Phytoplankton community structure in the lower reaches of a Mediterranean river (Aliakmon, Greece)

Phytoplankton communities from the lower reaches of the Aliakmon River, Greece, have been studied over a period of one year. Seasonal phytoplankton dynamics was described on the basis of its species composition and abundance, biomass and chlorophyll-a concentrations. Phytoplankton assemblages comprised diatoms (mainly Cyclotella meneghiniana, Nitzschia palea and Fragilaria ulna), cyanobacteria (mainly Pseudanabaena limnetica, Aphanizomenon sp. and Limnothrix cf. redekei) and green algae (various species of Scenedesmus and Chlamydomonas, Coelastrum microporum and Monoraphidium contortum). Many benthic diatoms were also present. An important growth of euglenophytes was observed during summer, mainly Lepocinclis steinii, L. ovum, Euglena intermedia, E. viridis and Phacus caudatus. The spring maximum of centric diatoms extended into the summer. The qualitative and quantitative composition of the phytoplankton is compared to communities of other European rivers. With regard to phytoplankton assemblages, Aliakmon resembles more the relatively fast-flowing, individual-poor European rivers. Its widely varying discharge volume, turbidity and shallowness, seem to be the main factors controlling phytoplankton communities in its lower reaches.