Reproductive potential of Daphnia galeata in relation to food conditions: implications of a changing size-structure of the population

A field study was conducted in the highly eutrophic, biomanipulated Bautzen Reservoir to investigate the influences of food conditions on fecundity of Daphnia galeata during spring and early summer in four consecutive years. By defining a relation between body size and maximum brood size, the realis ed reproductive potential (RRP) of Daphnia was expressed as percentage of a ctual egg numbers of the population in relation to calculated maximum egg n umbers. This parameter is independent of the size structure of the populati on. It was found to correlate generally with mean clutch size. However, dev iations occurred if size at maturity (SAM) declined. Extremely low SAM valu es (similar to0.8 mm) were recorded in 2 years, corresponding with increasi ng food concentrations. Low clutch sizes in these situations can be explain ed by small body sizes of ovigerous females, whereas RRP increased, thus re flecting good food conditions. From the comparison of different reproductiv e parameters, it is concluded that RRP is the most reliable one with regard to bottom-up influences in case of extremely variable size structure and p ossible predation impacts. Fecundity of D. galeata was clearly food limited during the clear-water stage. Relatively high POC-threshold concentrations of 0.26 mg C l(-1) suggest that food quality was low and may be equally im portant as food quantity in limiting egg production.