Reach- and mat-scale differences in Microcoleus autumnalis (cyanobacterium) accrual along velocity and nitrate gradients in three New Zealand rivers

Proliferations of the toxic, mat-forming cyanobacterium Microcoleus autumnalis are an increasingly recognized problem in cobble-bed rivers worldwide. This study explored how flow and nutrient concentrations influence mat expansion. Microcoleus autumnalis was inoculated into cobbles placed in runs, riffles, and pools in three rivers with different nutrient conditions, and mat size was monitored over 21 days. The following hypotheses were tested: (i) mat expansion will reflect cover increases at the reach scale; (ii) biomass and cover will be highest in high-velocity habitats; and (iii) under similar velocities, nutrient concentrations will be more important than other abiotic and biotic variables in determining expansion rates. Mat expansion accurately reflected the increase in reach-scale cover, and expansion was most rapid at intermediate water velocities (0.25-0.45 m.s(-1)). Mats persisted the longest in riffles. Accrual cycles were terminated earlier in runs than in riffles, as high expansion rates resulted in patches reaching maximum mat size rapidly. Although M. autumnalis accrual differed among rivers, this was attributed to differences in shear stress and grazing pressure rather than nutrient concentrations.