Fatty acids have been used as qualitative markers to trace or confirm predator-prey relationships in the marine environment for more than thirty years. More recently, they have also been used to identify key processes impacting the dynamics of some of the world's major ecosystems. The fatty acid trophic marker (FATM) concept is based on the observation that marine primary producers lay down certain fatty acid patterns that may be transferred conservatively to, and hence can be recognized in, primary consumers. To identify these fatty acid patterns the literature was surveyed and a partial least squares (PLS) regression analysis of the data was performed, validating the specificity of particular microalgal FATM. Microalgal group specific FATM have been traced in various primary consumers, particularly in herbivorous calanoid copepods, which accumulate large lipid reserves, and which dominate the zooplankton biomass in high latitude ecosystems. At higher trophic levels these markers of herbivory are obscured as the degree of carnivory increases, and as the fatty acids originate from a variety of dietary sources. Such differences are highlighted in a PLS regression analysis of fatty acid and fatty alcohol compositional data (the components of wax esters accumulated by many marine organisms) of key Arctic and Antarctic herbivorous, omnivorous and carnivorous copepod species. The analysis emphasizes how calanoid copepods separate from other copepods not only by their content of microalgal group specific FATM, but also by their large content of long-chain monounsaturated fatty acids and alcohols. These monounsaturates have been used to trace and resolve food web relationships in, for example, hyperiid amphipods, euphausiids and fish, which may consume large numbers of calanoid copepods. Results like these are extremely valuable for enabling the discrimination of specific prey species utilized by higher trophic level omnivores and carnivores without the employment of invasive techniques, and thereby for identifying the sources of energetic reserves. A conceptual model of the spatial and temporal dominance of group-specific primary producers, and hence the basic fatty acid patterns available to higher trophic levels is presented. The model is based on stratification, which acts on phytoplankton group dominance through the availability of light and nutrients. It predicts the seasonal and ecosystem specific contribution of diatom and flagellate/microbial loop FATM to food webs as a function of water column stability. Future prospects for the application of FATM in resolving dynamic ecosystem processes are assessed.

Fatty acid trophic markers in the pelagic marine environment.

Global warming and the loss of sea ice threaten to alter patterns of productivity in arctic marine ecosystems because of a likely decline in primary productivity by sea ice algae. Estimates of the contribution of ice algae to total primary production range widely, from just 3 to >50%, and the importance of ice algae to higher trophic levels remains unknown. To help answer this question, we investigated a novel approach to food web studies by combining the two established methods of stable isotope analysis and fatty acid (FA) analysis--we determined the C isotopic composition of individual diatom FA and traced these biomarkers in consumers. Samples were collected near Barrow, Alaska and included ice algae, pelagic phytoplankton, zooplankton, fish, seabirds, pinnipeds and cetaceans. Ice algae and pelagic phytoplankton had distinctive overall FA signatures and clear differences in delta(13)C for two specific diatom FA biomarkers: 16:4n-1 (-24.0+/-2.4 and -30.7+/-0.8 per thousand, respectively) and 20:5n-3 (-18.3+/-2.0 and -26.9+/-0.7 per thousand, respectively). Nearly all delta(13)C values of these two FA in consumers fell between the two stable isotopic end members. A mass balance equation indicated that FA material derived from ice algae, compared to pelagic diatoms, averaged 71% (44-107%) in consumers based on delta(13)C values of 16:4n-1, but only 24% (0-61%) based on 20:5n-3. Our estimates derived from 16:4n-1, which is produced only by diatoms, probably best represented the contribution of ice algae relative to pelagic diatoms. However, many types of algae produce 20:5n-3, so the lower value derived from it likely represented a more realistic estimate of the proportion of ice algae material relative to all other types of phytoplankton. These preliminary results demonstrate the potential value of compound-specific isotope analysis of marine lipids to trace C flow through marine food webs and provide a foundation for future work.

Tracing carbon flow in an arctic marine food web using fatty acid-stable isotope analysis.

Status and recommendations on marine copepod cultivation for use as live feed

Tetraselmis suecica andDunaliella tertiolecta were grown for 24 hr in the presence of14C sodium bicarbonate and then fed separately to batches of juvenile oysters,Crassostrea gigas, for 3 days.D. tertiolecta contained fatty acids no longer than C18; 22∶6ω3 was absent inT. suecica. Analysis of the oyster fatty acids by radio gas chromatography (GC) showed that oysters were able to incorporate some of the dietary14C label into long-chain fatty acids not supplied in the diet, e.g., C20 and C22 mono- and polyunsaturated fatty acids, and particularly 20∶5ω3. However, the low14C incorporation into fatty acids longer or more unsaturated than those supplied in the diet suggests that elongation and desaturation activity in young oysters is not sufficient to sustain optimum growth.

Fatty acid metabolism in young oysters, Crassostrea gigas: polyunsaturated fatty acids

The use of live prey is still necessary for a large number of raised ¢sh species. Small sized rotifers are usually used as live preys during the ¢rst days of feeding in small mouth ¢sh. An alternative to this is the use of copepods as prey for ¢rst feeding. In this study, the sizes, weight and biochemical contents of two copepods and one rotifer species raised on similar algal food conditions were compared. Rotifers contained a higher proportion of essential amino acids in the free amino acid (FAA) fraction (43%) than copepods (30^32%). However, rotifers had lower levels of important fatty acids like DHA (7% compared with 23^32% in copepods) and their DHA/EPA ratio was lower than that in copepods (0.54 compared with 1.35^1.63 in copepods). The FAA pattern of the preys tended to be species-speci¢c and its implications from an aquaculture point of view is discussed. In contrast, the-protein bound amino acids tended to be very conservative among the studied organisms. The second part of the work is focused on ‘the price’ of hatching in Acartia tonsa eggs before or after cold storage at 31C. The fatty acid contents in A. tonsa tended to decrease with the storage time. It also decreased with hatching of the nauplii, but its proportion compared with the dry weight remained constant.

Biochemical and technical observations supporting the use of copepods as live feed organisms in marine larviculture

The metabolic fate of radioactive linoleate and α-linolenate administered to the South Atlantic copepodParacalanus parvus was studied. The wild copepod was able to incorporate the labeled acids dissolved in seawater. The radioactive linoleate was elongated to 20∶2ω6 and 22∶2ω6 and desaturated by a Δ6 desaturase to 18∶3ω6. α-Linolenate was also desaturated by a Δ6 desaturase to 18∶4ω3 and elongated to 20∶3ω3. The copepod was able to convert α-18∶3 to 20∶5ω3 and 22∶6ω3.

Julia E. A. De Moreno

Papers

Fatty acid metabolism in the calanoid copepodParacalanus parvus: 1. Polyunsaturated fatty acids