## 4 Discussion
[PERSON] (2004) [45] found that three unionid species, namely _Anodonta anatina, Unio pictorum_, and _Unio tutmidus_, removed 45% of the suspended solids and produced feces and pseudofeces. No mussel died during the study period. Our previous studies reported that no _Unio douglasiae_ died in experimental chambers during daily administration of toxic cyanobacterial water for 3 months [8]. Thus, we present that _Unio douglasiae_ did not prefer _Microcytstis_-rich water as a nutrient or an energy source, due to its low nutritional value, and strong toxicity of _Microcytstis aeruginosa_. Previous studies have demonstrated that filter feeders, such as fish and mussels (i.e., _Corbicula fluminea_), are likely to disorganize particulate organic nitrogen and phosphorus and improve the biodegradation of POM through filter-feeding, digestion, and excretion [46, 47, 48]. In particular, excretion of a large amount of NH\({}_{4}\) is expected following the introduction of uninorul mussels to M-waters with cyanobacterial blooms [8, 49, 50, 51, 40]. The lethal NH\({}_{4}\) concentration (LC50) for juvenile unionids (_Lumpsilis caratum_) is 93-140 \(\mu\)g L\({}^{-1}\) for 10 days [52] or 157-372 \(\mu\)g L\({}^{-1}\) for 96 h [53]. Similarly, we observed remarkable increase in NH\({}_{4}\) concentration after mussel stocking (Figure 7). At 60 h, NH\({}_{4}\) concentration peaked to reach 198 \(\mu\)g L\({}^{-1}\), being 25 times higher than the control value; nonetheless, no mussel exposed to this M-water died over the experimental period (72 h). In SPM, BODY, and FECAL samples, the levels of newly biosynthesized carbon and nitrogen primarily increased during the initial 24 h of the experiment, with simultaneous increase in isotope accumulation into tissues. After 24 h, the uptake and accumulation of both \({}^{13}\)C and \({}^{14}\)N isotopes declined rapidly and did not increase until the end of the experiment. Thus, in mussels, nutrient uptake and accumulation may be accompanied by NH\({}_{4}\) excretion. Additionally, our findings can help elucidate the mechanisms through which uninorul mussels detect or reject _Microcyts aeruginosa_, and this information may be beneficial to overcome the hazards of ammonia and microcystis toxicity of water in small closed systems. Overall, indigenous mussel populations in Korea can be conserved and sustained even in streams and lakes with cyanobacterial blooms. There have been concerns about negative changes in water quality and biodiversity in aquatic ecosystems due to the outbreak of harmful cyanobacteria in rivers and estuaries in Korea [8]. The bivalve _Unio douglasiae_ has been used for food [5], and is surviving excellently even in situations where there are large outbreaks of cyanobacteria bloom. In this study, mussels showed to have potential of an effective controller exerting a rapid feeding on toxic _Microcyts aeruginosa_. On the other hand, pseudofeces produced after mussel feeding sink to the bottom of the water body and may provide food source for other benthic organisms. However, a toxic cyanobacterium would not be used as food in practice; rather, adverse effects on other organisms are expected due to the release of high concentrations of toxins and ammonia. Additionally, an accumulation of toxins through the food chain is also expected to cause direct or indirect damage to animals and humans.