Detecting anthropogenic disturbance in an environment with multiple gradients of physical disturbance, Manukau Harbour, New Zealand

Demonstrating spatial or temporal gradients of effects on macrobenthic comm unities can be a useful way of providing strong empirical evidence of natur al or anthropogenic disturbance. Gradient designs for environmental assessm ent are sensitive to change for point source data, enabling the scale of th e effects of a disturbance to be readily identified. If the spatial scale t hat is sampled from the point sourer is adequate. problems of selecting con trol sites can be avoided. However, sources of spatial variation in macrobe nthic communities, which are not related to the impact, can confound the us e of gradient designs. This can occur if the natural spatial structure over laps that of the gradient and cannot be identified either as a location or environmental covariable, The ability to detect point source impacts using a gradient design against natural spatial variability was tested using bent hic macrofaunal data collected from Manukau Harbour, New Zealand. Treated s ewage wastewater is discharged into the north west area of the Manukau Harb our. Sandflats in the vicinity of the outfall are also subject to physical disturbance from wind-waves and strong tides. Ordination techniques and the testing of a priori predictions were used to try and separate the relative effects of organic and physical disturbance on the benthic communities. Wh ile the occurrence of other environmental disturbances along a gradient of anthropogenic disturbance makes interpretation of community pattern more di fficult, the use of a gradient sampling layout, ordination analysis and the testing of a priori predictions enabled impacts of the anthropogenic and n atural environmental disturbances to be interpreted. Gradient designs, ther efore, provide a method of assessing complex impacts that operate over broa d spatial and temporal scales.