Assessment of Ecological Status of Standing Water Bodies in Bulgaria: Calibration of a New Multi-Metric for Macroinvertebrates

Reliable assessment of the ecological status of standing waters is key for the effective implementation of the Water Framework Directive and for supporting water management and conservation. Biological quality elements, including benthic macroinvertebrates, provide an integrated measure of ecological condition by reflecting the cumulative impacts of environmental pressures over time. However, assessment methods developed for running waters are not readily applicable to standing waters, highlighting the need for category-specific approaches. We conducted a large-scale survey of representative stagnant water bodies (lakes and reservoirs) on the territory of Bulgaria. The goal was to develop a relevant type-specific methodology for the ecological status assessment of standing waters based on macrozoobenthos. We sampled 79 monitoring lake sites between May and November 2020 through an adapted version of the multi-habitat sampling methodology developed within the European AQEM/STAR projects in accordance with applicable standards. A composite sample was taken from the littoral zone of lakes and reservoirs, representative of the substrates available at each location. The sites represent 19 lake types. Eleven biological indices for lakes and seven for rivers were calculated in order to identify the most suitable metric. For each of the metrics, the pressure–response relationship was estimated. Being the first operational national index based on macroinvertebrates, the proposed Bulgarian multi-metric index for lakes is an adapted version of the Hungarian multi-metric macroinvertebrate index. It consists of three metrics, which have been separately tested in Bulgaria for monitoring purposes as early as 2011–2012 as well as in subsequent scientific research. Ecological quality class boundaries for each type of water body were determined. The compatibility of the BMMI with the existing national monitoring programme would facilitate its future integration into the biological water assessment system.