Spatial and temporal pattern of development and decline of the spring diatom populations in Lake Stechlin in 1999

Padisák, Judit and Scheffler, W. and Sípos, C. and Kasprzak, P. and Koschel, R. and Krienitz, L. (2003) Spatial and temporal pattern of development and decline of the spring diatom populations in Lake Stechlin in 1999. ARCHIV FÜR HYDROBIOLOGIE BEIHEFT ADVANCES IN LIMNOLOGY, 58. pp. 135-155. ISSN 0071-1128

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Abstract

Integrated samples from the euphotic layer (mostly 0-25 m) were taken between January 1994 and December 2000 in the open water of Lake Stechlin. Monthly sampling was carried out in January-April 1999 and bi-weekly in May-June 1999 at depths of 0, 10, 20, 30, 40, 50 and 60 m. Phytoplankton, with special attention to diatoms, was qualitatively and quantitatively investigated. These data, together with water chemistry and zooplankton monitoring information, were used to describe spring development and decline of the phytoplankton. Results were compared to predictions of the PEG-model. i) The large variety in timing and species composition of early-season diatom growth allows one to conclude that in lakes, which can be either mono- or dimictic, it is primaliry not the trophic state that determines spring growth/composition, rather it is the presence, absence or duration of ice-cover and weather conditions during the onset of the thermocline. ii) Physical stresses (low temperatures, low light, complete turbulent mixing) upon phytoplankton in such lakes has been less severe than growth limitation set by scarcity of nutrients. Diatom growth starts soon after the complete autumnal overturn. Thermocline development, instead if initation of the spring bloom as stated in the PEG model, terminates growth since diatoms, cells sink rapidly into the hypolimnion. iii) „Life time” of unicellular planktonic centric diatoms was the longest int he stage when they turn to decaying or dormant cells. Empty frustules dissolve rather quickly. Frustules of planktonic pennatae diatoms were apparently more persistent than that if centrics. iv) Because of their low abundance, metazooplankton cannot play a significant role in the elimination of spring the phytoplankton crop. v) After the onset of thermocline, rejuvenating diatoms can be found in small numbers throughout the hypolimnion. This study demonstrates that one should be careful with using the term „vegetation period” since the bulk of growth of planktonic primary producers might occur during the isothermal period prior to onset of the thermocline. Moreover, driving forces during this period are largely determined by climatic factors or actual weather, such as the duration of the ice-cover or the onset time of the thermocline.

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