Relationship between N : P ratio and growth rate during the life cycle of calanoid copepods: An in situ measurement

The mean nitrogen and phosphorus contents of the calanoid copepod Mixodiapt omus laciniatus Lilljeborg were measured to the stage level throughout the ice-free period of a high mountain lake. Our results indicate large intrasp ecific variations in calanoid elemental composition. While mean N as dry we ight increased from 3.0 +/- 1.3 in nauplii to 6.0 +/- 2.1 in copepodites, m ean P content showed the opposite trend, varying intraspecifically from 0.9 8 +/- 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 paramete rs, the growth rate and body size, were associated with zooplankton stoichi ometry. Among all II 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 gro wth 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 rapi d growth rates. Overall, these results tend to support the hypothesis linki ng specific growth rate with P content for copepods, but these results also suggest that the validity of this hypothesis is robustness when difference s in the life history of copepods as a consequence of metamotphosis are acc ounted for. We suggest that the intra-stage variation in P content is assoc iated with peaks of intensive metabolic activity during the process of molt ing in copepods, and we emphasize the importance of new empirical evidence to examine this hypothesis further.