The seasonal changes in the adenosine triphosphate ($TP) ;and chlorophyll a (Chl a ) concentrations in two eutrophic lakes (Lake Peipus, Lake VGrtsjarv; Estonia) were studied in the course of two years. Comparative measurements were performed in the Baltic Sea. The main task was to examine how the ATP content of the water characterizes thc functional statc of the phytoplankton community. Primary prodnction, total phosphorus and total nitrogen were measured as supplementary parameters. The under-ice bloom in Lake V6rtsjLrv was also observed. The under-ice bloom, consisting mainly of the diatom Melosira islandica spp. kelveticn, started a t the bottom of the water body and spread over the wholc water column in a few days. The ATP content corre-Iatcd well with changes in the chla concentration of the bottom water, but in the uppermost regions of the water column no correlation was found. Obviously most of the cells had died beforereaching the surface layers. Seasonal observations showed an accumulation of "dead" chlorophyll in autumn. The Chl a/ATP ratio increased in the following order: sea-entrophic lake-hypereutrophic lake. The values of the index increased exponentially as thLe phytoplankton standing stock increased. 131 132 132
ATP determination in microplankton was introduced hy HOLM-HANSEN and BOOTH (1966) in 1966. Living organisms have a comparatively consistent ATP content, accounting on average for about 0.4 of t,he organic carbon content (HOLM-HANSEN, 1970; HOLM-HANSEN and PAERL, 1972). This fact serves as a basis for the determination of microplankton biomass using ATP assays. Assuming that ATP is contained 9 Int. Revue ges. Hydrobiol. 74 (1989) 2