Changes of Biological Quality Elements during rehabilitation activities in LTER site Srebarna Lake, (LTER EU BG 007 “SREBARNA”)

The Srebarna Lake is a shallow floodplain lake in NE Bulgaria, on the right bank of the Danube River, biosphere and managed reserve. Due to its importance as a unique ecosystem with diverse habitats and species richness it was designated as a Ramsar site (The Convention on Wetlands, RAMSAR. 1975), UNESCO Biosphere Reserve (Reserve 1977), a Monument of World Cultural and Natural Heritage (Heritage 1983), an Important Bird Area (Area 1989) and Natura 2000 protected zone (Natura 2000 protected zones 2008). The lake is characterized by an unstable water level, highly dependent on the level of the Danube River and is often subject to drastic variations, including risk of drying up. In 1948 the lake was completely isolated from the Danube River through embankment, and the ecosystem underwent significant changes, including increased eutrophication, hypertrophic conditions and severe algal blooms, accelerated habitat and species succession. In 1994 restoration activities started with the construction of a canal reconnecting the lake with the Danube River, intended to enhance the lake’s recovery. As a result of all these events, which have influenced the biodiversity of the reserve, the lake is subject to ongoing monitoring (as well as a site of the Bulgarian national hydrobiological monitoring network). This includes monitoring the water level, as well as hydrochemical and hydrobiological observations of the biological quality elements, as well as zooplankton. In 2022 and 2023, another restoration activities to improve connectivity were carried out: removing silt and detritus, cutting reeds, and building of an additional canal connection with the Danube River. For this purpose, intentional monitoring of aquatic biodiversity was conducted to track the changes in the composition and key parameters of the phytoplankton, zooplankton, macroinvertebrates and macrophyte communities and to guide and develop a remediation program if necessary. Changes in the species composition and structure of aquatic communities were monitored three times - before, during and after the rehabilitation activities, at four sampling stations (including а control one) for each of the activities. The following parameters were monitored: taxonomic composition (total number of taxa), species diversity (Shannon and Weaver 1949, trophic state index (TSI) (Carlson 1977), Hungarian lake phytoplankton index (HLPI), total abundance (ind./L) and biomass (mg/L) of zooplankton, Hungarian multimetric macrozoobenthic index for lakes (HMMI_lakes), Biological monitoring working party (BMWP), number of the macroinvertebrate families, Evenness index e (Pielou 1966), Dominance index s (Simpson 1949), relative abundance of macrophytes, Reference index RI (Gecheva et al. 2013). The monitoring results showed that some of the parameters studied changed negatively during the rehabilitation activities and therefore affected the community structure and ecological state of the lake. However, recovery processes occur at different rates for the individual biological elements – phytoplankton, macrozoobenthos, macrophytes and zooplankton, depending on the degree of deviation, but also on several other factors, such as seasonality, hydromorphological features, climate changes, etc.