As for many other regions, environmental and biodiversity monitoring of the brackish Baltic Sea suffers from low species resolution for several taxa. One such case is the benthic larvae of midges Chironomidae (Diptera), which are estimated to constitute about 30% of the macrozoobenthos species of the Baltic Sea and are important indicators of environmental quality. We assessed the usefulness of COI (cytochrome oxidase I) gene barcoding to improve species resolution and its potential for implementation in monitoring programmes. Neighbour-Joining, Maximum parsimony and Bayesian-inference analyses all provided high congruency with morphological analyses of adult males for almost all 42 species studied. Barcoding was helpful to elucidate some cases of taxonomical difficulties, such as synonyms. In contrast to the high identification accuracy when using our local database, there were a number of cases where matching with GenBank and BOLD provided puzzling results. For reliable species identification at least 15-30 specimens from 5-10 well-distributed sites within the geographical range of the species might be needed in a database to adequately cover the intraspecific variability of chironomids. Implementation of DNA barcoding, as applied here, in monitoring would result in an increase from at present less than 10% to more than 90% successful chironomid species identification of Baltic Sea benthic samples, as it also would for many nearby lakes. Routine monitoring of benthic environmental samples based on Next-Generation sequencing techniques would provide a cost effective way to obtain a taxonomically much more complete assessment of environmental quality and biodiversity, as required by EU directives and national legislation.

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Infaunal predators participate in structuring benthic comnlunities by affecting species negatively or positively. The impact of the predatory isopod Saduria entomon (L.) on the bivalve Macoma balthica (L.) was studied in parallel in the laboratory and the field. The hypotheses tested were (1) S. entomon preys on newly settled and small M. balthica and (2) S. entomon affects the recruiting macrofauna significantly. In the laboratory several experiments with 1 specific size class of M. balthica per experiment were performed. Sizes ranged from newly settled postlarvae to 3.2 mm long specimens. Analysis revealed that S. entomon preys actively on M. balthica above a certain minimum size (0.8 mm), and on all the larger sizes tested. In a d l t i o n , pilot experiments showed that 34 mm long S. entomonwere able to open and eat 17 mm long M. balthica, a prey half as large as the predator Recruitment, establishment and early succession were studied in the field. Boxes with and without enclosed S. entomon were sampled after 3 mo. The assemblages that had developed were similarly diverse and abundant in both treatments. However, M. balthica decreased significantly when S. entornon was present.

Behavioural responses of 2 Baltic benthic crustaceans to chemical substances from predators were studied using infrared video-recording in the laboratory. This is the f~rst study of behavioural responses to species-specific chemical substances in a 3-trophic-level food chain. Exposure to chemical substances from a predatory fish, the short-horned sculpin Myoxocephalusscorp~us (L.), ca.used the isopod Saduria entomon (L.) to remain buried in the sediment most of the time and decreased its foraging success on the amphipod Adonoporeia affinis (Lindstrom). M. a f f~n~s decreased its swimming activity when exposed to water from S. entomon feeding on M. affinls, whereas water from unfed S, entomon had no such effect.

Recent advances in molecular biomonitoring open new horizons for aquatic ecosystem assessment. Rapid and cost-effective methods based on organismal DNA or environmental DNA (eDNA) now offer the opportunity to produce inventories of indicator taxa that can subsequently be used to assess biodiversity and ecological quality. However, the integration of these new DNA-based methods into current monitoring practices is not straightforward, and will require coordinated actions in the coming years at national and international levels. To plan and stimulate such an integration, the European network DNAqua-Net (COST Action CA15219) brought together international experts from academia, as well as key environmental biomonitoring stakeholders from different European countries. Together, this transdisciplinary consortium developed a roadmap for implementing DNA-based methods with a focus on inland waters assessed by the EU Water Framework Directive (2000/60/EC). This was done through a series of online workshops held in April 2020, which included fifty participants, followed by extensive synthesis work. The roadmap is organised around six objectives: 1) to highlight the effectiveness and benefits of DNA-based methods, 2) develop an adaptive approach for the implementation of new methods, 3) provide guidelines and standards for best practice, 4) engage stakeholders and ensure effective knowledge transfer, 5) support the environmental biomonitoring sector to achieve the required changes, 6) steer the process and harmonise efforts at the European level. This paper provides an overview of the forum discussions and the common European views that have emerged from them, while reflecting the diversity of situations in different countries. It highlights important actions required for a successful implementation of DNA-based biomonitoring of aquatic ecosystems by 2030.

Field studies in the Baltic Sea indicate that the growth rate of the deposit-feeding benthic amphipod Monoporeia affinis (Lindström), which is found in densities of up to ≅25 000 ind. m -2