Effects of land-use pattern and physiochemical conditions on phytoplankton communities in a German lowland catchment

To understand the lowland riverine phytoplankton community (of species and functional groups), and to investigate the effects of abiotic factors on phytoplankton variance during the high-flow season, 59 sites of the Treene catchment, northern Germany, were visited in December 2014, where 327 species belonging to six taxonomic groups and 21 phytoplankton functional groups were observed. Bacillariophyta, especially benthic pennales (corresponding to the functional group TB), were dominant in most of the study area. Cyanobacteria (specifically indicated by the functional groups M and S1), which are typically found in lake environments, were also found in the main stream of the River Treene. Furthermore, Euglenophyta (represented by the functional groups W1 and W2) showed a higher percentage in the headwaters. A redundancy analysis indicates that the land-use pattern and physiochemical condition co-contributed to the phytoplankton community variance. The total explained variance (68.4 %) of phytoplankton data was partitioned into three parts: purely physiochemical factors contributed to 24.1 % of the explanation, followed by land-use characteristics (15.6 %), and their shared effects (28.7 %). The functional groups W1 and W2 exhibited a strong positive correlation to the percentage of agricultural land use. The percentages of urban land use and phosphate concentration in the stream were interrelated with the functional groups Lo and M, which indicates mesotrophic to eutrophic water conditions. The unexplained variance (31.6 %) may result from hydrological regimes, which will be taken into account in subsequent studies.