We measured ecosystem metabolism and organic matter transport seasonally in five streams draining catchments dominated by native forest, exotic pine plantation, grazed tussock grassland, or developed pasture. All streams are tributaries of the Taieri River in southeastern New Zealand. Whole-stream metabolism was estimated by both twostation and single-station open-channel methods, allowing comparison between these techniques. Transfer of oxygen across the stream surface was estimated using reaeration coefficients determined from three different procedures: tracer gas injections (propane), analysis of the oxygen record, and empirical velocity-depth equations. Measurements of gross primary production (GPP) and community respiration (CR) showed differences among streams that reflected method rather than ecosystem process. The ratio of GPP:CR and net ecosystem metabolism, however, showed strong concordance among methods, suggesting that bias involved in standardizing estimates to areal units may be more important than differences among methods. The two-station approach appeared to cope more adequately with steep slope, high bed roughness, and low GPP than did the single-station method. However, in tranquil and productive streams, both methods worked well. When differences between methods were accounted for, results emphasized how differences in terrestrial landscapes may markedly affect ecosystem processes within streams. Shading by the heavy canopy at the native forest site, turbidity at the pasture site, and the valley walls at one of the tussock grassland sites appeared to limit GPP. CR was high in the native forest site, due to a large supply of organic material from the riparian zone. Concentrations of seston were highest at the developed pasture site, apparently a result of intensive grazing and associated bank failure. The organic content of seston was highest in the native forest site and lowest in the pasture site. Organic carbon spiraling length in the pasture site was longer than average for a stream of its size, whereas spiraling length in the native forest site was short when compared to similar-sized streams elsewhere. Changes in catchment land use and riparian vegetation, as well as reach-specific geomorphic factors, alter light availability and organic matter supply, which are fundamental factors controlling organic matter production, respiration, and transport in streams.

Physiological evidence is presented that visual receptive fields in the primate eye are shaped like the sum of a Gaussian function and its Laplacian. A new 'difference-of-offset-Gaussians' or DOOG neural mechanism was identified, which provided a plausible neural mechanism for generating such Gaussian derivative-like fields. The DOOG mechanism and the associated Gaussian derivative model provided a better approximation to the data than did the Gabor or other competing models. A model-free Wiener filter analysis provided independent confirmation of these results. A machine vision system was constructed to simulate human foveal retinal vision, based on Gaussian derivative filters. It provided edge and line enhancement (deblurring) and noise suppression, while retaining all the information in the original image.

1. The impact of agricultural activities on waterways is a global issue, but the magnitude of the problem is often not clearly recognized by landowners, and land and water management agencies. 2. The Pomahaka River in southern New Zealand represents a typical lowland catchment with a long history of agricultural development. Fifteen sites were sampled along a 119‐km stretch of the river. Headwater sites were surrounded by low‐intensity sheep farming, with high‐intensity pasture and dairying occurring in the mid‐reach and lower reaches. 3. Water clarity decreased significantly from about 6 m in the headwaters to less than 2 m in the lower reaches. Benthic sediment levels increased significantly downriver, peaking at 35 mg m−−2 below several tributaries with high‐intensity agriculture in their catchments. Periphyton levels were also significantly greater in the lower reaches than the headwaters, and coincided with increased nitrogen (DIN) and phosphorus (SRP) concentrations. 4. Macro‐invertebrate species richness did not change significantly throughout the river, but species composition did with Ephemeroptera, and to a lesser extent, Plecoptera and Trichoptera dominating the headwater sites (where there was high water clarity, and low nutrient and periphyton levels). Downriver these assemblages were replaced by molluscs, oligochaetes and chironomids. 5. Canonical correspondence analysis indicated that agricultural intensity and physical conditions associated with agriculture activity (e.g. impacted waters, high turbidity and temperature) were strongly associated with the composition of benthic assemblages at differing reaches down the Pomahaka River. 6. The present results indicate that quantifying agricultural intensity within a catchment, particularly relative livestock densities, may provide a useful tool for identifying threshold levels above which river health declines.

No abstract