Slovak Republic, after the accession to EU and adoption of the „Water Framework Directive“, is obliged to develop a system of the ecological status and ecological potential assessment of the surface water bodies. Development of the system of the ecological status and ecological potential assessment for water reservoirs is the up–to–date issue at present. These are considered as the rivers with modified category. One of the research tasks of WRI is to create the database with representative analyses results of selected biological quality elements, which are able to reflect the changes really in ecological potential and to select the suitable evaluative metrics. On the basis of the reservoirs survey in previous years, the partial aim was to test the sampling method with colonization samplers (application of artificial substrates) in selected reservoirs. The data were obtained also by means of commonly used deep–water sampler (Birge–Ekman sampler), with the aim to evaluate a possibility of using these sampling techniques in ecological potential assessment.

Since the protection of the colonization samplers against stealing and damage could not be ensured in all surveyed reservoirs, only the samples of benthic invertebrates from the water–supply reservoir Hriňová were evaluated.

Sampling of benthic invertebrates using artificial substrates was tested for the purpose of the water quality assessment in the rivers (Woodiwiss 1976; Kahlaf & Tachet 1980; Watton & Hawkes 1984; De Pauw et al. 1986; Cziernawska–Kusza 2004) and in the lakes as well (e.g. Delong & Payne 1985; Schmude et al. 1998; Klemetsen & Elliot 2010).

Colonization samplers (artificial substrates) for benthic invertebrates sampling were installed twice a year – for obtaining the spring and autumn assemblages. The samples were taken by the deep–water sampler (Birge–Ekman sampler) at the same time. The sampling was performed in three sampling points (Figure 1), which were sufficiently distant from the banks to prevent damage, emerging or sludging during the decrease of water level. Both sampling techniques were applied in 10 m depth, where even the anoxic conditions do not negatively affect the diversity. The sampling procedure was carried out according to STN EN ISO 9391 (sampling of benthic invertebrates in deep rivers). Artificial substrates were placed on the bottom for several weeks (min. one month). Purified natural bottom substrate bound in synthetic net was used as an artificial substrate. It was put and fastened in the bucket, of volume 10 L, with holes to allow penetration for organisms. The length of rope, which was centrally mounted, corresponded with maximal operating water level in reservoir. Its location was marked by the signalling buoy. The samples were taken by the boat; spilling water with organisms was sieved using mesh size 500 μm. The protective net and artificial substrates were thoroughly washed and the captured organisms were preserved by 4 % formaldehyde in sampling bottles (STN 757715).

There were 68 taxa in total found in the Hriňová reservoir using both techniques during spring and autumn seasons. 24 taxa were recorded by means of deep–water sampler, 12 during the spring and 20 in the autumn season. Representatives of Chironomidae family and bristle worms (Oligochaeta) dominated, (Figure 2). Representatives of other temporal and permanent fauna were present in a negligible amount (max. 1 taxon). By using colonization samplers 49 taxa were found in total, 21 in the spring season, 38 during the autumn. A dominant group was presented in most cases by representatives of temporal fauna, with prevailing Chironomidae larvae. Also Cziernawska–Kusza (2004) found this group in much larger numbers on artificial substrates compared to hand–net sampling method. Oligochaeta prevailed within permanent fauna. Considerably more diversified assemblages were recorded in colonization samplers (approximately double values), where besides Oligochaeta and Chironomidae also other representatives of permanent and temporal fauna were examined. The occurrence of these taxa was isolated by Birge–Ekman sampler, while by colonization sampling they occurred regularly and in higher number of taxa (Table 1). According Cziernawska–Kusza (2004) oligochaetes and chironomids represent a selective community collected by basket–type samplers although these groups are generally considered important as water quality indicators (Watton & Hawkes 1984).

Comparing 2 seasons, the apparent difference in number of individuals of the present taxa was recorded. Markedly higher quantities were obtained during the autumn samplings, when number of individuals reached the hundreds of specimens (more than 400 ind.), while in the spring the numbers did not exceed 50 ind. (Figure 3).

Concerning the trophic guilds presence, during the spring period mainly collectors of fine organic matter dominated using both sampling techniques. They were represented by typical representatives of Chironomidae and Oligochaeta (Figure 4). Also predators occurred in mass density, specially represented by predacious chironomids (Figure 4, Table 1).

The number of filterers raised significantly in autumn, mainly in artificial substrates, what was given by higher concentration of photosynthetic planktonic organisms after summer season. A mass occurrence of Chironomidae representatives of genus Glyptotendipes and the presence of genus Endochironomus and small mussels of Pisidium genus dominated. Regarding sampling technique, in contrast to the “Birge–Ekman” samples – with prevailing fine sediment, in the colonization samples with prevailing mesolithal (stone size of 6 – 20 cm) we found also grazers, scraping the phythobenthos from the surface of solid substrates. They occurred more frequently in autumn, after summer increase of temperature and intensity and duration of sunlight. Artificial substrates were preferred also by shredders, but in low abundance, because they are more demanding to sufficiently high concentration of oxygen (Gammarus fossarum, caddis flies larvae, stonefly Leuctra nigra), which is better assured in colonization samplers. Coarse organic matter as the main food of shredders is trapped and present in artificial substrates as well. While De Pauw et al. (1986) found that the nature of substrates is not a crucial factor for macroinvertebrate colonization, Schmude et al. (1998) state that the more complex 3–dimensional artificial substrate, with its grater heterogeneity and surface complexity and interstitial space, supports a more diverse and abundant macroinvertebrate community. By Birge–Ekman sampling only a fine muddy, clayey and sandy substrate with almost anoxic conditions was present. Coarse organic matter was absent. It is well known that the reduction of habitat complexity reduces local invertebrate diversity (Schmude et al. 1998). Occurrence of parasitic leech Helobdella stagnalis was examined – only in colonization samplers, apparently due to the higher oxygen concentration and availability of host – fish.

Based on list of taxa, evaluative metrics were calculated using ASTERICS 3.1.1. software (AQEM Consortium 2002). For the purpose of evaluation of the ecological conditions in Hriňová reservoir, selected metrics were chosen and applied on the results obtained from both sampling methods (Table 2). When selecting metrics, we focused on their ability to reflect a diversity of assemblages, degree of organic pollution, hydro–morphological and overall degradation of surveyed water body. Selected metrics clearly showed differences between two sampling methods; in artificial substrates case the values were more realistic which was connected to higher representativeness of the sampled assemblage for given water reservoir.

According to the results and compared to deep–water sampling, the method of colonization sampling of benthic invertebrates performed in autumn was considered as the more suitable and effective for the acquisition of the relevant data for ecological potential assessment in water–supply reservoirs. De Pauw et al. (1986) show that artificial substrate sampler can replace the usual samples obtained by means of a handnet, and provide a correct assessment. Moreover the main advantage of colonization technique application is that sampling efforts can be better standardized, as artificial substrates offer the same habitats for colonization by organisms at all sampling sites (Cziernawska–Kusza 2004). Also the selected evaluative metrics used for water quality evaluation showed the more objective picture in case of colonization samplers.