engleski

Long-term changes of phytoplankton in the floodplain waters of Kopački Rit Nature Park

Kopački Rit Nature Park is one of the largest preserved natural floodplains of the Middle Danube stretch. This internationally important wetland provides a diversity of water and wet biotopes continuously changing depending on the inflow of river water. The first complete checklist of its phytoplankton flora listing 303 species, 45 varieties and 14 forms was compiled by Dragica Gucunski (1973) whose results encouraged future research on the Kopački Rit phytoplankton and gave large contribution to the development of algology in Croatia. The investigations of phytoplankton community structure and ecology have been conducted continuously for several decades in order to define the ecological state and changes in this complex river-floodplain system. So far, more than 600 phytoplankton taxa have been listed. In general, the seasonal phytoplankton succession pattern is characterised by diatoms in early spring, late autumn and winter, Chrysophyceae later in the spring and autumn, “clear-water phase” and dominance of Cryptophyceae in May, as well as the development of chlorococcal green algae and Cyanobacteria in summer. It is interesting to note the appearance of a “red tide” caused by massive development of Peridinium species in the autumn of 1999. Invasive species Cylindrospermopsis raciborskii (Wol.) Subba Raju, a tropical element in temperate waters, was detected in the summer of 2003. Since then, massive blooms have occurred periodically. Changes in phytoplankton communities depend mostly on flooding dynamics and recent investigations showed dual impact of flooding on phytoplankton development. The stimulative effect in spring is seen through nutrient enrichment, while later in the summer flooding acts as a disturbance causing water column mixing, nutrient dilution and washout effect. Current research is focused on phytoplankton classifications based on species morphological and functional traits using different classification systems. Results show that the flood phase in the river-floodplain system is characterised by diatoms and chlorococcal green algae from different functional and morpho-functional groups characteristic for turbid and well-mixed waters, while large amount of nutrients and hydrological stability enabled the domination of Cyanobacteria. Extreme hydrological events such as long-term dry periods or extreme floods which are more frequent in this part of the Danube, result in the changes of phytoplankton abundance and community structure from “turbid state” to a “clear state” showing the possible consequences of global climatic changes.

phytoplankton; Kopački Rit Nature Park; river-floodplain system

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