We measured concentrations of essential fatty acids (EFAs) in four size categories of planktonic organismsseston (10-64 m), microzooplankton (100-200 m), mesozooplankton (200-500 m), and macrozooplankton (Ͼ500 m)-and in rainbow trout (Oncorhynchus mykiss) in coastal lakes. Size-dependent patterns in concentrations of specific fatty acids (FAs) are important for ecosystem function, because planktivorous fish and some invertebrates are size-selective predators. We demonstrate that the retention of individual FAs differs among the four size categories of planktonic organisms in our study systems. Changes in individual EFA concentrations within the planktonic food web were similar in all coastal lakes sampled, which indicates the generality of our findings. Although concentrations of arachidonic acid, eicosapentaenoic acid (EPA), and linoleic acid increased steadily with plankton size, the concentration of ␣-linolenic acid decreased slightly in larger size fractions of zooplankton. Concentrations of another EFA, docosahexaenoic acid (DHA), declined sharply from mesozooplankton to the cladoceran-dominated macrozooplankton size class. Our results indicate that the retention of EFAs, as a function of plankton size, is related, in part, to the taxonomic composition of planktonic food webs. We suggest that, in general, zooplankton exhibit an EPA-retentive metabolism with increasing body size, whereas different taxonomic groups within the planktonic food web retain DHA differently. Finally, we conclude that EPA is highly retained in zooplankton, whereas in rainbow trout DHA is highly retained.

Although food quality is thought to play an important role in the survival of marine organisms, the extent of natural variability in food quality over long time scales remains poorly characterized. We present a 6 yr time series of fatty acid data from the calanoid copepod Neocalanus plumchrus, an important contributor to mesozooplankton biomass in the Strait of Georgia and the northeast Pacific Ocean. Fatty acid profiles indicate significant spatiotemporal differences in the diet of this copepod. Spatially, oceanic specimens display fatty acid signatures characteristic of omnivorous copepods while coastal animals display primarily herbivorous, diatom-based signatures. Temporally, the fatty acid profiles of coastal N. plumchrus shifted from an omnivorous oceanic diet to an herbivorous, diatom-based diet between 2001 and 2006. The ratio of diatom to flagellate fatty acid markers increased over time, peaking from 2005 to 2006. The composition of flagellate markers also changed from primarily dinoflagellate markers (rich in docosohexaeonic acid) to green algal markers (poor in this essential fatty acid). The diet of N. plumchrus as deduced from fatty acids correlates with phytoplankton community composition. The abundance of coastal N. plumchrus in the Strait of Georgia was strongly correlated with the ratio of docosahexaeonic acid to eicosapentaeonic acid in the lipids of these copepods. We also discuss the potential for an imbalance of essential fatty acids supplied by a diatom-rich diet to limit the growth and survival of N. plumchrus in the Strait of Georgia.KEY WORDS: Copepod · Diapause · Diet · Docosahexaeonic acid · Eicosapentaeonic acid · DHA · EPA · Fatty acids · Food quality · Neocalanus Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 382: [151][152][153][154][155][156][157][158][159][160][161] 2009 which are retained in their zooplankton predators and can be used as qualitative tracers of dietary source (Dalsgaard et al. 2003). Diatoms, for example, are characterized by high concentrations of eicosapentaeonic acid (EPA,, 16:1n-7 and by the presence of polyunsaturated fatty acids (PUFA) containing 16 carbon chains (16PUFA), whereas dinoflagellates are characterized by high concentrations of the essential PUFA docosahexaeonic acid (DHA, and PUFA containing 18 carbons (18PUFA, specifically, 18:4n-3) (Thompson et al. 1992, Viso & Marty 1993, Graeve et al. 1994, Stevens et al. 2004a. Essential PUFAs such as DHA and EPA are important for the physiology of marine copepods and have been shown to affect the efficiency by which energy is transferred in food webs (Müller-Navarra et al. 2000, St. John et al. 2001. Other examples of fatty acid trophic markers of phytoplankton, microzooplankton, bacteria and calanoid copepods are provided in Table 1.The use of fatty acid trophic markers in calanoid copepods has been verified in the laboratory and the field, and has succeeded in establishing trophic relations among and within different species of copepods and...

Carbon stable isotope analyses have been widely used to estimate terrestrial carbon contribution to lake secondary production. In such approaches, phytoplankton is tacitly assumed as a single, isotopically homogenous source. Such assumption might be valid if (i) zooplankton do not feed selectively on specific algal taxa within bulk phytoplankton, or (ii) although zooplankton do feed selectively, the variability in the d 13 C values amongst the different algal taxa is small compared with the variability between the d 13 C values of bulk phytoplankton and terrestrial end-members. In a summer field study of six coastal lakes of British Columbia, Canada, we tested these assumptions using gut pigments and stable isotope measures on zooplankton and particulate organic matter. Results revealed that filter-feeding cladocerans positively selected cryptophytes, shown to be substantially 13 C-enriched compared with bulk phytoplankton and even with the terrestrial end-member. Comparing a classical two-source mixing model and a model that accounted for algal isotopic heterogeneity, we showed that the use of a classical two-source model can result in an overestimation of terrestrial contribution to zooplankton secondary production. Résumé :Les analyses en isotopes stables du carbone ont été largement utilisées pour estimer la contribution du carbone d'origine terrestre à la production secondaire des lacs. Dans ces approches isotopiques, le phytoplancton est tacitement considéré comme une source unique et isotopiquement homogène. Cette supposition peut être valide sous l'hypothèse selons laquelle (i) le zooplancton ne se nourrit pas sélectivement de certains taxons algaux au sein du phytoplancton ou (ii) même si le zooplancton se nourrit de façon sélective, la variabilité des d 13 C entre les différentes taxons algaux n'excède pas la variabilité des d 13 C entre la source algale globale et la source terrestre. Nous avons testé ces hypothèses au cours d'une étude de terrain menée sur six lacs côtiers de Colombie-Britannique (Canada) en été en analysant les pigments du tube digestif et la composition isotopique du zooplancton et de la matière organique particulaire. Les résultats montrent que les cladocères filtreurs sélectionnent les cryptophytes, dont la composition isotopique est particulièrement enrichie en 13 C par rapport au phytoplancton total et même à la matière organique d'origine terrestre. En comparant les estimations issues d'un modèle de mélange classique à deux sources à celles d'un modèle prenant en compte l'hétérogénéité isotopique des algues, nous montrons que le modèle classique à deux sources conduit à une surestimation de la contribution du carbone terrestre à la production secondaire zooplanctonique.