Lake Balaton (Hungary) is the largest shallow lake in Central Europe (between the northern latitudes 46 62’ and 47 04’, and between the eastern longitudes 17 15’ and 18 10’) that has a large surface area (596
km2 at mean level) and a mean depth is 3.25 m. The progressive eutrophication of the lake was perceived in the early seventies. In parallel with the accelerated eutrophication, the phytoplankton assemblage was observed to shift from eucaryotic algae (diatoms and dinoflagellates, especially
Ceratium hirundinella) to a dominance of N2-fixing cyanobacteria. The appearance of new
N2-fixing species was also observed. From 1975 to 1994 the development of summer blooms of
N2-fixing cyanobacteria became a regular phenomenon in the western part and sometimes in the eastern part of the lake. In the western part of the lake the maximum chlorophyll-a concentration generally exceeded 100 µg
l-1. During the latest bloom (1994) the whole water body of the lake was almost a pure culture of the cyanobacterium
Cylindrospermopsis raciborskii (Woloszynska Seenayya et Subba Raju (95% of total phytoplankton biomass). Since 1995 the phytoplankton biomass, particularly
N2-fixing cyanobacteria, has decreased dramatically, i.e. the maximum chlorophyll-a concentration has not exceeded 70 µg
l-1. During this time the contribution of dinoflagellates to the phytoplankton biomass has increased again.
N2-fixing cyanobacteria in Lake Balaton generally form planktonic populations for only one-three months a year otherwise they exist as akinetes accumulating in the surface layer of the lake sediment. Earlier studies have used indirect counting methods (the number of germinated filaments in the germination experiment) for the determination of akinete abundance in the sediment.
The aim of my study was
(i) to develop a direct counting method for determine the abundance of akinetes in sediment
(ii) to determine the akinete abundance of
C. raciborskii and the total akinete abundance of N2-fixing cyanobacteria in the sediment of Lake Balaton
(iii) to compare the data of available akinete abundance with the changes of the trophic state of Lake Balaton
(iv) to determine the interactive effects of temperature and light on growth, cell morphology, pigment composition, C:chlorophyll-a ratio, C:carotenoid ratio and C:N ratio composition of the most abundant cyanobacterum species
(Cylindrospermopsis raciborskii, Aphanizomenon flos-aquae RALFS.) of Lake Balaton
(v) to determine the dynamics of different iron species (total iron, dissolved iron and biologically available iron [chelatable iron]) in Lake Balaton and in four inflows of the lake
(vi) to develop the easy
molecular methods for proving of the iron limitation of the phytoplankton.