 0, respectively. Localities with one or more specimens of a very sensitive organism are then assigned the acidification score of 1, a locality with moderately sensitive taxa present have the score of 0.5, and so on. Localities with only tolerant species have a score of 0. Using a fixed set of representative localities in a watershed, the mean index for the watershed is calculated on each sampling date. Changes in the index can then be followed over years. Two slightly different acidity indices based on the same data exist. The one described above is Index 1 (3). Index 2 is a more conservative index. It takes into account the abundance of the mayfly Baetis rhodani (Pictet), the dominating sensitive species in the southern and western Norwegian rivers, compared to the abundance of tolerant stoneflies. For example, while Index 1 will give the value of 1 if only one specimen of B. rhodani is found in a locality, Index 2 can take any value between 0.5 and 1 based on the abundance of B. rhodani as a ratio of the abundance of tolerant stoneflies (4). There are other general univariate measures that are useful for detecting environmental stress or recovery, but they are not as sensitive as multivariate methods (5–7). However, the aim of this paper is to examine whether chemical recovery is evident in benthic community structure, not just as changes in acid sensitive taxa as indicated by specialized acidity indices. We use a direct multivariate method, Redundancy Analysis (RDA) to evaluate the 2 different approaches for identifying a recovery process. It should be stressed that we do not try to identify any recovery target, or how far from such a target we are in the Nausta watershed. The method used only indicates whether a process of recovery is going on. However, the method may give us an indication of how strong a recovery process is as compared to other factors affecting the abundance of the benthos.