Volunteers have become more and more involved in monitoring the quality of the environment in which they live. Traditionally, water quality monitoring is the field of professionals at water authorities, however, community initiatives have been undertaken to monitor abiotic conditions in waterbodies. To date, biological water quality assessment based on data collected by volunteers remains scarce. A citizen science project on biological water quality assessment was launched in the Netherlands in 2018. In this project, volunteers collect macroinvertebrates from in a nearby waterbody, identify and count the number of specimens, and register their catch through a web portal to instantaneously receive a water quality score based on their data. This study compares the data from the volunteers with the data from professionals focussing on type of sampled waterbody, sampling period, observed animals and water quality.

The analyses show that volunteers and professionals seldomly sample the same waterbody, that there is some overlap in sampling period, and that volunteers more frequently sampled smaller water bodies and more urban waters. The citizen science project is thus yielding useful spatially and temporally complementarity data. The assessments by volunteers and professionals likely differ in character and thoroughness. Volunteers collected significantly lower numbers of animals per sample and fewer animals from soft sediments but more mobile individuals from the open water column. As a result, the water quality scores between volunteers and professionals differ. To bridge these differences, new tools and processes may need to be developed to further increase the value of monitoring biological water quality by volunteers for professionals.

The data from the citizen science project were retrieved from the original website 'waterdiertjes.nl' and cover the period 2018-2020. Only relevant fields are included in the dataset being location code, year, month and date of samping, user ID, watertype, latin name of taxa and abundance. Multiple recordings at the same location on the same date and nearly time were aggregated into one sampling. Each line equals the abundance of one taxon at a specific location and specific date. Taxonomy follows the British Freshwater name trail (Orton R, Bebbington A, Bebbingon J. 1996. The freshwater name trail. Field Studies Council Publications, Telford, UK.)

The data from the professionals were retrieved from the original website 'waterkwaliteitsportaal.nl' and cover the period 2013-2019. Only macroinvertebrate recordings were retrieved that were collected following the multihabitat approach (Barbour, Stribling, Verdonschot. 2006. The multihabitat approach of USEPA’s rapid bioassessment protocols: benthic macroinvertebrates. Limnetica. 25: 839-850) with a standardized sampling length of 5 m. The data were aggregated to the same taxonomic level as used by the citizen science project 'waterdiertjes.nl' and thus following the British Freshwater name trail (Orton R, Bebbington A, Bebbingon J. 1996. The freshwater name trail. Field Studies Council Publications, Telford, UK.). All other taxa were disregarded.