While the knowledge about spatial structure of zooplankton communities in large rivers has been relatively well studied, little is known on the longitudinal spatial variation of zooplankton in small, slowly flowing fishless streams. In these streams, changes in zooplankton communities along entire length of the stream can be quite different than in those where young planktivorous stages of fish reduce the abundance of zooplankton. The aim of this study was to answer the following questions. What is the spatial pattern of the taxonomic groups of zooplankton in the slow-flowing stream? Do the small tributaries have an impact on the zooplankton community in the main stream? What biotic variables (content of chlorophyll a, vegetation coverage, macroinvertebrates abundance) and abiotic variables (temperature, dissolved oxygen, pH, conductivity, N-NO3, N-NO2, N-NH3, TN, P-PO4, TP, width, depth, current velocity, discharge) most affect the zooplankton community in a small stream? This study was performed at six sites along a small (1 km long, mean width 1.7 m; mean depth 0.3 m; mean current velocity 5.9 cm s-1; mean discharge 2.6 cm3 s-1, mean vegetation coverage 52%) fishless agricultural-meadow stream and at one site in its two tributaries. The stream was searched with the use of electric fish gear to make sure there were no fish. Zooplankton samples were collected each month throughout the years 2008 and 2009. The main factors which affected the zooplankton communities were hydrological conditions, especially current velocity. Water current in the stream impeded the movement of rotifers and juvenile copepods. Adult copepods were able to manage in the current, over the entire length of the stream. Cladocerans probably were only able to persist in the last section of the watercourse flow, where the velocity was the lowest (3.3 cm s-1) and where the content of chlorophyll a was the highest (56.2 μg L-1 ). The two tributaries had only a small effect on shaping of the zooplankton communities in the main stream. On the basis of Pearson correlations it can be concluded that macroinvertebrate had a low ability to reduce the density of zooplankton, all significant correlations between the abundance of potamozooplankton and that of macroinvertebrates were positive.
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