Investigations on pelagic food webs in mountain lakes - aims and methods

The mean nitrogen and phosphorus contents of the calanoid copepod Mixodiaptomus laciniatus Lilljeborg were measured to the stage level throughout the ice-free period of a high mountain lake. Our results indicate large intraspecific variations in calanoid elemental composition. While mean N as dry weight increased from 3.0 ± 1.3 in nauplii to 6.0 ± 2.1 in copepodites, mean P content showed the opposite trend, varying intraspecifically from 0.98 ± 0.26% in nauplii, to 0.87 ± 0.21% in copepodites and 0.51 ± 0.16% in adults. Thus, the mean N:P ratio increased ontogenetically from 3.3 in nauplii to 13.3 in copepodites and 24.6 in adults. Two ontogenetic parameters, the growth rate and body size, were associated with zooplankton stoichiometry. Among all 11 copepod stages, growth rate was positively related to %P and negatively related to %N and N:P ratio. A two-part analysis of these relationships, before and after metamorphosis, showed that the nauplii growth rate explained nearly all the variance in naupliar P content. A high P content in nauplii may reflect a high content of RNA, translating into rapid growth rates. Overall, these results tend to support the hypothesis linking specific growth rate with P content for copepods, but these results also suggest that the validity of this hypothesis is robustness when differences in the life history of copepods as a consequence of metamorphosis are accounted for. We suggest that the intra-stage variation in P content is associated with peaks of intensive metabolic activity during the process of molting in copepods, and we emphasize the importance of new empirical evidence to examine this hypothesis further.