although it controls for spatial variation, natural temporal variation is confounded with potential effects of impacts. b-a studies were conducted by larkum and west to monitor long-term changes in seagrass meadows in botany bay. australia. aerial photography and field surveys revealed that posidonia australis had undergone a steady decline over 40years concomitant with residential and industrial development in the catchment (watershed). however, zostera capricorni was found to have increased in abundance, colonizing many sites previously occupied by p.australis. the changes in these species distributions were coincident with development, but cannot be ascribed to a cause and effect relationship. larkum and west state taht care must be taken not to confuse man-induced effects with natural cyclic events.by sampling along a gradient of disturbance, short and burdick were able to demonstrate that impacts to zostera marina distribution over time were due to residential development (with the disturbance identified as nutrient input). in this case, the problem of temporal variability was excluded by stratifying sampling along the impact gradient. the confounding sources of spatial and temporal variability associated with both c-i and b-a designs, respectively, can be overcome through combining both methods into a baci design