Hydrobiologia 424: 109–122, 2000. C.S. Reynolds, M. Dokulil & J. Padis´ ak (eds), The Trophic Spectrum Revisited. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 109 Seasonal variation in phytoplankton composition and physical-chemical features of the shallow Lake Doïrani, Macedonia, Greece M. Temponeras 1 , J. Kristiansen 1 & M. Moustaka-Gouni 2 1 Botanical Institute, Dept. of Phycology, University of Copenhagen, Øster Farimagsgade 2D, DK-1353 Copenhagen K., Denmark 2 Institute of Botany, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece Key words: phytoplankton, freshwater, shallow lake, seasonality, Ceratium monoceras Abstract Phytoplankton species composition, seasonal dynamics and spatial distribution in the shallow Lake Doïrani were studied during the growth season of 1996 along with key physical and chemical variables of the water. Weak thermal stratification developed in the lake during the warm period of 1996. The low N:P ratio suggests that nitrogen was the potential limiting nutrient of phytoplankton in the lake. In the phytoplankton of the lake, Chlorophyceae were the most species-rich group followed by Cyanophyceae. The monthly fluctuations of the total phytoplankton biomass presented high levels of summer algal biomass resembling that of other eutrophic lakes. Dinophyceae was the group most represented in the phytoplankton followed by Cyanophyceae. Diatomophyceae dominated in spring and autumn. Nanoplankton comprised around 90% of the total biomass in early spring and less than 10% in summer. The seasonal dynamics of phytoplankton generally followed the typical pattern outlined for other eutrophic lakes. R-species (small diatoms), dominant in the early phase of succession, were replaced by S-species (Microcystis, Anabaena, Ceratium) in summer. With cooling of the water in September, the biomass of diatoms (R-species) increased. The summer algal maxima consisted of a combination of H and M species associations (sensu Reynolds). Phytoplankton development in 1996 was subject to the combined effect of the thermal regime, the small depth of mixing and the increased sediment-water interactions in the lake, which caused changes in the underwater light conditions and nutrient concentrations. Introduction Among the early information on Lake Doïrani was a publication by Bruno Schröder on the Macedonian lakes, containing species lists of phytoplankton from three net-samples, collected between 1891 and 1918 (Schröder, 1921). Ten years later, S. Stankovic pub- lished a broader limnological study of the lakes in the area, containing geological, hydrographical, phys- ical and chemical data (Stankovic, 1931). A thorough study on the zooplankton in lake Doïrani was conduc- ted in 1951–1952 (Popovska-Stankovic, 1954). More recent data on physical and chemical features of the lake are given by Mourkidis (1985). The present work is the first study on the phyto- plankton of Lake Doïrani to include quantitative ana- lyses, spatial and temporal variations and key environ- mental factors. Study site Lake Doïrani is located in Macedonia north of Thes- saloniki (see Figure 1), traversed by the border between Greece and FYROM (Former Yugoslav Re- public of Macedonia). The lake is at lat. 41 ◦ 15 ′ N and long. 22 ◦ 15 ′ E. The geological nature of the area is karstic and the lake-basin is a doline. The present lake is the remnant of a much larger pleistocene lake, the Peonie, with a surface area of 127 km 2 (Stankovic, 1931). The surface area of the lake is now about 28 km 2 , whereof one third is in Greece. The inflow consists of underwater springs (Stankovic, 1931), apart from run- off from the catchment. There is currently no surface outlet from the lake. By an agreement between the two states, the level has been kept within a certain range since 1956 by the