Elemental and isotopic fractionation of carbon and nitrogen by marine, planktonic copepods and implications to the marine nitrogen cycle

Particle-grazing copepods, primarily Temora longicornis and T. stylifera, and seawater with natural particles were collected from the northwest Gulf of Mexico. Control and ammonium-enriched aliquots of seawater were incubated in triplicate for 2 days, copepods added and the incubation continued for 2 days. Analyses were made of dissolved nutrients (nitrate, ammonium and phosphate), suspended particles (chlorophyll a and phaeopigments, C, N, τ13C, τ15N), and copepod bodies and feces (C, N, τ13C, τ15N) and the rates of egg and feces production were estimated. Primary production Δchlorophyll a, C, N) was enhanced by N enrichment, indicating its initial N limitation. The rates of egg and feces production were greatest for copepods in N-enriched seawater, indicating food-limited ingestion and egg production. Elemental (C:N) and isotopic (τ13C, τ15N) fractionation by copepods occurred following ingestion of suspended particulate matter (spm) and during the production of tissue (b) and feces (f): C:Nf>C:Nspm >C:Nb, τ13Cb>τ13Cf>τ13Cspm, and τ15Nf>τ15Nb>τ15Nspm. In a second experiment, N-enriched and N-deficient phytoplankton were fed to Acartia tonsa and again C:Nf>C:Nspm>C:Nb. These data indicate that copepods in the present study (i) used nitrogen more efficiently than carbon for tissue production and (ii) produced tissue and feces enriched and excreta depleted in 13C and 15N relative to the suspended particulate matter. The implications of these results to the marine nitrogen cycle are discussed.