Which one of the following is not a bioindicator of water pollution?

Stone flies are not bioindicators. Bioindicators provide a range of techniques to assess the impcts of air pollution from reactive nitrogen (N) compounds on statutory nature conservation sites. They complement physical monitoring of atmospheric concentrations and deposition and risk assessment based on the critical loads approach by providing sitebased information on atmospheric Nconcentrations, $\mathrm{N}$ deposition and/ or ecological impacts.