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

Abstract

Thirty sites were sampled in 3 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 (Vmax), ATP, chlorophyll a (Chl a), and concentrations of N and P compounds. Bacterial populations per milliliter fluctuated 3-fold (6.4-19.4 .times. 105) along the Waikato River and were lower and more consistent in the 2 South Island rivers (1.46-2.55 .times. 105). In contrast, Vmax varied 5000-fold in the Waikato River, from a characteristically oligotrophic value of 0.0035 .mu.g .cntdot. l-1 .cntdot. h-1 (Lake Taupo outlet) to a eutrophic value of 18.4 .mu.g .cntdot. l-1 .cntdot. h-1 at the Mihi bridge. Vmax for the 2 South Island rivers ranged from 0.0091-0.189 .mu.g .cntdot. l-1 .cntdot. h-1. ATP, Chl a, Kjeldahl N, nitrate N, and total P concentrations for the 20 sites on the Waikato River varied in a similar way to the Vmax 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 2 South Island rivers. Most parameters were significantly correlated with each other for the Waikato River samples. The strongest correlations were between Vmax and bacterial numbers and between Vmax and nitrate N. 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 N, and between ATP and bacterial numbers.