Species composition, spatial distribution, and the seasonal and interannual dynamics of phytoplankton in brown-water lakes enclosed with reed-belts (Neusiedlersee/Fertő Austria/Hungary)

摘要

Phytoplankton of 12 reed-belt enclosed inner ponds of the large, shallow, alkaline, turbid lake, Neusiedlersee (Austria/Hungary), was studied in the last 13 years. The paper describes the dominant algal groups and their seasonal and interannual quantitative development. Interannual changes are related to those of some physical and chemical variables.ud1.The rather rich algal flora can be recorded in the inner lakes. However, most of the species have benthic or periphytic origins. Thus, as in the open lake, the inner lake phytoplankton is characterized by only few dozens of major planktonic species. In inner lakes with small surface areas (1-5 hectares) quantitative contribution of non-planktonic algae (large diatoms, homocytic blue-green and filamentous green algae) can be around 40%.ud2.In small lakes with brown water that is transparent to the bottom, planktonic flagellates (mostly Cryptomonas and Rhodomonas species) represent the most important group. In bigger lakes in which the water is turbid as a result of stirred up inorganic sediment, non-motile planktonic algae, mostly diatoms, are important. In one of the lakes coccal green algae were the most abundant.ud3.Preliminary records show that in transparent, brown-water lakes circadian rhytms of flagellates should not escape consideration in further studies.ud4.The seasonal development of phytoplankton was characterized by a mid- or late summer peak biomass. Considerable spring bloom of algae was not observed.ud5.The average volume (volumetric biomass [µm(3) 1(-1)] divided by the number of individuals) of species peaked in spring or midsummer and reached its seasonal minima during the seasonal peak biomasses. Consequently, the peak biomass in these lakes comprise algae that are easily grazeable for even the smaller, non-selecting zooplankton species.ud6.A coccal green/diatom peak developed each year with high degree of regularity in lakes in which the seasonal succession was studied in detail. However, on species level, seasonal development appeared to be quite unpredictable: the peak biomass was provided by different species each year. In this respect the enclosed lakes differ greatly from the open water of Neusiedlersee, in which an extraordinarily low level of seasonality can be observed.ud7.The interannually observed increase in phytoplankton peak biomass in both lakes (Haider-Seppl-Poschen-Lacke, Ruster-Poschen) coincided with a drying-out period. This can be well demonstrated by the increasing trend of the conductivity records. PO (4)(3-)-P increased and disssolved N forms (NO(3)(-)-N and NH(4)(+)-N) decreased during the study period. A a consequence, N/P ratio decreased significantly in both lakes. An increase in dissolved oxygen was recorded in one of the lakes. These changes can be considered as consequences of parallel physico-chemical and biological changes. Experimental investigations are necessary for a better understanding of the real causal interconnections. Nevertheless, the results of this study unequivocally prove that the recurrent drying-out periods significantly affect the planktonic habitats and plankton communities in a wetland system.