Trend and noise: long-term changes of phytoplankton in the Keszthely Basin of Lake Balaton, Hungary

Lake Balaton is the largest shallow lake in Central Europe (length: 77.9 km; average width: 7.2 km; surface area: 593 km'; mean depth: 3.14 m, theoretical retention time: 3-8 years; catchment area: 5182 km'). The Zala River, which enters the lake at the SW shore, drains an area of2622 km'. As a result of the increased phosphorus load, the originally mesotrophic lake underwent rapid eutrophication in the 1960s-1970s (HERODEK 1984). Between 1975 and 1981, the total P load was estimated as 2.47 g m-' yearin the western part of the lake, and 0.31 g m-' yearin the eastern part (SOMLYODI & JoLÁNKAI 1986). As a consequence of the morphometric and loading conditions, a sharp trophic gradient has developed in the lake. In the early 1980s, a large-scale eutrophication management program was started (ISTVÁNOVICS et al. 1997), which included the installation of a large water qualiry protecting reservoir (more than half of its 80-km' surface area being wetland) at the inlet of the major river. So far, the whole management program has resulted in approximately 50-60% reduction of the biologically available P (BAP) load of the lake. This load reduction was expected to slighdy inhibit the development of phytoplankton in the lake (HERODEK 1984). In parallel to load reduction, the humic material load of the Keszthely Basin increased, since the entire discharge of the largest river has been filtered through a swampy wedand area. From this pattern, it is expected to see: l. A trendlike increase of phytoplankton biomass during the eutrophication phase (prior to 1985) combined with a proliferation of heterocytic cyanobacteria. 2. A trendlike decrease of phytoplankton biomass after restoration has started (1985). 3. An increase in the biomass share of actually or potentially mixotrophic species (especially f! agellates) as a consequence of increased organic load of the basin, due to humic materials input from the wedand of the protecting reservoir. Phytoplankton species have short doubling times under natural conditions populations at the log phase of increase usually exhibit doubling times of 2-6 days depending on their size, morphology and intrinsic capacities. It was shown in many studies dealing with the succession of phytoplankton that stochasticity of weather conditions and other unexpected events may interfere with regular patterns of succession, sometimes only reversing its sequence but in other cases shifting it, which results in a very different successional pathway. Moreover, the actual biomass of phytoplankton in shallow lakes is often dependent upon the release of internally stored P, of which some has also been weather dependent. Therefore, it is also expected that 4. There will be a 'noise' on the trends that can be forecasted from expectations numbered here from l to 3. This paper describes phytoplankton changes in the 19 years between 1982 and 2000 in the Keszthely Basin of the lake. Special attention will be paid to Cylindrospermopsis raciborskii, a bloom-forming subtropical cyanobacterium. Earlier data indicated that the occurrence of its blooms depends both on hydrometeorological conditions and the P-pool of the sediments (PADISÁK & ISTVÁNOVICS 1997, PADISÁK 1998). The basic question is whether the period of water quality restoration can really be attributed to successful management o r is i t merely a consequence of an ideal combination of hydrometeorological circumstances.