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.

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Fatty acids (FA) are inextricably linked with key physiological and biochemical processes and are thus integral to proper ecosystem functioning. FA not biosynthesized effectively by animals are termed essential fatty acids (EFA). These EFA are important "drivers" of ecosystem health/stability and are therefore highly conserved in aquatic food chains. Aquatic organisms have been and continue to be our primary source of readily available EFA. However, overfishing and our burgeoning population may be acting in concert to threaten our access to this source of EFA. Here, we review the marine FA synthesis/transport cycle and traditional and nontraditional sources of EFA. Our review suggests that, while some traditional sources of marine oils (e.g., tuna) are in steady decline, other sources (e.g., krill) and technologies (e.g., heterotrophic fermentation) hold great promise for maintaining our access to EFA. We provide a minireview which illustrates that EFA contribute to our health and well-being. Finally, there is growing evidence that EFA have been an important force in our past evolution, leading us and others to speculate that an unbroken link exists between EFA, our present health, and, in all likelihood, our continuing evolution.

We used a scanning spectroradiometer to conduct underwater optical surveys of 44 waterbodies during the ice-free seasons of three consecutive years in wetlands and lakes in central Saskatchewan, Canada. The waterbodies ranged widely in dissolved organic carbon (DOC) concentration (4.1-156.2 mg L Ϫ1) and conductivity (270-74,300 ohms cm Ϫ1). Although penetration of UV radiation (UV-R; 280-400 nm) in these systems was largely a function of DOC concentration, as has been reported previously, UV-R penetrated more deeply in saline waterbodies than in freshwater systems with similar DOC concentrations. Power models representing our K dUV-B or K dUV-A versus DOC relationships were described by K dUV-B ϭ 0. . UV-B radiation (280-320 nm) is not expected to penetrate deeply (typically Ͻ50 cm) in prairie lakes and wetlands because of high intrinsic DOC concentrations. However, the central plains are characteristically windy and this, coupled with the shallowness of many of these systems, suggests that biota may still be at risk from present-day and future-enhanced levels of UV-B (which may result from ozone depletion). Moreover, this risk may be exacerbated in saline systems. This could be significant, especially because saline waterbodies are often highly productive and represent important North American staging areas for shorebirds and waterfowl.Ultraviolet radiation (UV) has influenced the evolution of life on earth since it first appeared. Even in recent times, in 1 Corresponding author. AcknowledgmentsWe thank J. C. Mollison of the Instrument Technology Services for designing and constructing the parallelogram swing arm used to deploy the Optronics minicosine and submersible sphere sensors. We are grateful to H. I. Browman, Institute of Marine Research, Storebø, Norway, for supplying us with the underwater immersion correction factors used with the submersible sphere and for his sound advice on several technical aspects of the Optronics OL-754 meter. We thank R. Young and C. Rapp/C. Johnson, Optronics Laboratories Inc., Orlando, Florida, for providing us with the immersion correction factors for the minicosine sensor and for answering our technical questions, respectively. We are indebted to R. A. Bourbonniere and K. Edmondson (NWRI-Burlington) for performing DOC analyses on the 1998 samples. This research was made possible by funding through Environment Canada's National Water Research Institute to M.T.A. and R.D.R.