EFFECTS OF SALINITY ON METABOLISM AND LIFE-HISTORY CHARACTERISTICS OF DAPHNIA-MAGNA

In the Baltic Sea area, the cladoceran Daphnia magna is commonly found in brackish water rockpools and it has been suggested that salinity is one of the niche dimensions that affects the distribution of the species. The salinity tolerance of D. magna was studied both in physiological and life history experiments. The experimental salinities were freshwater, 4 parts per thousand S and 8 parts per thousand S . The highest respiration and ammonium excretion rates were measured in the freshwater treatment with decreasing respiration and ammonium excretion rates at higher salinities. The lowest O/N ratio (oxygen consumption to ammonium excretion), describing the metabolic status of an organism, was obtained at 8 parts per thousand S, although the only significant differences were detected when comparing to 4 parts per thousand S treatments. Individual growth rate, reproductive output and population growth rate were highest at 4 parts per thousand S. At 8 parts per thousand S growth and reproduction were reduced as compared to freshwater and 4 parts per thousand S. The life history parameters in the performed experiments indicated higher fitness (expressed as r) as well as more favourable conditions for growth and reproduction at 4 parts per thousand S, whereas the O/N ratio was more difficult to interpret and, in this case, gave a less clear picture of the salinity influence.