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Monitoring data were used to assess causes behind a recent shift from a clear-water to a turbidwater state in Lake Major, a 10 ha shallow lake in Hungary. In 1999-2000, fish manipulation was conducted in this hypertrophic lake. Reduced fish stock resulted in clearing water and the development of a dense ([80% coverage) submerged vegetation in 2005. During the recent abrupt shift, which occurred in 2007, submerged vegetation subsequently declined after a two-year period of clear water and abundant vegetation. An intense decay of macrophytes within the lake produced a rapid transition between the clearand turbid-water states. During the clear-water state in 2005-2006, the most important variables predominantly correlating with macrophyte cover were Secchi transparency, temperature and TN, while TN, temperature, Secchi depth and chlorophyll-a were the most significant variables during the turbid-water state in 2007. Nitrogen may play a significant role in the cover of submerged macrophytes when TP is moderate. We argue that several factors in concert are necessary to initiate a shift. Water temperature likely has contributed to triggering shift through inter-year-dependent changes in cover of macrophytes, with fish recruitment having key roles in the dynamics of shallow lakes.

To reduce external nutrient loading by the greatest nutrient transporter to Lake Balaton, the Kis-Balaton Water Protection System commenced operation in 1985. Cyanobacterial blooms (Cylindrospermopsis raciborskii, Aphanizomenon sp. and Anabaena sp.) cause nitrogen loading via nitrogen fixation, which can exceed the total external N-load to the reservoir during the summer. Nitrogen fixation of phytoplankton in the system in 2009 was measured using the 15 N-isotope technique. The light dependence of fixation was described by an exponential saturated curve which was then used to calculate daily nitrogen fixation. The total amount of nitrogen fixed during summer months (July-August) exceeded 170 tons for the 21 km 2 of open water area, which was more than three times higher than the external load by inflows. Total nitrogen: total phosphorus mass ratios (2.5−11.5) favoured the development of a N 2 -fixing cyanobacterial bloom, the contribution of which to the total algae biomass varied between 50 and 90%. The estimated primary production of phytoplankton was used to calculate the contribution of nitrogen fixation (0.01−28%) to algae nitrogen demand. Nitrogen fixing cyanobacteria were found to play a key role in addressing the nitrogen demand of phytoplankton.

A food web manipulation experiment was started in 1999 in a small shallow eutrophic (Major Lake as a part of the wetland area, the Kis-Balaton Water Protection System, KBWPS). The development of submerged macrophytes, the structure and biomass of phytoplankton and crustacean plankton responded rapidly to the removal of 60% of omnivorous cyprinid fish. An increase in transparency and a decrease in the concentrations of chlorophyll-a, phytoplankton and phosphorus occurred simultaneously with the increased presence of submerged macrophytes, which covered 45% of the lake area compared with <10% during the premanipulation period. The success of fish manipulation demonstrated the potential of this measure as a short-term management strategy. Our data also indicated that the clear water state was not stable in the long term. As long as phosphorus level is high, there is a risk that macrophytes will not successfully establish for longer period.

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