What is the impact of “liming” of streams and rivers on the abundance and diversity of fish and invertebrate populations? (systematic review)
Calcium carbonate has been applied extensively to mitigate the impacts of surface-water acidification caused by “acid rain” but there are still uncertainties about the effects on fish and invertebrates. For the first time, this systematic review summarises the best available evidence on the impact of liming on invertebrate assemblages and fish populations in rivers.
A systematic search for relevant articles used terms describing rivers, the biota of interest and the intervention. All retrieved articles were scanned for relevance using specified inclusion criteria. Included articles were appraised critically; study methods were identified along with the presence of confounding factors. The main findings of the studies were extracted individually for each outcome of interest (fish abundance, fish diversity, invertebrate abundance, etc.) and the response calculated as the ratio of diversity or abundance in the limed sample to that in the unlimed control. A random effects meta-analysis was carried out on the log of the ratios.
Thirty-four independent studies met the stringent selection criteria, from which data were subsequently extracted. Only a minority had both control and baseline data (a Before, After, Control, Impact, BACI study design). Hence, there was a risk of bias due to differing baselines between the treatment and control groups and factors other than liming changing during the course of the study. All studies were included in the analysis but sensitivity of the results to the study design was tested.
Over all studies liming increased fish abundance, with mean response ratio of 1.7 (C.I 1.3 – 2.1). However, the effect varied between studies and fish abundance was predicted to decrease in around one fifth of limed rivers; the interval predicted to contain 95% of true study effects was 0.7 to 4.3. The variability was partially explained by the average duration of liming treatment and where salmon and trout occurred together they were impacted differently. There were no significant differences between studies of different experimental design.
Liming had no effect on invertebrate abundance over all studies. Including only BACI designed studies, liming on average reduced invertebrate abundance (mean response ratio = 0.78, CI= 0.61 – 0.99). The mean effect of liming on the number of invertebrate taxa was a significant increase (mean effect (response ratio) = 1.17, CI=1.03-1.33). However, again the effect varied significantly between studies and the prediction interval overlapped no effect. Effect of liming on acid sensitive invertebrate abundance was positive (mean response ratio = 1.96, CI=1.12-3.44), however, the prediction interval still overlapped no effect. There was a significant increase in the number of acid sensitive invertebrate taxa after liming (mean effect= 2.58, CI=1.65-4.02) largely reflecting just one study.
Liming is usually an effective intervention for increasing fish populations in streams and rivers affected by anthropogenic acidification but cannot be guaranteed in all situations. Positive effects are more likely in longer-term applications and on salmon, although again this is not guaranteed. Liming generally increased the abundance and taxonomic richness of acid sensitive invertebrates but effects were variable and for all invertebrate taxa combined liming may decrease abundance.
Acidification of streams and rivers has long been of concern to environmental managers. Significant changes in the pH of water bodies are primarily caused by increased acid deposition since the industrial revolution. This phenomenon has been popularly termed ‘Acid Rain’. Considerable efforts to manage industrial emissions that were contributing to acid rain have been made since the 1980’s. Subsequently, some streams and rivers have recovered naturally, at least in terms of their chemical characteristics (Davies et al. 2005; Reynolds et al. 2004). Biological recovery, in terms of fish and invertebrates, has often been slow to follow on (Ormerod & Durance 2009).
Acidified waters frequently undergo changes in stocks of fish and invertebrates (Herrmann et al. 1993). Land and river managers have developed tools to mitigate the effects of acidification and maintain biologically healthy water courses (e.g. Ormerod et al. 1990). One typical intervention is to add calcium carbonate (lime) to the system in order to raise the pH. Several methods of ‘liming’ have been used; from stream source (Miller et al. 1995) and point source dosing using lime dosers (Fjellheim & Raddum 2001), to catchment scale applications (Dalziel et al. 1994).
One of the best ways to monitor the effectiveness of these liming interventions uses multiple long-term case studies (Ormerod & Durance 2009). However, funding of such long-term studies is often unavailable and only a few are successfully established (Fjellheim & Raddum 2001). More often, monitoring of the effects is over shorter time scales (Simmons & Doyle 1996; Weatherley & Ormerod 1992).
This systematic review aims to find the best available evidence on the effectiveness of liming in restoring suitable habitat in streams and rivers for invertebrates and fish.