Monitoring temporal changes in the biology of acid waters
Authors: LANCASTER J. , REAL M. , JUGGINS S. , MONTEITH D.T , FLOWER R.J. , BEAUMONT W.R.C. 
Published: November 2003

1. Assemblages of epilithic diatoms, macrophytes, benthic macroinvertebrates and fish were monitored over a 5‐year period (1988–92) in ten lakes and nine streams in the U.K., as part of the U.K. Acid Waters Monitoring Network. All organisms were categorized according to their functional or morphological characteristics and integrated to describe the food web at each site. In general, the flora and fauna of all sites were typical of oligotrophic, softwater systems subject to various degrees of acidification.

2. Salmonids were the only fish caught at any site and brown trout were the dominant species. With only 5 years of data it was not possible to test for directional changes in fish populations at each site. Among sites, fish density was positively associated with pH, and this did not vary between lake outflows and streams or between fish of different age class (0 +, 1 +). Condition factor, reflecting fish health, was not associated with pH among sites, but was negatively associated with fish density and, on average, was higher for fish in streams than those in lake outflows.

3. Variability in the diatom, macrophyte and invertebrate data sets were quantified in three ways using multivariate techniques: species turnover or replacement (temporal variation due to directional change), persistence (the reciprocal of between‐year variability) and within‐year variability (heterogeneity between sample replicates). For all groups, turnover was relatively low and persistence was high. The least persistent macrophyte assemblages occurred in stream sites and this may reflect high inter‐annual variation in the cover of filamentous algae which are prone to scouring. Within‐year variability was higher than turnover or between‐year variability for the diatoms and invertebrates, and highest values were recorded for lake invertebrates.

4. Redundancy analysis, RDA, was used to test the diatom, macrophyte, invertebrate and food web data for evidence of directional changes over time and its significance was assessed using Monte Carlo permutation tests. These tests appeared robust to temporal and spatial variability in the data set. Significant trends could be identified in some data sets despite considerable between‐replicate and non‐linear between‐year variability.

5. Significant linear trends in at least one biological group were found at eight lake and seven stream sites. Only one lake and one stream had significant trends in all four groups. These trends represent changes in the flora and/or fauna, but they can be interpreted in several different ways. Only six sites showed trends that were consistent with our knowledge of species’ responses to water chemistry: three indicated increased acidity and three indicated decreased acidity. At only one site were the biological results consistent with observed chemical changes and there was disagreement at the other five. Of the other nine sites that showed biological changes, two appeared to reflect known physical habitat disturbances; the other seven remain unexplained.