Picoeukaryotes dominate the phytoplankton of Lake Balaton-the largest shallow lake in Central Europe-in the winter period. We examined the annual dynamics of picoplankton abundance and composition in the lake in order to establish if the picoeukaryotes merely survive the harsher winter conditions or they are able to grow in the ice-covered lake when the entire phytoplankton is limited by low light and temperature. Lake Balaton has an annual temperature range of 1-29°C, and it is usually frozen between December and February for 30-60 days. In the spring-autumn period phycocyanin and phycoerythrin rich Cyanobacteria are the dominant picoplankters, and picoeukaryotes are negligible. Our five-year study shows the presence of three types of picophytoplankton assemblages in Lake Balaton: (1) Phycoerythrin-rich Cyanobacteria-the dominant summer picoplankters in the mesotrophic lake area;(2) Phycocyanin-rich Cyanobacteria-the most abundant summer picoplankters in the eutrophic lake area and; (3) Picoeukaryotes-the dominant winter picoplankters in the whole lake. The observed winter abundance of picoeukaryotes was high (up to 3 9 10 5 cells ml -1 ), their highest biomass (520 lg l -1 ) exceeded the maximum summer biomass of picocyanobacteria (500 lg l -1 ). Our results indicate that the winter predominance of picoeukaryotes is a regular phenomenon in Lake Balaton, irrespective of the absence or presence of the ice cover. Picoeukaryotes are able to grow at as low as 1-2°C water temperature, while the total phytoplankton biomass show the lowest annual values in the winter period. In agreement with earlier findings, the contribution of picocyanobacteria to the total phytoplankton biomass in Lake Balaton is inversely related to the total phytoplankton biomass, whereas no such relationship was observable in the case of picoeukaryotes.
The abundance and composition of autotrophic picoplankton (APP) were studied between February 2003 and March 2004 in Lake Balaton. Water samples were taken fortnightly in the eutrophic western basin and mesotrophic eastern basin. Our study, which took more than one year, revealed pronounced seasonal pattern of the picoplankton abundance and composition. According to our results there were three types of picoplankton in Lake Balaton: 1. Phycoerythrin-rich coccoid cyanobacteria (PE), dominant summer picoplankters in the mesotrophic lake area; 2. Phycocyanin-rich cyanobacteria (PC), the most abundant summer picoplankters in the eutrophic lake area; 3. Picoeukaryotes, dominant winter picoplankters in the whole lake. The observed abundance of picoeukaryotes (3 × 10 5 cells ml -1 ) was one of the highest ever found. Our study confirms that in Lake Balaton the colonial autotrophic picoplankton (colonial APP) become dominant in summer in the nutrient limited period. We have found strong negative relationship between the concentrations of available nitrogen forms and the colonial APP abundance. IntroductionSeasonal dynamics of marine and freshwater autotrophic picoplankton (APP) have been reported by several studies (CRAIG, 1984;WEISSE, 1988;STOCKNER, 1991;PICK and AGBETI, 1991;OCHS and RHEW, 1997;MALINSKY-RUSHANSKY et al., 1997;HEPPERLE and KRIENITZ, 2001;JASSER and ARVOLA, 2003), but the factors that regulate their abundance are little known. According to the latest results it seems there is a general trend in the autotrophic picoplankton (APP) dynamics: in the majority of temperate lakes having different depth and trophic state they showed a biphasic distribution.In a four-year study in the meso-to eutrophic Lake Constance WEISSE and KENTER (1991) found that in spring the autotrophic picoplankton (APP) abundance increased with the water temperature until the abundance reached the first maximum in April. Later in May it significantly decreased, and the second peak appeared in late summer, in August. The results reported by CALLIERI and PINOLINI (1995) from the subalpine Lake Maggiore were similar to the results obtained by WEISSE and KENTER (1991). They noticed that the first peak was in April, but the second was later, in late autumn-winter. The seasonality of autotrophic picoplankton (APP) in the shallow and dystrophic Lake Flosek (JASSER, 1997) was biphasic Internat. Rev. Hydrobiol.
The biological availability of dissolved organic carbon (DOC) was experimentally studied in water samples collected at the mouth of the River Zala and in the western and eastern basin of Lake Balaton (Hungary) in four seasons. The water samples were filter-sterilized and inoculated with the in situ bacterial population. The concentration of fulvic and humic acids were analyzed at the beginning of the experiment. The bacterioplankton biomass and DOC concentration were measured on day 0 and 28. The decrease in the DOC concentration and the ratio of the bacterial C/initial DOC concentration showed the microbial utilization of DOC. No seasonal changes in DOC availability were found at any of the sampling stations. The DOC bioavailability was higher in the river than in the lake water. A strong positive correlation was found between the bioavailability and the humic properties of DOC. The observed changes in the organic matter composition of Lake Balaton support the view that much of the not readily utilizable 'refractory' DOM resides in the nonhumic pool in standing waters.
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