Phytoplankton growth and phosphate uptake ( for P limitation ) by natural phytoplankton populations from the Loosdrecht lakes ( The Netherlands ) ’

Phosphate uptake capacity of natural phytoplankton populations was used as a physiological indicator of P limitation in the Loosdrecht lakes. During a substantial period of the growth season of 1983, P-limited growth was observed in Lake fireukeleveen and Lake Loosdrecht, but P limitation was less severe in the latter. In Lake Breukeleveen the maximum initial phosphate uptake rate (V,,) varied between 0.3 and 5.7 pg P (pg Chl)-’ h-l during P-limited growth, whereas in Lake Loosdrecht, V,, varied between 0.3 and 2.1 pg P (pg Chl)-’ h--l. Both at the end of June and the beginning of July, growth was not P limited in either lake. Phytoplankton in Lake Loosdrccht was not limited by phosphorus in September. The frequent occurrence of P limitation led to the prediction that reduction of the P loadings of Lake Loosdrecht will result in a reduction of the phytoplankton biomass. Algal bioassays can provide valuable information relevant to the understanding and prediction of the effects of nutrient additions or removal (e.g. by sewage addition or diversion of diluted water) on lakes (Claesson and Forsberg 1980; Goldman 1978). Here we discuss various bioassays and report a study of phosphorus-phytoplankton interactions in the highly eutrophic Loosdrecht lakes (The Netherlands), 20 km southeast of Amsterdam (52”20’N, 5’5’E). The total catchment area of the lake system is 44.4 km2 (Fig. 1); average depth of the lakes is 2 m. The mean hydraulic retention time’of the lakes is about 1 year (Van Liere et al. 1984). Inputs of industrial and agricultural wastewaters (60 t P yr-‘) have increased eutrophication. During the past 30 years phytoplankton biomass has increased, leading to partial disappearance of the submerged macrophytes and charaphytes (Best et al. 1984). The seston mass now varies seasonally from 10 to 30 mg literI. Phytoplankton diversity has diminished and the lakes are dominated by cyanobacteria (de KIoet et al. 1984). Measures aimed at reducing P loadings began in 1984. Eighty percent of the exter’ This investigation was supported by the Foundation for Fundamental Biological Research (BION) which is subsidized by the Netherlands Organization for the Advancement of Pure Research (Z.W.O.). nal P load to the lake system came from the River Vccht. In 1984, the Vecht was disconnected from the Loosdrecht system, to be replaced with water from the Amsterdam-Rhine canal after removal of phosphorus by coagulation with FeCl,. To estimate the results of the restoration program, we studied seasonal changes in phosphorus availability for 1 year before the diversion. The manner in which algae experience their growth environment is reflected in the adaptation of physiological properties. We measured phosphate uptake capacity of the natural phytoplankton to study the availability of phosphorus for growth. We thank A. Konopka, D. W. Tempest, and S. J. Tarapchak for comments and suggestions, and also E. van Liere and J. H. Gons who provided the chlorophyll data. U. G. DijkstraStam provided technical assistance. Indicators of nutrient limitation Algae use light energy to construct new cell material from elementary nutrients such as carbon, nitrogen, and phosphorus. When one of these becomes limiting, its rate of supply will determine the algal growth rate. To decide whether phosphorus limits the growth rate of algae in a particular lake, we can measure soluble reactive phosphorus (SRP) concentrations (Kappers 1984), but this provides no information about the availability of other nutrients. Enrichment