The growth of the human population brought about the global intensification of aquacultural production, and aquaculture became the fastest growing animal husbandry sector. Effluent from aquaculture is an anthropogenic environmental burden, containing organic matter, nutrients, and suspended solids that affect water quality, especially in water bodies of high biodiversity and conservation value. Water quality assessment often relies on bioindicators, analysing changes in taxonomic diversity of various freshwater organismal groups. Stepping beyond taxon diversity, we used functional and phylogenetic diversities of rotifers to identify factors affecting their community organization in response to an aquaculture effluent gradient in the largest oxbow lake in the Carpathian Basin, Hungary. Sampling was carried out three times per season at five points along a 3.5 km section of the oxbow lake, including the point of effluent inflow. We used eight traits to evaluate functional diversity: body size, trophi type, feeding mode, protection type, body wall type, corona type, habitat preference, and tolerance level. Functional and phylogenetic distances among the 24 species identified indicated trait conservatism. Rotiferan diversity increased with increasing distance from the point of influx in spring and summer. Among the factors affecting community organization in spring and summer, we find examples of environmental filtering, while in autumn the role of biotic interaction is more frequent. Under nutrient-rich conditions in spring and summer, organisms belonging to the same functional group were dominant, while under oligotrophic conditions more diverse but less abundant groups were present. Considering functional and phylogenetic traits allowed us to identify organising forces of rotifer communities in the largest oxbow lake of the Hungarian Lowland.

The Kákafok oxbow lake, stretching between the south Hungarian settlements of Szarvas and Békésszentandrás (46° 51' 14.9''N, 20° 30' 44.6''E) is 29 km long and was separated from the River Körös during the 19th-century river regulations. Five sampling points (K1 – K5) were selected along a 3.5 km long section of the oxbow lake. The first of these is the introduction point for effluent water from an intensive African catfish (Clarias gariepinus) warm-water farm and the experimental fish ponds of the Research Centre for Aquaculture and Fisheries – Hungarian University of Agriculture and Life Sciences. The other four sampling sites were located at different distances from this point: at 500m (K2), 2500m (K3), 3000m (K4), and 3500 m (K5) from the point of effluent. An intensive fish survey along a ca. 2km long section prevented us from sampling there.

Rotifers were sampled three times during each season: in spring (20 April, 10 May, 1 June), summer (29 June, 21 July, 10 August), and autumn (30 August, 21 September, 24 October) of 2016. Each time, 50 l of surface water at each point was filtered through a 50 µm mesh plankton net, concentrated to 100 ml, preserved by the addition of formaldehyde (4% final concentration) and stored at 4° C until identification. Counting was performed with a 5 ml counting chamber using a microscope (40-125x magnification). Species were identified using standard keys (Bancsi 1986, 1988).

REFERENCES

• Bancsi, I., 1986. Identification keys for Rotifera I. Vízügyi Hidrobiológia, 15. Országos Vízügyi Hivatal, Budapest. [In Hungarian]
• Bancsi, I., 1988. Identification keys for Rotifera II. Vízügyi Hidrobiológia 17. Országos Vízügyi Hivatal, Budapest. [In Hungarian]