FLOOD AND VELOCITY EFFECTS ON PERIPHYTON AND SILT ACCUMULATION IN 2 NEW-ZEALAND RIVERS

Effects of water velocity on the accumulation of periphyton and silt w ere investigated in two unenriched rivers (Tongariro and West Kowai Ri vers) of New Zealand. Artificial substrata were incubated for up to 5 weeks in areas of moderate (0.2-0.5 m s(-1)) and high (0.6-0.9 m s(-1) ) velocity. In the Tongariro River, periphyton accrual was significant ly higher in moderate velocities than in high velocities, but not in t he West Kowai River. The accumulation of silt was significantly higher under the moderate velocity treatment in both rivers. Silt densities were closely correlated with periphyton chlorophyll a and stepwise mul tiple regression analysis indicated that periphyton biomass was a more significant determinant of silt densities than water velocity. Neithe r chlorophyll a nor silt densities showed any significant variation wi th depth within the ranges measured (0.2-0.6 m). Several floods occurr ed during the study period and these, together with water velocities g reater than c. 0.3 m s(-1), tended to affect periphyton ash-free dry m ass (AFDM) and silt more than they affected chlorophyll a. It appeared that growing periphyton were resisting the effects of high shear stre ss and that unattached inorganic and organic particles were flushed fr om the periphyton matrix during higher flows. Spatial variation in per iphyton and silt accrual as functions of velocity and depth were too v ariable to be useful for prediction of abundance and distribution usin g hydraulic-habitat models.