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Planktiska alger i Sjöar. Bedömningsgrunder

Willen, Eva


Classifications of the ecological status of Swedish lakes based on total biomass and composition of phytoplankton are presented. The period of evaluation refers to the late summer succession stage (July 15–August 31) and a lake´s epilimnion layer. The Utermöhl analysis method has to be used preferably with the technique published in the Swedish EPA´s Handbook of Environmental Assessment at present only in a Swedish version ( Concerning the species richness used for evaluations in a gradient of acidity is that technique necessary. Classifications are made of the following indictors in a 5-degree scale from high status (=reference conditions) to poor/bad status: • total biomass of phytoplankton • proportion (%) of cyanobacteria • proportions (%) of chrysophycean algae • trophic plankton index (TPI) • species richness Indicators 1–4 refer to a trophic gradient and no. 5 to a gradient of acidification/acidity. The basis of the assessment work is late summer samples from 480 lakes with thorough taxonomic resolution and countings, where slightly more than 50% was considered as references according to an elaborated manual common for a broad range of assessment work in freshwater environments, and including several organism groups. The manual is presented in table 2 of this work. The assemments comprise three regions in Sweden each separated in clear and humic lakes (borderline value 30 mg Pt/l): alpine lakes above the tree-limit, Sweden north and south of limes norrlandicus. Lakes invaded by the slime-producing alga Gonyostomum semen are treated separately. A special section of these assessment criteria describes the construcion of a new trophic plankton index which includes indicator scores given to 57 taxa with prevalence in nutrien-rich lakes and 34 taxa which mainly occur in nutrient-poor lakes. The index is scored in a ± range of 3 meaning 6 different indicator classes from –3 to 3. Finally a recommendation of getting a mean value of three summers is given for a more reliable assessment, due to the inter-annual weather-induced variations of plankton abundances and community structures. Three is the least number of seasons to get some idea of variations even if classifications are possible based on data from just one sample and summer period. A period of testing activities is necessary for a final establishment of the strength of values given for biomasses, class proportions and index scores


växtplankton; trofiindex; vattenkvalitetsbedömning

Published in

Rapport / Sveriges lantbruksuniversitet, Miljöanalys
2006, number: 2006:4
Publisher: Institutionen för Miljöanalys