Forestry affects the abundance of Phormidium-dominated biofilms and the functioning of a New Zealand river ecosystem

Toxic cyanobacterial proliferations in water bodies can cause serious environmental and public health issues, as well as having economic effects. Increased inputs of nutrients and fine sediment caused by forestry have been hypothesised as possible causes of increased Phormidium-dominated proliferations in New Zealand rivers. Little is known about the effect of these proliferations on river ecosystem functioning. In the present study, we evaluated five sites along the Maitai River (New Zealand) differing in pine plantation cover of their catchments. We hypothesised that Phormidium biofilms would trap more sediments and recycle more phosphorus than diatoms, that Phormidium proliferations would increase with forestry cover in the catchment and that the varying abundance of Phormidium would affect river ecosystem functioning. Phormidium did not trap more sediment or recycle more phosphorus (measured as alkaline phosphate activity) than diatom biofilms. However, the cover of Phormidium did increase with the proportion of forestry in the catchment. Organic matter decomposition rates (measured as loss of tensile strength of standard cotton strips) varied very little among sites, whereas river ecosystem metabolism increased with the abundance of Phormidium, especially in the lower part of the river. The results of the present study suggest that pine forestry does promote Phormidium biofilm abundance and affect ecosystem functioning in the Maitai River.