In 1976 Daphnia, a genus that had been inconspicuous in the zooplankton of Lake Washington, suddenly became dominant. The mean summer transparency of the lake doubled. The major change affecting the SUCCESS of Duphniu evidently was a decrease in the abundance of the predatory Neomysis mercedis in the mid-1960s. Daphnia probably did not increase at that time because of the continued persistence of significant quantities

Increased external nutrient loads of anthropogenic origin, especially those of phosphorus (P), were one of the major causes of eutrophication during the first half of the twentieth century in Europe. They led to deterioration of lake ecosystems, particularly including noxious blooms of (potentially toxic) cyanobacteria. From the 1970-1980s, strategies to decrease the phosphorus loads from sewage were increasingly implemented, among them are the ban of phosphates in detergents, the expansion of sewer systems and improvement in wastewater treatment to remove nutrients. Case studies of eight lakes, whose response to point source reduction of phosphorus was observed over decades, show that a pronounced reduction of the phosphorus load from point sources can be achieved either by the diversion of inflows carrying high loads, by upgraded sewage treatment, or by phosphorus precipitation in the major tributary directly before its inflow into the water body. Outcomes demonstrate that in order to effectively control cyanobacterial blooms, the measures taken need to reduce in-lake concentrations of total phosphorus below 20-50 lg L -1 , with this threshold varying somewhat between lakes depending in particular on hydromorphological and biological conditions. Whether and when load reduction succeeds in controlling cyanobacteria depends primarily on the load

Life history responses are expected to accompany climate warming, yet little is known how long-term effects of climate and environmental change affect the seasonal dynamics of planktonic organisms. We used an historical data set from Lake Washington (U.S.A.) to quantify population responses of a calanoid copepod (Leptodiaptomus ashlandi) to long-term changes in temperature and resource availability and explore potential mechanisms for the responses. Increasing water temperatures (annual mean increase of 1.5uC in the upper 10-m water volume) and longer stratification periods (about 4 weeks) were observed between 1962 and 2005, coincident with a pronounced decline in Leptodiaptomus densities. However, production was maintained because of an increase in the production to biomass ratio and a life cycle shift in Leptodiaptomus from an annual to a 6-month cycle. Cross-wavelet analyses demonstrated that the annual thermal forcing of copepod recruitment observed during the first two decades of the study weakened substantially, leading to more stochastic population dynamics during the past two decades. This shift from one to two generations per year was most likely produced by a longer and warmer growing period combined with changing fluctuations in resource (phytoplankton) availability. Climate change can lead to higher-frequency voltinism in ectothermic organisms and to temporal reorganization of their population dynamics.

The ability of Daphnia pulicaria and Daphnia thorata to grow and reproduce when given the same concentrations of single species of algae was compared. The 10 algae selected included some of those most frequently found in the guts of Daphnia in Lake Washington. Clearing rates were determined and consumption of food expressed as number of cells ingested per day, cell volume, and carbon content. Cryptomonas erosa and Stephanodiscus hantzschii supported the highest egg production and increase in biomass. Colonial forms (Asterionella formosa, Fragilaria crotonensis, Tabellaria fenestrata, and Melosira italica) were less favorable as food. Growth and reproduction were lowest with Chlorella sp. and Melosira italica tenuissima. Daphnia p&curia grew and reproduced faster than D. thorata with 7 of the 10 food species. The greater success of D. pulicaria than of D. thorata in the lake may be explained partly by its ability to use more effectively the energy available in a variety of foods.Daphnia pulicaria has been continuously present in Lake Washington since May 1976, forming relatively dense populations for a few weeks each year and becoming scarce in winter but not entirely disappearing. Other species of Daphnia have formed peaks at different times of year and have not been continuously present, most notably Daphnia thorata and, to a lesser extent, Daphnia galeata (Edmondson and Litt 1982). Our purpose here was to explore the question of how these differences could be related to food supply. For this, we compared the ability of D. pulicaria and D. thorata to use different species of algae.Studies of the feeding rate and rate of assimilation of different species of radioactively labeled algae by animals (Arnold 197 1;Infante 1973;Hayward and Gallup 1976) describe the ability to obtain and incorporate food, but cannot of themselves indicate the effect of a given species of food on the development and maintenance of a population. Because population dynamics are more directly related to growth rates and reproductive rates, we measured the early growth and reproduction of Daphnia when fed single species of algae.