Developing an efficient macrofauna monitoring index from an impact study — A dredge spoil example

Data from an initial impact study were used to develop a labour-efficient index for biological monitoring of dredge spoil disposal. Each of 12 indicator species was assigned a score, based primarily on the ratio of its abundance in control vs impacted samples. The ‘Index’ value was the average score of those indicator species present in the sample. The Index was tested during a second survey. Sample processing for Index calculation took only 25% of the time required to process all macrofauna. The Index value provided a sensible, semi-quantitative indication of impact on benthic macrofauna. Index values of <2, 2–6 and > 6 were indicative of ‘severe’, ‘patchy’ and ‘no’ impact respectively. Mean Index value correlated with mean macrofaunal richness (r = 0.93, P < 0.05) and abundance (r = 0.92, P < 0.05). While the Index is site- and impact-specific, the process of developing efficient monitoring tools from an initial impact study should be widely applicable.