To store or to drain - To lose or to gain? Rewetting drained peatlands as a measure for increasing water storage in the transboundary Neman River Basin.

Agriculture continues to place unwanted pressure on peatland functionality, despite international recognition calling for their conservation and restoration. Rewetting of peatlands is often the first step of restoration that aims towards improving the delivery of ecosystem services and their benefits for human well-being. Ongoing debates on peatland restoration in agricultural landscapes raise several issues based on the valuation of benefits achieved versus the costs of peatland restoration. Using the transborder Neman River Basin in North-Eastern Europe, this study aimed to quantify and evaluate the gains provided by peatland rewetting. To achieve this, this study estimated i) possible changes in water storage capacity from peatland restoration, ii) the value of expected benefits from restoration and iii) costs of restoration measures at the overarching basin level. Applying multiple assumptions, it was revealed that rewetting drained peatlands in the Neman River Basin could increase water retention by 23.6-118 M m3. This corresponds to 0.14-0.7% of the total annual Neman River discharge into the Baltic Sea. Unit increase of water retention volume due to rewetting ranged between 69 and 344 m3·ha-1. The estimated water retention value ranged between 12 and 60.2 M EUR·year-1. It was also shown that peatland rewetting at the scale of Neman River Basin would cost from 6.8 M and 51.5 M EUR·year-1 depending on the selected scenario. Applying less expensive rewetting measures (non-regulated outflow from ditch blocks), the economic gains (as water storage ecosystem service of rewetted peatlands) from rewetting exceed the costs of rewetting. Thus, rewetting peatlands at a river-basin scale can be considered technically and economically efficient measures towards sustainable management of agricultural landscapes. The novel methodology applied in this study can be used when valuing trade-offs between the rewetting of drained peatlands and leaving them drained for the uncertain future of wetland agriculture.

Microhabitat Conditions and Inter-Species Competition Predict the Successful Restoration of Declining Relict Species Populations.

The local populations of relict plant taxa living near the limits of their geographical range are particularly vulnerable to extinction. For example, Salix lapponum is one of the rarest and most endangered glacial relicts in Western and Central Europe. In Poland, the number of its sites has dramatically decreased over the past few decades, prompting us to take conservation measures focused on saving endangered populations. During a field experiment aimed at the reconstruction of the downy willow population in the Knyszyn Forest (NE Poland), 730 individuals of the species were planted in four different natural sites. The seedlings were obtained by micropropagation from parts of vegetative individuals taken from the most abundant population of this species in eastern Poland (Lake Bikcze). The success of the reintroduction, measured by the number of individuals that survived 2-3 years in the wild and took up growth, was about 67%, however, with low flowering efficiency (7.5%). Additionally, monitoring showed significant differences in plant survival and growth rates under different habitat conditions prevailing at the site and with different cover from competing species, especially tall grasses. However, the restoration projects for relict shrub species should include periodic removal of competing plants and protection of plants from trampling and browsing by herbivorous mammals to increase reintroduction success.

The Chinese pond mussel Sinanodonta woodiana demographically outperforms European native mussels.

Unionid mussels are essential for the integrity of freshwater ecosystems but show rapid worldwide declines. The large-sized, thermophilic Chinese pond mussel Sinanodonta woodiana s.l., however, is a successful global invader, spread with commercially traded fish encysted with mussel larvae; its negative impacts on native mussels are expected. Here, we exploit a natural experiment provided by a simultaneous introduction of S. woodiana and four species of native unionids for water filtration to a pond in north-eastern Poland. Sinanodonta woodiana established a self-sustaining population and persisted for 19 years in suboptimal thermal conditions (mean annual temperature, 7.4 °C; mean temperature of the coldest month, - 3.7 °C, 73-day mean yearly ice-formation), extending the known limits of its cold tolerance. Over four study years, its frequency increased, and it showed higher potential for population growth than the native mussels, indicating possible future dominance shifts. Outbreaks of such sleeper populations are likely to be triggered by increasing temperatures. Additionally, our study documents the broad tolerance of S. woodiana concerning bottom sediments. It also points to the importance of intentional introductions of adult individuals and the bridgehead effect facilitating its further spread. We argue that S. woodiana should be urgently included in invasive species monitoring and management programmes.

Lake-stream transition zones support hotspots of freshwater ecosystem services: Evidence from a 35-year study on unionid mussels.

Securing adequate supply of high-quality water is of increasing global importance and relies in large part on ecosystem services provided by freshwater biota. Unionid mussels are important keystone species and habitat engineers that shape freshwater ecosystems through water filtration, nutrient cycling and provision of habitats; their rapid global declines result in dramatic losses of ecosystem functions. Maintenance and enhancement of the services they provide depend on the identification of their crucial habitats. Following theoretical assumptions, this study analyses the importance of lake-stream transition zones for unionid mussels, based on data collected in 1984 and 2019 from an undisturbed stream flowing through five consecutive lakes. Mussel distribution matched the distribution of host fish and was strongly influenced by lakes: densities were highest near lake outlets, reaching 290 ind. m-2 (14.7 kg m-2) in 2019, and declined with downstream distance following a negative power function. This pattern was spatially consistent and sustained over time. All six unionid species native to north-central Europe were present, but common species (Anodonta anatina, Unio pictorum, U. tumidus) contributed about 80% of individuals and were responsible for most of the ecosystem services provided by unionid mussels. Estimated 1.9 × 106 mussel individuals inhabiting 3.2 km of stream length filtered a water volume equivalent to the total stream discharge approximately 2.5 times daily. Aggregations of spent shells, up to 17 kg m-2, accumulated downstream of lakes, forming extensive shell and mussel beds, providing habitats and contributing shell hash that improved stream-bed conditions. Globally invasive Dreissena polymorpha was present at low densities and did not spread or increase in abundance, indicating a long-term biotic resistance of the natural native community. Our study underscores the importance of undisturbed lake outlets, longitudinal connectivity of riverine ecosystems, and of common mussel species in maintaining freshwater ecosystem functionality and provision of vital services.