Changes in the volume and composition of phytoplankton after experimental acidification of a humic lake

Abstract

In connection with the HUMEX project carried out in Lake Skjervatjern, near Førde, in the western part of Norway, the effects of the experimental acidification on the phytoplankton volume and composition were investigated. The lake was divided into two basins by a thick plastic curtain. One of the basins, Basin A, and its entire catchment was acidified by a mixture of ammonium nitrate and sulphuric acid, while Basin B acted as a reference. The results of the phytoplankton analyses from both basins in the investigation period are presented. In Basin B, the control basin, the succession of the main groups of phytoplankton throughout the growth season remained almost identical unit 1993. The results from the acidified Basin A in 1991 and 1992 show marked changes in the phytoplankton composition and percentage of the main algae groups. The percentage of the green algae decreased, especially the species Oocystis submarina v. variabilis, while the dinoflagellates, mainly the species Peridinium umbonatum (P. inconspicuum), and the cryptomonads increased. In 1993, however, and even more in 1994, there was a marked change in the composition in the control Basin B compared to previous years, with less green algae and more chrysomonads. The total volume was higher in Basin A in 1994 than in Basin B, especially in the autumn, while it was more or less the same in both basins in 1993. In the acidified Basin A, the dominance of the dinoflagellates was more pronounced in 1993 and 1994 than in previous years. The reason for the change in composition of phytoplankton registered in the control Basin B in 1993 and 1994 is hard to explain. As the changes in the composition of Basin A came immediately after the treatment of the Basin started, it is obvious that the addition of nitrogen, and especially the nitrate, through the treatment, was important in regulating the composition in this acid and humic lake. An increase in phosphorus seems to contribute more to the level of the total biomass of phytoplankton than to the observed changes in the composition.