Planktonic diatoms in a floodplain lake (Kopački Rit Nature Park, Croatia) under extremely high flooding conditions on the River Danube

Vertical distribution of species composition and abundance of phytoplankton in a shallow floodplain lake, Lake Sakadaš, (Nature Park Kopački Rit, Croatia) and in the Danube River (at 1388 r. km) were studied during the period March - November 2006, with emphasis on the diatom flora. Integrated phytoplankton samples were collected from the whole water column in the lake and in the Danube. In order to find out the vertical distribution, phytoplankton samples were collected every meter of the depth from the surface till the bottom of the lake. Quantitative analysis of phytoplankton was done according to the Utermöhl method. Data were analyzed using multidimensional statistic methods. Extremely high flooding during the whole spring and major floods in August characterized the hydrological conditions. Water depth of Lake Sakadaš varied between 4 – 8.25 m depending on the inflow of Danube water. The obtained results indicated that floodwater had altered the physical and chemical environment of the lake. The correlation of measured physico-chemical parameters between the Danube and the lake was high (r = 0.96, p < 0.01). The low mean phytoplankton abundances established in Lake Sakadaš (6.28x106 ind. l-1) as well as in the Danube (5.81x106 ind. l-1) with diatoms and chlorophytes numerically dominant, correspond with the extremely high flooding of the lake and high water discharge of the Danube. The total diatom biodiversity was higher in the Danube (99 species) than in Lake Sakadaš (83 species). Principal Component Analysis (PCA) showed that seven diatom species (Cyclotella radiosa, Epithemia turgida, Navicula menisculus, Nitzschia acicularis, N. heufleriana, N. vermicularis, N. sygmoidea) represent 35.29% of the variance described with PC1 axis which explained over 85% of the total variance in the species data. It is interesting to note that the invasive species Didymosphenia geminata was found in the River Danube almost in all samples. The highest abundance of diatoms was established in March and in November, both in the lake and in the Danube. According to Fuzzy Cluster Analysis (FCA), a similar distribution of phytoplankton in the water layers was established only in March, when diatoms represented from 74.54 – 92.31% of the total abundance. In all other months, vertical distributions of phytoplankton varied substantially with depth, and data series were optimally divided in 19 classes with fuzzy coefficient 0.21. The rank-abundance curve is best fitted by log-normal function (AIC=15577523 ; BIC=1557729) which indicates that influence of abiotic factors on phytoplankton abundance was equable during the investigation period.