Ecology and conservation value of aquatic vegetation of gravel pit lakes in the Drina Riverfloodplain | Ekologija i konzervaciona vrednost vodene vegetacije šljunkara u plavnom području reke Drine

Gravel pit lakes in the river floodplains represent a kind of ecological paradox. Gravel exploitation was recognised as important factor significantlyaffecting aquatic habitats and biodiversity. On the other hand, gravel pit lakes are valuable biodiversity refugiums, potentially supporting rarae species and habitats. The aim of this dissertation was to determine the most significant and relevant hydromorphological parameters in structuring macrophyte assemblages in gravel pit lakes along the Drina River floodplain and to determine the correlation between selected parameters and macrophyte quantitative indices. The research was carried out at the 18 gravel pit lakes (60 survey sectors) in Crna Bara, Badovinci and Lipnicki Sor and four natural fluvial lakes (13 survey sectors), in the Drina River floodplain during the summer months of 2015, 2016, 2017 and 2018.Macrophyte vegetation was recorded in all 18 gravel pit lakes, in total supporting 31 taxa. The most abundant species, with highest tot al cover value were Potamogeton nodosus,Ceratophyllum demersum subsp. demersum, Myriophyllum spicatum, Najas marina and Chara globularis. Fluvial lakes supported 13 macrophyte taxa with Vallisneria spiralis, Elodea canadensis, Callitriche palustris,Potamogeton natans and Nuphar lutea as constant and dominant species. The values of all macrophyte quantitative indices found to be significantly higher in the gravel pit lakes compared to the fluvial ones. The cluster analysis revealed 14 aquatic vegetation groups (VG). At 16 out of 18 gravel pit lakes 13 vegetation groups were revealed: VG1Ceratophyllum demersum, VG2 Ceratophyllum demersum- Valisneria spiralis, VG3 Chara contraria, VG4 Chara globularis, VG5 Elodea canadensis, VG6 Elodea nuttallii, VG7 Najas marina, VG8 Najas minor, VG9 Nitellopsis obtusa, VG10 Nuphar lutea, VG11 Potamogeton nodosus, VG12 Potamogeton natans, VG13 Potamogeton pectinatus), Natural fluvial lakes supported 4 vegetation groups: VG5 Elodea canadensis, VG10 Nuphar lutea, VG12 Potamogeton natans and VG14 Typha latifolia. All gravel pit lakes can be characterized as water bodies with good to maximal ecological potential, while all the fluvial ones can be characterized as water bidies with poor to moderate ecological status. The values of total suspended supstances, chemical and biological oxygen demand, total organic carbon and nitrates were significantly higher in the natural fluvial lakes compared to the gravel pit ones. Measured level of physico-chemical parameters indicating mesotrophic character of gravel pit lakes in Badovinci and mesoeutrophic in Lipnicki Sor, while all the gravel pits in Crna Bara could be characterized as eutrophic. Similar range values were calculated for LHQA for gravel pit and fluvial lakes (36 – 49). However, natural lakes showed significantly higher values for LHMS score. The above mentioned, indicates higher anthropogenic pressures on natural fluvial lakes compared to gravel pit ones. Physico-chemical and hydromorphological parameters together explained about 57 % of the total variance of macrophyte assemblages with 16.57 % of the shared effect. After accounting for the effects of physico-chemical parameters (17.02 %), hydromorphological variables explained around 23 % of the total variance. The most significant water quality variables were: oxygen saturation, total organic carbon, surfactants, ,electroconductivity, pH and total alkalinity. The The most significant hydromorphology variables for structuring macrophyte assemblages were: riparian vegetation structural complexity, diversity of natural landcover types in riparianzone, shore structural habitat diversity, diversity of natural littoral zone, maximal lake depth, lake surface area, relative depth ratio, lake distance from r iver main channel and lake age.Hydrologycal parameters were explained 8.38 % of variance in structuring macrophyte assemblages. The most significant hydrology variables were the number of floods in vegetation season in first year when vegetation was sampled, and the number of spring floods in all four research years. These results confirm the direct destructive influence of summer floods on aquatic vegetation, as well as the indirect impact of spring floods, due to the impact on trophic status of water. Gravel pit lakes in te Drina River floodplain represent an optimal habitat for rare and threatened macrophyte flora. Of the total macrophyte species recorded, 30 % were categorized as protected or threatened. At least one strictly protected, protected or threatened species was recorded in each gravel pit lake. Significantly higher values of conservation indices (C and Csp score) found to be significantly higher in the gravel pit lakes compared to the fluvial ones. High habitat diversity and conservation value of the sites have been recorded according to the National Rulebook, Annex I of Habitats Directive (NATURA 2000), Resolution no. 4 of the Bern Convention (EMERALD) and the European Red List of Habitats. Values of selected lake attributes can be used for early-design phases of future gravel extraction in the Drina River floodplain area, and in other similar sites. Therefore, general recommendations are that two gravel pit types should be excavated within the single extraction area in order to support pioneering charophyte vegetation and vegetation of typical eutrophic lowland floodplain lakes as well. The first hydromorphological lake type, suitable for stonewort species, should be excavated up to 100 m from river main channel, saving a surface area up to 1000 m 2 and a relative depth ratio > 5 %. The second gravel pit type should be located about 300 m from river main channel, with preferable maximal depth inrange 3–4 m (at least 2 m depth), and a lake surface area between 10000 m 2 and 20000 m 2 (at least 4000 m 2 ). Relative depth ratio may vary, but should be less than 5 %. Generally, all sites should be designed with the minimal impact to the riparian and shore zones. These proposed measures would considerably increase lake habitat diversity and their conservation potential. Creating gravel pit lakes as proposed in this dissertation would allow spontaneous recultivation of exploitation fields, remediation in order to improve water quality and renaturalization of habitats, which will significantly reduce, or completely eliminate, the costs of terrain technical recultivation.