Sudden and gradual responses of phytoplankton to global climate change: case studies from two large, shallow lakes (Balaton, Hungary and the Neusiedlersee Austria/Hungary)

This paper analyses two phytoplankton long-term datasets; both are from large, temperate shallow lakes. The main difference between them is that phytoplankton growth in Lake Balaton remained severely P-limited despite P-driven eutrophication during the last 30 years, whereas extremely high turbidity causes a permanent light limitation in Neusiedlersee and therefore an increase in P-loadings did not result in a similar increase in phytoplankton biomass. Neusiedlersee is a (slightly) saline inland lake. In Lake Balaton, the blue-green alga Cylindrospermopsis raciborskii blooms invariably if the July-august temperature deviates positively from a 30-year average by ca. 2 °C. A supposed global warming is predicted to cause a higher frequency (but not intensity!) of these blooms. Neusiedlersee is very shallow and therefore regulation techniques cannot prevent water levels sinking in successive dry years. Annual averages of phytoplankton seem to follow quite a regular, wave-like cyclicity. Such cycles can be recognised in the population records of the characteristic species. Similar changes were seen in changes of water level, conductivity, inorganic-P, inorganic N-forms and nutrient ratios. How phytoplankton species can follow a climatic cycle that covers 200 to 500 generations has not yet become clear. Because of reasons discussed in the paper, neither of the two cases can be generalised; each is quite individual

Estimation of minimum sedimentary inoculum (akinete) pool of Cylindrospermopsis raciborskii: a morphology and life-cycle based method

Cylindrospermopsis raciborskii undergoes characteristic morphological changes during its annual population development. Primary filaments ( those that derive directly from the akinetes) possess the following morphological features: the trichomes have one or two acuminate ends; they are narrower (2.4 - 2.6 mum) than older filaments, their lengths vary in a wide range (from 40 to 300 mum); some of them are straight but others are slightly coiled even in populations that later consist exclusively of straight filaments; cell walls between the cells are not or only hardly visible; the filaments have a fine granulation but no gas-vacuoles, polyphosphate- or any other contrasting bodies are seen. The secondary filaments deriving from the first division of primary filaments are similar but with only one acuminate end. This way, the number of germinating akinetes can be estimated as the number of primary filaments + 1/2 number of secondary filaments. The described morphological method to estimate inoculum size of C. raciborskii has limitations. ( 1) The number of akinetes that we get in this way is only a minimum number since germination is not synchronous under field conditions and probably not each akinete germinates that is present in the sediments. ( 2) The method is applicable only if germination occurs suddenly ( relatively synchronously). This is often the case in a temperate region where germination is triggered by temperature. ( 3) The method cannot be used in tropical or subtropical lakes where water temperature is permanently rather high and akinetes do not develop or develop only very rarely. ( 4) Being an a posteriori method it cannot be used as predictive tool. Estimations for Lake Balaton are in a good agreement with results of studies with other methods ( germination in laboratory cultures and direct counting) and have the advantage that they do not need time-consuming experimenting or difficult direct countings