WATER-QUALITY IN A POLLUTED LOWLAND STREAM WITH CHRONICALLY DEPRESSEDDISSOLVED-OXYGEN - CAUSES AND EFFECTS

The Whangamaire Stream (North Island, New Zealand) has high concentrat ions of nitrate nitrogen (NO3--N), biochemical oxygen demand (BOD5), a nd Kjeldahl nitrogen (TKN) as a result of catchment land use practices . The lower reaches of the stream drain intensively farmed land and ha ve dissolved oxygen (DO) levels of 10-50% saturation. The dominant rip arian vegetation, Apium nodiflorum, provides a large organic loading b y intercepting nutrients in run-off and then decaying in the stream ch annel. Water quality and reaeration aspects of the stream were studied in order to explain the observed low DO levels. Measurements of the r eaeration coefficient at 20 degrees C, K-2(20), using methyl chloride (CH3Cl) as a gas tracer, yielded values of 1.1-3.0 d(-1) for the upper part of the study reach and 15.5-16.2 d(-1) for the lower reach (over all average 12.5 +/- 2.5 d(-1)). These were in agreement with values i nferred from single-station diurnal curve analysis, which also showed that respiration was dominant in the lower reach where photosynthetic activity was inhibited by shade. The relatively large reaeration coeff icients ensure that parts of the stream do not become anoxic at night time. Better riparian management and reduced nutrient inputs are likel y to improve stream water quality.