Freshwatermussels are undergoing rapid global declines due to habitat loss and fragmentation, among other factors,
but little is known about the effects of small hydropower plants. Here we assessed the impact of small hydropower
plants on the abundance and size structure of the imperilled pearl mussel Margaritifera margaritifera.
For this, we sampled 66 sites in three Portuguese rivers (Mente, Rabaçal and Tuela) located upstreamand downstream
of dams andwithin the reservoirs. Pearl musselswere significantly more abundant upstreamthan downstream
of dams (97.4% more) or within reservoirs (98.5% more). In addition, juveniles were mostly found
upstream of dams. The most significant environmental alterations that explained the observed patterns were related
to changes in sediment characteristics (accumulation of fine sediments and organic matter in reservoirs)
andwater chemistry, most notably suspended solids (highest values in reservoirs) and dissolved oxygen (lowest
values in reservoirs). Overall, results show that small hydropower plants can deeply affect pearl mussel populations:
specimens almost disappeared from the areas within the reservoirs and sites located downstream only
retained adults without signs of recent recruitment. Future management measures devoted to the conservation
of pearlmussels should take into account the results reported here to avoid the construction of newdamsin pearl
mussel rivers; improve management of the river flow in downstream areas; and consider the decommissioning of dams in pearlmussel riverswith a particular attention devoted to the re-naturalization of river sections under
the influence of the reservoir and downstream areas.
This work was supported by: i) POSEUR-03-2215-FC-000096 and
ICNF funds under project CP01-MARG-QUERCUS/2018; ii) Project Reviving
Douro Basin funded by MAVA, Fondation pour la Nature and iii)
European Investment Funds by FEDER/COMPETE/POCI – Operational
Competitiveness and Internationalization Program, under Project
POCI-01-0145-FEDER-006958 and National Funds by FCT - Portuguese
Foundation for Science and Technology, under the projects UID/AGR/
04033/2013 and UID/AGR/00690/2019. FCT also support MLL with a
doctoral grant (SFRH/BD/115728/2016).
