L. Forró szerk.: Miscellanea Zoologica Hungarica 10. 1995 (Budapest, 1995)

Gulyás, P.: Rotatoria and Crustacea plankton of the River Danube between Bratislava and Budapest

At the Bratislava sampling site the abundance varied between 30-1874/100 1 in 1992, was 20-756/100 1 in 1993, while its value in 1994 was only 12-382/100 1. First maxima were measured by the end of April - early May, that was accounted for some large population of Brachionus, Keratella and Polyarthra species. This is followed by a smaller population peak in July and August. In the investigated overall river section the smallest abundances were measured here in three consecutive years, averaging about 168 in 1992, 163 in 1993, and finally 58 in 1994. Compared to the upstream sampling location, the Rajka and Hrusov sampling sites only showed increase in abundance in 1993. In this year the number of individuals per 100 liter varied between 26-2064, and 40-1090 (yearly averages: 256 and 272). In the other two years similar values were measured to the Bratislava sampling point (yearly averages: 128 and 53). Alteration of the flow of the Duna water in the fall of 1992 was followed by an expressed decrease in abundance, which continued until the spring of 1993. Thereafter no significant differences were observed in the abundance values at the altered river section. Abundance maxima were observed at late spring and early summer (April-June), and also in late summer early autumn (July-September). In prolonged small water periods, usually early September, apart from the Rotatoria some plankton species were becoming also highly abundant (Bosmina longirostris, Daphnia cucullata, Diaphanosoma brachyurum, Thermocyclops oithonoides). Downstream of the diverted section, at Medve a well-expressed increase in abundance was observed. This was especially characteristic in 1992. In this year number of individuals per 100 liters varied between 62-6648 in the vegetation period, yearly average was 413. Large values were due to Daphnia cucullata and Thermocyclops oithonoides individuals. In 1993 no such maximum was observed, (maximum value was 2259 indVlOO liter), but average values were similarly high (373 ind./100 liter). All over 1994 smaller individual numbers were found (yearly average 105 ind./lOO liter), but in the vegetation period the populations were three-four times larger, than at the altered river section. Maximum periods in individual numbers were more prolonged, from early March until the middle of June, that was followed with a smaller peak from the middle of July till the end of October. As a result of further decreasing water flow velocities downstream of Medve individual numbers at Komárom also showed a further increase (255 in 1992, 405 in 1993, 106 in 1994 ind./lOO liters). In the periods between March and October the individual numbers were usually 2-3 times higher here, compared to the values at Medve. The numbers of individuals per 100 liters in the vegetation period of 1992 were 140-1894, in 1993 96-2384, while in 1994 36-1224 were detected. These increasing tendencies are clearly visible in Figs 1-3. Large numbers are due to the Rotatoria, and larvae of Copepoda characteristic to eutrophic waters. Largest numbers of individuals were found at Szob and Nagymaros sampling sites. Individual numbers per 100 liter about 1000 and above were characteristic for several months. Even at early spring and late fall larger Rotatoria and Crustacea populations were found here, than at upper river sections. This tendency is clearly shown by the yearly averages at Szob in 1992: 236, 1993: 456, 1994: 334, while at Nagymaros: 1992: 533, 1993: 712, 1994: 343 indVlOO liter. It can also be seen in the figures, that opposite to the formerly described sections - large individual numbers are formed in the second half of the year and at early spring and early summer. Individual numbers of Rotatoria and Crustacea populations at the sampling point upstream of Budapest were similar to that of Nagymaros. No significant increase was observed on this 35 rkm long river section. Yearly average individual numbers were 501 in 1992, 619 in 1993, and 344 ind./100 liter in 1994. Population dynamics showed similar pattern to the upper sections.