Crustacean plankton abundance in the Danube River and in its side arms in Hungary

The spatial distribution and seasonal dynamics of the crustacean zooplankton were studied in the Danube River and in its side arms near Budapest, Hungary. Microcrustaceans were sampled biweekly from October 2006 to November 2007 at eleven sites. Thermocyclops crassus, Moina micrura and Bosmina longirostris added up to 57.6% of the total density. Comparisons of the different water bodies stressed the separation of the eupotamal and parapotamal side arms. Densities in the side arms were one respectively two orders of magnitude higher as compared to the main channel, which was relatively poor in plankton. There were remarkable longitudinal and transversal variations in the abundance of the major zooplankton groups (cladocerans, adult copepods, copepodites, nauplii) and dominant species (t-test, P < 0.05). However, no general pattern was observed, the spatial distribution depended on the examined objects. There were statistically significant seasonal differences in zooplankton abundance (Tukey-test, P < 0.05). Water residence time and water discharge were not found to be related to zooplankton abundance, but water temperature was positively correlated with microcrustacean density.