SHORT COMMUNICATION Stoichiometry of some marine planktonic crustaceans

Atomic C:N ratios in calanoid copepods were generally below the Redfield ratio (6.6), except for the fifth copepodid stage of Calanus sp. The C:P ratios in copepods were generally close to, or higher than the Redfield ratio of 106, but low C:P ratios were found in Acartia clausi (63 ± 7) and in the cladocerans (Podon sp.: 34 ± 5 and Evadne sp.: 59 ± 22). Recently, the indirect effect of an apparent invariable stoichiometry of zooplankton on the algal community has attained interest in freshwater (Elser et al., 1988; Sterner, 1990; Hessen and Andersen, 1992; Elser and George, 1993; Elser and Hassett, 1994), and somewhat in marine systems (Elser and Hassett, 1994; Gismervik, 1997). Indeed, the stoichiometry of plankton has been an issue in marine ecology for centuries (e.g. reviewed by Corner and Davis, 1971), but little attention has been paid to community effects of zooplankton stoichiometry on nutrient regeneration. The elemental ratios of zooplankton are generally near those of phytoplankton growing at maximum rates (Gismervik et al., 1997). Under nutrient stress, however, the elemental composition of phytoplankton is highly variable (Harrison et al, 1977; Jahnke et al, 1986; Jahnke, 1989; Ki0rboe, 1989), leading to enhanced differences between zooplankton and phytoplankton stoichiometry. As zooplankton may have higher assimilation efficiencies for the limiting element compared to carbon (Olsen et al, 1986), they may retain nitrogen and/or phosphorus in their tissue and thereby enhance nutrient limitation of the phytoplankton. Analyses of elements in marine copepods have mainly been restricted to carbon and nitrogen content (Bamstedt, 1986), with little emphasis on phosphorus (but see Butler et al, 1969 and Bamstedt, 1986). The purpose of this study was to examine the range and interspecific differences of C:N:P ratios in marine crustaceans. I also wanted to test if marine cladocerans comprise lower C:P ratios than the copepods, as have been found in freshwater (Andersen and Hessen, 1991). Zooplankton were collected on six occasions in 1994 from the Oslofjord (Norway), and analysed for carbon, nitrogen and phosphorus. Copepods were collected around mid-day from the surface water (maximum 40 m) by a WP2 net equipped with a closed cod-end. Some additional deep tows (-100-60 m) were taken in December 1995 to collect Calanus spp. CV over-wintering stages. Prior to this investigation, I analysed both live and frozen samples for P-content, and this revealed significant differences among the treatments for two of the species (Mann-Whitney (/-test, P < 0.05 for Calanus spp. and Pseudocalanus sp.) (Table I). Thus, further analyses were only performed on live samples. Copepods and cladocerans were lightly anaesthetized with carbonated sea water and picked individually, quickly dipped in distilled water, and placed in tin capsules for C:N © Oxford University Press 279 at Penylvania State U niersity on Feruary 1, 2013 http://planfordjournals.org/ D ow nladed from