Bacterial populations, heterotrophic potentials, and water quality in three New Zealand Rivers

Abstract

Abstract Thirty sites were sampled in three New Zealand rivers (Waikato, Maitai, and Wakapuaka) during late summer 1977. Samples were collected from just below the surface at mid river or in the tailraces below hydro‐electric dams. Parameters measured included bacterial numbers (direct counts), heterotrophic potential ( V max ), adenosine triphosphate (ATP), chlorophyll a (Chi a ), and concentrations of nitrogen and phosphorus compounds. Bacterial populations per millilitre fluctuated threefold (6.4–19.4 × 10 5 ) along the Waikato River and were lower and more consistent in the two South Island rivers (1.46–2.55 × 10 5 ). In contrast, V max varied 5000‐fold in the Waikato River, from a characteristically oligotrophic value of 0.0035 μg. l −1 ·h −1 (Lake Taupo outlet) to a eutrophic value of 18.4 μg. l −1 ·h −1 at the Mihi bridge. V max for the two South Island rivers ranged from 0.0091 to 0.189 μg. l −1 · h −1 . ATP, Chi a , Kjeldahl nitrogen, nitrate nitrogen, and total phosphorus concentrations for the 20 sites on the Waikato River varied in a similar way to the V max and bacterial data. There were large peaks at the Mihi bridge, lower values for the dam tailraces and significant increases for the sites below Hamilton. Concentrations for these parameters were lower and more consistent along the lengths of the two South Island rivers. Most parameters were significantly correlated with each other for the Waikato River samples. The strongest correlations were between V max and bacterial numbers and between V max and nitrate nitrogen. In the Maitai and Wakapuaka River series these correlations were also significant, but the only other significant correlations recorded there were between ATP and nitrate nitrogen, and between ATP and bacterial numbers.