Fifty shades of grey: Variability in metric-based assessment of surface waters using macroinvertebrates

Since the introduction of the European Water Framework Directive (WFD) in 2000, every member state is obligated to assess the effects of human activities on the ecological quality status of all water bodies and to indicate the level of confidence and precision of the results provided by the monitoring programs in their river basin management plans (European Commission, 2000). Currently, the statistical properties associated with aquatic monitoring programs are often unknown. Therefore, the overall objective of this dissertation is to quantify the variability and accuracy associated with biological assessment based on macroinvertebrates. This dissertation shows that the variability in metric values applied in biological assessment is often high. We should face the issue of high variability by gaining a better understanding of ecosystem functioning and unraveling cause-effect mechanisms, as well as, by developing more cost-effective sampling and sample processing methods. A short-term solution to reduce variability and improve the performance of currently applied assessment systems in the Netherlands would be the implementation of quality assurance and quality control procedures. Additional standardization of the sampling and sample processing protocol is required, especially in terms of sorting effort. In the long run, water managers need to consider applying probability sampling to draw statistically sound conclusions at water body/national level. Combining probability sampling with a relatively cheap sampling and sample processing method to assess ecological status (‘Quick Scan’ method) will result in more cost-effective monitoring programs.