It is evident that the low spatial variance in diversity exhibited by the benthic and epibenthic community in the Zone (MESL 2011) would have major implications for the Power analysis. It is the variance around the mean that is the main driver for the estimated sample number and, as in this case where that variance is low, Power analysis informs us that a very high statistical Power is possible with a relatively small number of samples. This is most evident in the plots of ESN against EDCP which show that the lower limit of sample size that can be estimated from a Power analysis (n = 2) is reached before an EDCP of 50% is reached. This low community variation has positive implications for the EIA, as any changes in the community structure should be easily detected.
Although the relationship between detectable change and sample number shows very high Power, we cannot, however, assume that any post impact survey will show the same low levels of variance. Increasing levels of variation in future may compromise Power and therefore a conservative approach is advised. This is because increasing variation in spatial patterns of diversity is one potential outcome of development. Nevertheless, we have shown that the current survey protocol will detect change well below the proposed 50% (doubling or halving of diversity) that is deemed appropriate for many biological communities, we can assign high confidence that it will highlight any changes in the communities present in East Anglia THREE and FOUR and the cable corridor (once adequately surveyed).
The levels of variance found also have implications beyond this study. The low EDPC/high effect size relationship has resulted in a high effect size being selected as suitable for this study; that is an es of 0.8 consistently relates to a percentage change substantially below the 50% threshold. It is important however, to note that an effect size of 0.8 may not be applicable to Power analysis conducted for other benthic surveys, as they may demonstrate considerably greater spatial variation in diversity than found here. Indeed, the use of an effect size of 0.8 for other studies may potentially compromise those studies to the extent that major change is not perceived, or that an unmanageable number of samples are required. In the absence of regulator guidance, we therefore recommend the tailored approach retaining the 50% detectable change level to Power Analysis for similar studies.
5.2 Benthic Surveys
It is clear from the high Power and low EDCP values shown that the number of benthic stations sampled in undertaken in East Anglia THREE and FOUR are adequate to assess any change in community structure. Indeed the level of change detectable is quite low (estimated as low as 10% for Simpson metrics in East Anglia THREE).
Assuming that the homogenous ecological diversity recorded across the Zone also applies to the unsurveyed areas, then it is assumed that the cable corridor will demonstrate similar high Power/low EDCP levels based on the variance from the
