The influence of aquatic macrophytes on the hydraulic and physico-chemicalproperties of a New Zealand lowland stream

The effects of macrophytes on hydraulic and physico-chemical variables were examined by conducting tracer experiments with SF6, CH3Cl and rhodamine WT in a stream before and after complete removal of plants from a 180 m reach . Whakapipi Stream has high average biomasses (up to 370 g dw m(-2)) of mac rophytes (predominantly Egeria densa) that, on average, cause summer veloci ties to be lowered by 30% and depths increased by 40%, compared to a plant- free channel. Manning's roughness coefficent was consistently higher by 0.1 3 and longitudinal dispersion coefficients were more variable (CV = 52%, cf . 20% when plants removed), when macrophytes were present. Stream dissolved oxygen (DO) and temperatures were unevenly distributed, possibly as a resu lt of transient storage zones attributable to plant biomass. Surface water in macrophyte patches was 1-5 degrees C warmer than water in channels or be neath the plants near the bed of the stream, and DO was 2-28% of saturation higher at the top of the plants than in channel water and up to 7% higher than in bottom water. Effects of increased small-scale turbulence on the re aeration coefficient, K-2(20), were cancelled by increased stream depth and reduced velocity so that it varied little with flow. Application of a sing le-station diurnal curve model, DOFLO (Dissolved Oxygen at Low Flow), to co ntinuous monitoring data gave values of K-2(20) in broad agreement with tho se measured by the gas tracer method and showed that rates of gross photosy nthetic production in daylight (10-27 g m(-2) d(-1)) and respiration at 20 degrees C (19-37 g m(3) d(-1)) were high by comparison with other rural str eams. Streams with smaller K-2(20) values than Whakapipi Stream but with si milar levels of productivity and community respiration would show more pron ounced diurnal variations in DO and even be anoxic at times.