Influence of seasonal variation on bioassessment of streams using macroinvertebrates

One of the objectives of the Water Framework Directive is to standardise bioassessment of surface waters. Water managers prefer cost efficient methods such as sampling only once a year for the purpose of surveillance monitoring. A higher level of standardisation could be reached if samples from the same area were collected in the same time period, but which time period would be most suited for taking samples? Many European countries have some general understanding about the preferred time period for sampling; however, in many cases scientific background is lacking. The aim of this study was (1) to examine the variation in macroinvertebrate community composition between months, (2) to assess the effect of this variation on 37 biological indicator metrics and the Dutch AQEM assessment system in particular, and (3) to determine whether a preferred time period for sampling can be indicated for lowland streams in the Netherlands