| Understanding the movement and feeding habits of marine animals is crucial when managing their populations. The molecular, and stable carbon isotope composition of fatty acids from an organism provides time-integrated information on its dietary intake. Hence, when spatial differences in the quality of seston exist it should be able to trace these differences up into higher trophic level organisms. The presented study evaluates the applicability of 13C/12C ratios of individual fatty acids, as natural tags and dietary markers in marine pelagic organisms. In addition, the use of 13C/12C ratios of bulk sample, as well as fatty acid composition data in examining the movement, and diet of animals are further explored.; Samples of particulate organic matter, zooplankton, larval fish and juvenile salmon collected during three cruises off the west coast of Vancouver Island were analyzed. The fatty acid composition, stable carbon isotope ratio of either bulk sample, or individual fatty acids could typically distinguish samples collected in continental shelf waters from off-Shelf samples. The differences in fatty acid composition between the adjoining food webs seem to be mainly caused by the different contribution of diatom-derived material to the base of the food web. The higher 13C/12C ratios found in the diatom-richer seston in shelf waters were not simply caused by the higher contribution of diatoms. Instead, stable carbon isotope data on individual fatty acids indicate that growth conditions favouring diatom growth caused 13C-enrichment in algae other than diatoms as well.; The relative abundance of polyunsaturated fatty acids, such as docosahexaenoic acid (22:6n-3), were found to increase with trophic level. Whereas the abundance of saturated, and monounsaturated fatty acids was higher in organisms from lower trophic levels. This suggests that the fatty acid composition may be a useful trophic level indicator. However, literature data indicate that these trends observed in seston, zooplankton, larval fish and juvenile salmon, do not hold for larger organisms and adult life stages.; The variation in 13C/12C ratios of individual fatty acids from almost 200 samples from 3 cruises were compared. A large range (typically about 7‰) in the δ13C values of fatty acids is observed within single samples. The variation in δ 13C between the individual fatty acids was found to be reproducible, independent of the quality of seston, and in accordance with patterns reported by other studies. This suggests the presence of common underlying mechanisms, most likely biosynthetic effects, producing the semi-predictable offsets between the δ13C of fatty acids. The factors identified here as having potentially the largest impact on the δ13C seem to be desaturation, different timing of lipid class synthesis during the growth cycle of autotrophs, and perhaps also the proportion of PUFAs synthesized via an alternative (polyketide synthase-catalyzed) pathway.; The δ13C of essential fatty acids in zooplankton and larval fish did not prove to reflect the δ13C of the same fatty acids in the seston better than other, non-essential fatty acids. As natural tags, δ13C values of the bulk and/or the fatty acid composition were found to be similarly successful. However, when an animal moves into an area with isotopically distinct food, an unusual difference between the δ13C values of fatty acids that exhibit different turnover rates can be an indication for recent diet shift. When the various turnover rates are well constrained an estimate of the timing of the diet switch may be possible. |
