Environmental DNA (eDNA) analysis allows non-invasive and cost-effective monitoring of species distribution and composition in aquatic ecosystems. Benzalkonium chloride (BAC) treatment is an inexpensive and simple method for preserving macrobial eDNA in water samples, which is suitable for maximizing both the number of sampling replicates and water volume. However, its preservation performance has been evaluated in a limited manner by species-specific assays, targeting short fragments of mitochondrial DNA in freshwater and brackish ecosystems. Here, we examined the performance of BAC in preserving eDNA in seawater samples, targeting different fragment lengths of mitochondrial and nuclear eDNA, and community information inferred by eDNA metabarcoding. First, we quantified the time-series changes of Japanese jack mackerel (Trachurus japonicus) eDNA concentrations in experimental tanks and inshore seawater to compare the yields and decay rates of eDNA between BAC treatments. As a result, BAC addition increased the eDNA yields at the start of the experiment and substantially suppressed the initial phase of rapid degradation but not the subsequent phase of slower degradation. In addition, we performed eDNA metabarcoding targeting fish community, showing that BAC addition suppressed the decrease in species richness, where the number of fish species hardly varied throughout the day. Findings of the present and previous studies indicate high versatility of BAC in preserving qualitative (species richness) and quantitative (copy number) information on aqueous eDNA under various environmental conditions. BAC should therefore be used to minimize the false-negative detection of eDNA, regardless of target genetic regions, fragment sizes, environmental conditions, and detection strategies.

Changes in the thermal structure of lake ecosystems have been documented as a precursor of climate change, but the dynamics of biomass distribution, which fundamentally determines species conservation, have been less studied. An interdisciplinary approach was used to demonstrate the influence of climate-driven changes on the biomass distribution of two species (Gymnogobius isaza and Palaemon paucidens) in Lake Biwa. In field surveys in 2016–2017 (full water circulation) and 2019 (partial water circulation), environmental DNA concentrations of these species were used as proxies for biomass to measure 43 and 47 sites sampled at the lake bottom, respectively. A structural equation model was used to estimate the correlation between species biomass and environmental parameters. The species-environment relationship was applied to species biomass distributions under existing and future environments calculated by the model. Differences between the species were found in their responses to climate change. The biomass distribution of G. isaza will benefit in the future if full water circulation occurs, although it appears to be independent of water circulation at present. Partial water circulation enlarges the distribution area of P. paucidens, but its biomass will be low in the future, regardless of the extent of water circulation. These findings advance the knowledge of how species respond to climate change and suggest special attention should be given to species such as P. paucidens, which is currently abundant but sensitive to climate change. Furthermore, they emphasize the potential application of interdisciplinary methodologies for improved species conservation.