Incorporation of spatial gradients into benthic impact assessment

Ellis, Joanne I. (Joanne Irene) (1997) Incorporation of spatial gradients into benthic impact assessment. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Before-After Control Impact (BACI) sampling designs are commonly used in environmental impact assessment and are considered the most effective for detecting changes due to anthropogenic disturbances. These designs handle local spatial variability through randomized placement of samples into a treatment stratum and one or more control strata. While BACI designs based on agronomic block procedures are appropriate for disturbances that have defined boundaries, these designs suffer from serious limitations when applied to point source disturbances where the scale of disturbance is unknown. When a contaminant disperses with distance from a point source it is suggested that a 'gradient' design will be more sensitive to change than randomized placement of samples. This requires allocating samples according to distance, rather than by random placement within randomly placed strata (blocks). This thesis develops the use of gradient designs for environmental impact assessment. -- Gradient versus random sampling designs were compared using data from an oil field in the North Sea. The gradient sampling designs were more powerful than a randomized block design for point source disturbances, and enabled the scale of the disturbance to be readily identified. A gradient layout also avoids the problem of selecting adequate control sites, and gradient designs lend themselves to constructing mechanistic models that generate testable predictions of attenuating effects with increasing distance. The assumptions of gradient designs are that no natural gradients exist or that natural gradients do not effect community response to the anthropogenic disturbance. This assumption was tested using data where an increasing physical gradient due to increased wind/wave and tidal disturbance existed with distance from a sewage outfall at Manukau Harbour, New Zealand. -- Statistical methods to analyze data from gradient designs are reviewed. A method using regression to estimate the magnitude of spatial gradients successfully separated sensitive and tolerant species. Ordination and gradient techniques differed in their ability to generate and test causal models and to identify the extent of the disturbance. Ordination techniques were more sensitive at detecting community change, while gradient methods lead to quantitative predictions than can be readily tested.

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