Ecological evaluation of the Small Negovan Lake after habitat restoration

The study presents result on the changes in the ecological status of the sandpit lake during first six years after its restoration. The lake is located on the remnants of an extensive wetland with high biodiversity and conservation value, which was drained in mid 30s of the 20th century. Later canalization and embankment of the river resulted in the hydrological cut-off and complete drying up of the wetland. Fortunately, the territory was simultaneously exploited for aggregate production, thus retaining water and providing refuge to a number of the original flora and fauna of the wetland. After the exploitation of bottom material was ended, the sandpit lake regained a semi natural appearance, partly due to a series of projects. The most recent and complete restoration took part in the period 2013-2015 with the aim to restore and improve the ecological conditions in the lake. Subsequently (2016-2020), a monitoring program was launched, to assess the effect of the restoration activities on the macrophytes community (hydrophytes and helophytes), part of which are presented here. The results suggest pronounced deterioration of the status after the interventions, but the impacts are overcome in the next two to three years.


Introduction
At the beginning of the 20th century, at the place of the Small Negovan Lake (SNL), there was "Negovansko blato", which was a natural riverside wetland of high importance for the biological diversity and was part of the so-called "Swampy Triangle".In the 1930s, the drainage of wetland began, and later the correction of the Lesnovska River was made and lakes were dug for the extraction of aggregates, sand and gravel [1].As a result of deforestation, extraction of aggregates, illegal dumping of waste and the corrections of the Lesnovska River, the riverside ecosystems have lost their natural character.One of the main current problems of the SNL is the continuing drop in the water level due to the subsidence of the river bottom as a result of the periodical dredging of the Lesnovska River and to an illegal drainage canal dug up by the owners of the agricultural lands surrounding the lake.Although the canal was promptly filled up, the disturbance of the hydrological regime continues.This, as well as the prolonged summer droughts, led to a loss of the supra-littoral habitat and inwards expansion of the reeds.Today, the former sandpit lakes in the region are used for recreational fishing.
Data in the existing literature on plant biodiversity in the region, as well as data on their relationship to aquatic habitats, are sporadic and incomplete.Among the first sources of information is the work of Jordanoff [2], in which the author describes the higher vegetation of the bogs in Bulgaria.In the book by Michev and Stoyneva [3], on the inventory of wetlands in Bulgaria and their biodiversity, the data 1305 (2024) 012008 IOP Publishing doi:10.1088/1755-1315/1305/1/012008 2 provided for the SNL quarry lake, mentions 14 species of aquatic macrophytes, while for the neighboring quarry lake the authors mention 11 species of aquatic macrophytes.
The most detailed information on the composition of the higher aquatic vegetation in the quarry lakes is provided in the work of Tzonev et al. [4] on the inventory of biodiversity on the territories of some quarry lakes in Bulgaria.The authors describe the SNL lake as a habitat of Annex I of the Bulgarian Biological Diversity Act [5] and of Annex I of Directive 92/43/EEC [6] with code 3150: Natural eutrophic lakes with Magnopotamion or Hydrocharitiontype vegetation.
In the 70s and 80s there were some attempts to rehabilitate the abandoned sandpit lakes and to transform them into recreational areas.As a part of this there were several habitat restructuring attempts through the reintroduction of macrophytes [7], [8], [9].
The most recent and complete restoration took part in the period 2013-2015 through a "Demonstration Project for restoration of priority habitat type wetlands and species of European importance along Lesnovska River (near the Negovan Village)", a project of the Sofia Municipality supported by EU funds (Operational Programme Environment, Priority axis 3: Preservation and restoration of biodiversity; Project № 5103020-С-013/2012).The aim of the project was to restore and improve the ecological condition of degraded ecosystems to a state close to the one in which they existed before anthropogenic intervention.Subsequently (2016-2020), a monitoring program was launched, to assess the effect of the restoration activities, part of which was the assessment of the ecological status according to the adopted in Bulgaria Reference Index based on macrophyteshydrophytes and helophytes, presented here.

Material and methods
The studied lake falls under the classification of type L5, denoted as "Riparian lakes and marshes, characterized by mesotrophic to eutrophic conditions," according to the Bulgarian water body classification system.The lake is situated at an elevation of 514 meters above sea level and exhibits the following morphometric features: an area of 0.066 km², a maximum width of 210 meters, a maximum length of 550 meters, a maximum depth of 2.30 meters, and a coastline length of 4656 meters.The lake is situated within the temperate-continental climate zone, experiencing peak temperatures in July-August with an average of 21.6°C and minimum temperatures in January-February averaging -0.5°C.The region receives an average annual precipitation of 676 l/m².The primary sources of lake water come from underground sources and precipitation.
The monitoring was carried out annually between July and October from 2014 to 2020, based on the macrophytes as a biological quality element.A description of the aquatic macrophyte vegetation was made along 6 transects (stations).During the restoration period the monitoring was done on three sampling stations in the old parts of the lake.Later, the inventory of the species diversity of the macrophyte community and an assessment of the ecological status of the SNL was carried out on six points, the three in the old parts, and other three in the newly constructed maze of canals and internal shallow lakes and coves (figure 1).Other two sampling stations monitored until 2017 were removed from the monitoring scheme in 2018, due to the complete overgrowth by emergent vegetation (mainly by reeds) and a new point was added in one of the canals -point 3. The aquatic vegetation was assessed following the standard: EN 14184:2003 Water quality -Guidance for the surveying of aquatic macrophytes in running waters, and the determination of the Reference Index for flowing and standing waters according to Schaumburg et al. [10], [11], Gecheva et al. [12], Belkinova and Gecheva [13].
The assessment of the ecological state of the waters was carried out in accordance with Regulation No. H-4/14.09.2012 [14] on the characterization of surface waters.
Determination of the plants follows the main generalizing editions for Bulgarian flora [15], [16], [17], [18] and the scientific name for taxa according to the World Flora Online (WFO) Plant List [19].
Our study included the following protected species: Acorus calamus, Aldrovanda vesiculosa, Hippuris vulgaris, Marsilea qualrifolia, Menyanthes trifoliate, Nymphaea alba, Nymphoides peltata, Nuphar lutea and Utricularia australis.The study involves only visual examination and abundance estimates of the macrohytes species.We did not sample the protected species of aquatic plants.

Results and Discussion
During the field observations, the changes of the macrophyte community in the old parts of the lake, as well as the colonization of aquatic plants in the newly created areas, were monitored.A total of 51 macrophytes, both, hydro-and helophytes were observed, including the newly introduced 15 species, during the project implementation as well as in the period 2016-2020 (table 1).
At the end of the restoration project (2014), the macrophytes in the newly formed areas had a poorer composition (17 species) in comparison to the old parts of the lake (22 species).In the following years, the number of species in the lake steadily increased, but with higher rate in the newly formed parts where currently the number of species exceeds the diversity in the old ones by eleven species (table 1).
Immediately after the completion of the excavation works, the development of macrophytes began in the newly formed parts of the SNL.First to colonize the bare ground of the new habitat was Equisetum ramosissimum and the macrophytes Utricularia australis and Persicaria amphibia, as well as Phragmites australis and Typha latifolia into the shallow littoral zone.The fast spread of the macrophytes and the emergent species was aided by floating fragments of talus and rhizomes spread into the water when reshaping the south bank of the old lake.Massive development into the new habitats of the invasive species Elodea nuttallii was observed by 2016, as well as the appearance of Stuckenia pectinata, Potamogeton crispus, Ceratophyllum demersum, Myriophyllum spicatum, Chara spp.among others.Of the introduced species, Nymphoides peltata, Nymphaea alba and Potamogeton lucens also developed well.In 2017, a detailed mapping of the macrophytes was carried out as part of the monitoring of the introduced species.The survey indicated that ten of the new species for the lake did not survive more than a couple of years.Exceptions are Nymphaea alba, Nymphoides peltata, Potamogeton lucens, IOP Publishing doi:10.1088/1755-1315/1305/1/0120084 Ranunculus trichophyllus and Lemna trisulca.Another seven species, that were overlooked in the previous years, were described for the first time in the lake (Cyperus fuscus, Juncus conglomeratus, J. gerardii, J. effusus, Leersia oryzoides, Scirpoides holoschoenus, Sparganium erectum and pseudacorus).The last two in the list are very common along the Lesnovska River, separated from the SNL by just a dike, which makes it relatively easy to cross.The main reason for the suppressed growth of the newly developing community is the rapid and dense overgrowth by the reeds.
In the spring of 2019, additional excavation works were carried out to deepen the canal surrounding the lake.Despite the increased depth, species that were previously widespread rapidly recolonized the canal once again.In 2020, the canal around the lake was further cleaned and deepened, which temporarily delayed the spread of emergent species in it.
In shallower sections, especially between the newly created islands, massive overgrowths by emergent species (Phragmites australis, Typha angustifolia, T. latifolia, T. laxmannii, etc.) was observed which narrowed and even completely clogged the canals, especially at low water level.The dense overgrowth and swampification, due to increased rates of pond succession, led to the permanent loss of inland water mirrors, respectively, of newly formed habitats and reduction of species diversity.Apart from the aforementioned species and mostly Phragmites australis, the invasive species Elodea nuttallii as well as Stuckenia pectinata are mainly responsible for the habitat degradation.

Assessment of the ecological potential
In the conducted monitoring of the ecological potential (EP) prior to the restoration project, the EP of the SNL was assessed as "good" (table 2).In 2015 and 2016, a decrease in the values of the Reference Index (RI) and the ecological quality ratio (EQR) was observed, indicating a deterioration of the ecological conditions.Apart from the massive excavation works during the restoration project, the deterioration of the lake is greatly due to the digging of an illegal drainage canal in 2015 in the neighboring agricultural areas, which led to a pronounced lowering of the water level in the lake.As a consequence, the aggressive and highly adaptable to changing environmental conditions invasive species Elodea nuttallii, developed massively.This led to a change in the ecological potential -from "good" to "moderate", and even in some of the points to "poor" in the following years.In 2020 a small improvement in the ecological potential was evident, especially in the inner areas of the lake (table 2).The values of the Reference Index, at the individual sampling points show significant differences between the years.After 2017 there is a slight improvement of the conditions in the lake, especially at stations 4 and 5, according to the values of the Reference Index.The macrophytes community at stations 2 and 3 are late to respond due to the additional interventions in the canal, surrounding the restoration area.Thus, by the summer of 2020 the ecological potential at point 4 was approaching the border values for "good" EP, and those at stations 2 and 3 -for "moderate" EP.In 2020 the development of the invasive species Elodea nuttallii was slowing down, showing lower abundances at most of the stations due to the stabilization of the macrophytes community.At the shallower places, the Elodea nuttallii clogged the canals, which retarded water circulation and promoted the mass development of filamentous algae, as well as overgrowth with emergent vegetation.Wherever Potamogeton lucens developed, it suppressed the mass development of Elodea nuttallii and its abundance was significantly reduced, thus it can be used to control the invasive species Elodea nuttallii.There is also an overall increase in the biomass and area covered by the protected species Utricularia australis, the species of the genus Chara, as well as Ranunculus trichophyllus, which leads to an increase in the values of the Reference Index and, respectively, the ecological potential.
Despite the observed improvement of the ecological potential of the lake after the restoration activities, some negative trends were noted in the Small Negovan Lake.Foremost, it was the reduced size of the habitats of hydrophytes at the expense of the helophytes, which, due to the apparently low water level for two years, succeeded to completely occupy most of the canals and artificially created small ponds in the extended lake habitats.Other worry trend was the population decline of the species of conservation importance, both in size and numbers.

Conclusions
In summary, it can be said that after the completion of the earth-excavation works, the species diversity of the newly constructed water bodies exceeds that found in the old sections of the Small Negovan Lake.Additionally, plant biodiversity is enhanced through the re-/introduction of suitable species.The ecological potential in the initial stages (2014/2015) was found to be "good", according to the macrophytes.Among the different factors affecting the macrophyte communities, the time lapsed since the last earth-excavation works and the proximity to the altered sites mostly determine the state of the communities.Thus, between 2016 and 2020 we have observed shifts in the ecological potential -from "good" to "poor" and finally to "moderate" at all stations.The "poor" state is typical to the altered stations for two to three years after the interventions.
The main threats to the development of macrophytes and, respectively, to a change in the ecological potential of the lake are related to the massive development of emergent vegetation and the associated overgrowth of water areas, lowering of the water level with subsequent terestrialization of the lake.

Figure 1 .
Figure 1.Surveyed points for assessment of the ecological condition of the Small Negovan Lake (SNL) based on macrophytes (1-6).

Table 1 .
Composition of aquatic plant taxa in the old and newly formed areas in SNL for the period 2014-2020.(Introduced species are highlighted in bold), O -old parts of the lake, N -newly formed areas.

Table 1 .
Composition of aquatic plant taxa in the old and newly formed areas in SNL for the period 2014-2020.(Introduced species are highlighted in bold), O -old parts of the lake, N -newly formed areas.

Table 2 .
Ecological potential, Reference Index (RI) and Ecological Quality Ratio (EQR) values for Small Negovan Lake for the period 2014-2020.

Table 2 .
Ecological potential, Reference Index (RI) and Ecological Quality Ratio (EQR) values for Small Negovan Lake for the period 2014-2020.