The Effect of Water Colour on Fish Communities in Boreal lakes

The water colour of numerous lakes is increasing, with consequences for biotic communities. The effects of water colour on the abundance of fish in boreal lakes were examined by evaluating gillnet catches in 78 lakes with a water colour range from 18.9 to 380.0 mg Pt L⁻¹. In all, 22 different fish species were captured. Perch (Perca fluviatilis) and roach (Rutilus rutilus) were the most abundant species. Water colour had no effect on fish species diversity. The captured fish biomass per unit effort (BPUE) varied between 192.5 and 5681.2 g net⁻¹ and decreased significantly with increasing water colour. With the exception of pikeperch (Sander lucioperca), the effect of water colour was significant for all the dominant fish species (bleak Alburnus alburnus, bream Abramis brama, perch, pike Esox lucius, roach, ruffe Gymnocephalus cernuus). The main factor behind the effect of water colour was probably the decreasing availability of food, as indicated by the decreased abundance of roach that is not vulnerable to low light intensity. The negative effect of water colour on the planktonic feeder bleak suggested that reduced visibility and prey capture rate in high-colour lakes also had a role. Total phosphorus had a positive effect on BPUE but did not compensate for the effect of colour. BPUE:chlorophyll a ratio for the total catch and for bleak, perch, and roach decreased significantly with increasing water colour, reflecting decreasing transfer efficiency from basal trophic levels to consumers. BPUE and BPUE:chlorophyll a ratio for total gillnet catch or for selected species could be potential fish-based indicators of lake brownification.