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We studied the seasonal dynamics of phytoplankton, bacterioplankton, heterotrophic nanoflagellates, ciliates, and metazoan plankton in the highly eutrophic polymictic Lake Kastoria (Greece), which has a history of toxic cyanobacterial blooms. An acute increase in the flushing rate of the lake during spring inhibited cyanobacterial biomass accumulation. During this transient oligotrophic period, which was characterized by abundant lake snow particles, the plankton food web was an inverted biomass pyramid (low autotrophic biomass and high heterotrophic biomass). Prokaryotes played a key role in these changes (cyanobacteria during periods of autotrophy and bacteria during periods of heterotrophy). In summer and autumn, toxic cyanobacterial blooms developed, and the microbial loop was weak. The microbial loop was weak because the heterotrophic nanoflagellates and nanociliates decreased to undetectable densities during the summer, when larger bacterivores (rotifers and small cladocera) were abundant. Toxic blooms may have a dual effect on heterotrophic nanoplankton: negative during the first bloom and postbloom period and positive during a following toxic bloom. Different species (Cylindrospermopsis raciborskii, Aphanizomenon spp., and Microcystis aeruginosa) and succession phases of toxic blooms may differentially affect the microbial food web structure.In pelagic systems, phytoplankton and bacterioplankton constitute the complementary functional components that primarily produce new particulate matter by autotrophy and heterotrophy. Their carbon pool represents the base of grazing food chains and the microbial loop. Thus, the relative dominance of each functional component has significant implications for food web structure and the function and bio- AcknowledgmentsWe would like to thank the two anonymous reviewers and Associate Editor R. Bachmann for their constructive criticisms and suggestions. We are grateful to U. Sommer for his critical comments and suggestions on drafts of this manuscript and to U. Christaki for her helpful comments. We thank L. Economou for linguistic suggestions and C. M. Cook for the critical reading of sections of the manuscript concerning cyanotoxins. This work was partially funded by the Municipality of Kastoria, Research Committee, Aristotle University of Thessaloniki, project 7468. We thank all participating members of this project.

The phytoplankton species composition and seasonal succession were examined in Lake Kastoria during the period November 1998-October 1999. A total of 67 species and 19 functional groups were identified. Only 4 out of the 67 species, all Cyanobacteria, were dominant (Limnothrix redekei, Microcystis aeruginosa, Cylindrospermopsis raciborskii and Aphanizomenon gracile). Diatoms were rare, not only in terms of species number, but also in terms of biomass (contributing < 5% to the total phytoplankton biomass) in relation to the rather low silicon concentrations throughout the year. The functional groups S1, S N , M and H1 were found dominant in the lake. The species A. gracile (functional group H1) behaved like the species Cylindrospermopsis raciborskii(functional group S N ) which is tolerant to mixing and poor light conditions. The phytoplankton seasonal succession showed similar patterns in all six sampling stations, both at the surface and the bottom water layer, with minor differences during Microcystis aeruginosa dominance. Two steady-state phases were identified within a year lasting for 4 months under relatively stable physical conditions. In these steady-states, the Limnothrix redekei persistent dominance under low light availability and low inorganic nitrogen has been explained by its specific ability such as buoyancy regulation to exploit resources in the water column. Moreover, high population densities over the winter and before the development of daphnids may contribute to the steady-state dominance of Limnothrix. Different niches separated vertically in the water column is one of the explanations for the Limnothrix-Microcystis steady-state when a replacement between the two species was observed in different water layers and areas of the lake. Long lasting steady-states of Cyanobacteria observed in Lake Kastoria and in other Mediterranean and tropical freshwaters may indicate influence of warm climate properties on phytoplankton dynamics. Pediastrum boryanum (Turp.) Menegh. J P. duplex Meyen J P. simplex Meyen J P. tetras (Ehrenb.) Ralfs J Pseudodidymocystis fina (Kom.) Hege et Deason Scenedesmus acuminatus (Lagerh.) Chod. J Scenedesmus sp. J Selenastrum sp.

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Phytoplankton and Nutrients in the River Strymon, Greece key words: phytoplankton, potamoplankton, chlorophyll a, nutrients, river Abstract Phytoplankton species composition, biomass, diversity, nutrients and chlorophyll a were studied at monthly intervals from December 1991 to December 1992 in a selected area of the river Strymon. SRP ranged from 53 to 182 pg-' I-' and DIN from 265 to 850 pg-' I-'. Nutrient values do not indicate strong anthropogenic effects. Chlorophyll a ranged from 1 .O to 35.3 pg-' I-' and followed the temporal distribution of total phytoplankton biomass. Phytoplankton biomass exhibited maxima in winterspring and summer (6.8 g m-3 in' December 1991, 4.8 g m-3 in April 1992 and 9.3 g m-' in August 1992) composed mainly of diatoms, chlorphytes, cyanophytes and dinophytes. Nanoplankton was the most important component of phytoplankton biomass (69.5%) revealing increased values in winter and early spring. Phytoplankton diversity ranged from 0.8 to 3.2. The hydrological conditions in the river Stryrnon seem to be appropriate for the algae to reproduce themselves in the running water and so, to develop as a true potamoplankton. However, significant populations of phytoplankton must have been carried out from the Kerkini reservoir, situated at the north of the sampling station. The phytoplankton species composition and their periodicity in the river resemble those of typical, large, lowland and nutrientrich rivers of Europe. CHLOROPHYCEAEChlamydomonas spp. Pandorina cf. murum (MUEL.) BORY Sphaerellopsis lateralis PASCH. Geminella miitabilis (NAEG.) WILLE Pseudosphuerr~cystis lacustris (LEMM.) NOVAK. Actinastrum hnntzschii LAGERH. Closteriopsis longissima (LEMM.) LEMM. Coelastrum astroideum DE-NOT. Coelastrum rnicroponim NAEG. Coelastrum reticulatum (DANG.) SEEN. Coenochloris pyrenoidosa KORS.

Cyanophytes dominated the phytoplankton of shallow Lake Mikri Prespa during the period May 1990 to September 1992 (76.5 and 52.0% of the total phytoplankton biomass in 1990–1991 and 1991–1992, respectively). Biomass peaks were observed in autumn (from 5.2 to 34.5 g m‐3) when low dissolved inorganic nitrogen, high phosphate phosphorus and low Zcu to Zmix ratio prevailed. The dominant species were Microcystis aeruginosa and M. wesenbergii. These represented 64 and 86% of the cyanophyte biomass in 1990–1991 and 1991–1992, respectively and revealed similar patterns of seasonality forming biomass peaks in late summer ‐ autumn. Small chroococcalean cyanophytes (< 2 μm) showed also similar temporal distributions. Of the filamentous cyanophytes, the most important species was Anabaena lemmermannii var. minor which formed peaks in late summer and autumn (2.6 and 1.1 g m‐3 in 1990 and 1992, respectively). Rainfall and the N:P ratio were probably the main factors influencing the seasonality of all of the filamentous cyanophytes in the lake. Cyanophytes, tended to increase at temperatures higher than 16 °C and at inorganic nitrogen concentrations lower than 100 μg 1‐‐1. The frequent mixing of the water column did not seem to prohibit the substantial increase of the group.

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