Effects of the water discharge on periphyton abundance and diversity in a large river (River Danube, Hungary)

This paper examines the relevance of intermediate disturbance theory in the context of in a large river the algal flora attaching to artificial substrata. It was observed that the theory is in good agreement with most of the communities analysed, however, with different frequency intervals compared to phytoplankton. Floods provided the most significant disturbances. As a rough generalization, we can say that in case of floods following each other in 8-14 days, respectively, the disturbance is of medium frequency. In case of 8 days, high frequency is more typical while in case of 14 days, low frequency is more characteristic. Owing to the fact that riverine periphytic algal organisms are attached in different ways and with differing efficiencies, the changes in the total mass occurring as a result of changes in water discharge lead to changes in diversity. The analysis of individual numbers can help in investigating the intensity of the disturbing effect on the algal communities. Negative correlation was found between water discharge and numbers of individuals, that is, the numbers of algae increase with low water discharge and decrease with high water level in the river. This second observation is valid only within a certain range of discharges (in this case up to 100 m3 s-1), which will be specific to particular rivers and individual reaches. Below the upper limit of the range, the increased water flows carry more nutrients to the organisms but with no severe wash-off effect. Beyond the upper limit (in this case in range 100-400 m3 s-1) the numbers of individuals is roughly in inverse proportion to the wash-off effect of the current, and the community is in equilibrium. In cases when discharge surpasses 400 m3 s-1 decrease in individual numbers is general. We suggest that in the environments similar to the study area, the periphyton of the artificial substrata (e.g., pontoons) undergo similar processes. In respect of the periphyton formed on the surface of the riverbeds in large rivers with considerable changes in water level, a more complex system can be supposed where part of the periphyton is periodically dried and flooded alternately

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

Hutchinson´s heritage: the diversity-disturbance relationship in phytoplankton

This paper introduces a collection of contributions presented at the 8th Workshop of the International Association of Phytoplankton Taxonomy and Ecology. It compares the substance of with what to limnologists is the more familiar ‘paradox of the plankton’ posed by G. E. Hutchinson. The utility of Connell‘s Intermediate Disturbance Hypothesis in plankton ecology is, potentially, more instructive but inherent difficulties in relating response to stimulus have to be overcome. A copy of the brief distributed to contributors before the workshop is appended