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Stable isotopes of carbon and nitrogen and fatty acid analyses are increasingly being used in combination to determine the trophic structure of marine systems. For stable isotopes, the variability in carbon and nitrogen isotopic signatures has long been recognised and has been characterised for some taxa. Whilst it is known that metabolic processes may influence fatty acid profiles, the spatial variability of fatty acid profiles has not been documented. Understanding at what scale these 2 biochemical tracers vary, and if the scale of variability corresponds between tracers, is crucial for the correct design and interpretation of combined tracers in trophic studies. This study is the first to examine spatial variability in fatty acid profiles per se, and in combination with stable isotope ratios in the same organisms at multiple spatial scales. We used a spatially hierarchical design which sampled across broad geographic regions, reefs within regions, and also between different parts of macroalgal plants common on temperate reefs. For stable isotopes of carbon and nitrogen, variability was greatest at intermediate spatial scales (between locations within regions, and sites within locations). In contrast, fatty acid profiles showed the greatest variation amongst individual replicates of lobster, abalone and macroalgae. This study demonstrates that for the increasing number of trophic studies using combined biochemical tracers, sampling design should cater to the differences in the variability of each tracer technique and allocate sampling accordingly. KEY WORDS: Stable isotopes · Fatty acid analysis · Nested hierarchical design · Multivariate variance components · Tasmania Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 410: [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] 2010 acids include individual fatty acids that are rare, and unique ratios of commonly occurring fatty acids, both of which can be reflected in the fatty acid profile of a consumer (Dalsgaard et al. 2003). Variation in the fatty acid profiles of organisms is associated with the metabolic process of the organism (Dalsgaard et al. 2003).Environmental factors have been identified as the primary mechanism influencing the fractionation rates (variability) of isotopic ratios. For example, light intensity (Hemminga & Mateo 1996, Alcoverro et al. 2001, temperature (Hemminga & Mateo 1996), CO 2 availability (Burkhardt et al. 1999), water depth (Grice et al. 1996), and nutrient source (Marguillier et al. 1997, Waser et al. 1998, Finlay 2004 influence isotope ratios of autotrophs by altering the rate of productivity. Such factors differ across a range of spatial scales, potentially altering the isotopic ratio of the consumers that depend upon them. As variations in fatty acid profile are associated with the metabolic processes of an organism, factors that have the capacity to alter metabolic processes arguably have the capacity to influence the variation in fatty acid profiles...

The southern rock lobster Jasus edwardsii is a commercial species that has benefited from the complete protection offered by no-take reserves, with higher abundances and larger animals recorded in reserves than in adjacent fished areas. What remains unclear is whether there is any change in the diet of lobsters in reserves, for example, as a result of increased intraspecific competition for food. We used combined chemical tracers to examine the diet of lobsters in fished and reserve areas in 2 bioregions in eastern Tasmania. δ 15 N values of lobsters were richer in fished than in reserve areas, indicating that lobsters eat a greater proportion of food items from higher trophic levels in fished areas. Mixing models suggest that ascidians, sea urchins and the turbinid gastropod were all important food sources for lobsters, but the importance of these food items differed between bioregions. This spatial variability may suggest that the small size of the reserve in one bioregion is inadequate at ensuring the diet of lobsters is protected from fishing pressure. Fatty acid profiles of lobsters supported the importance of these food sources to lobsters. Differences between bioregions, or inside and outside of reserves, were not apparent using fatty acids. The present study highlights that lobster fishing has the capacity to alter the trophic status of prey for generalist predators and suggests that fatty acid analyses may be limited in detecting changes in the dietary composition of such generalist feeders. KEY WORDS: Effects of fishing · Food webs · Marine protected areas · Stable isotopes · Fatty acids Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 388: [169][170][171][172][173][174][175][176][177][178][179][180][181][182][183][184] 2009 lishment of marine reserves in which complete cessation of fishing is enforced (Edgar & Barrett 1997, Shears et al. 2006. Increases in the abundance of J. edwardsii have been linked with trophic cascades in reserves that shifted from sea urchin-to algal-dominated reef habitats (Shears & Babcock 2003). What remains unclear, however, is the potential dietary shift of lobsters due, in part, to greater lobster density after reserve establishment (e.g. via intraspecific competition) that may have unknown effects on other components of the reef community.Chemical tracers such as stable isotope and fatty acid analyses are means by which the diet of lobsters in fished and reserve areas may be determined. As lobsters macerate their food upon ingestion, the chemical tracer approach has advantages over conventional methods of dietary analysis, such as gut content analysis where identification is often labour intensive, requires taxonomic expertise and soft bodied organisms may be overlooked (Sheppard & Harwood 2005). Moreover, chemical tracers identify food that is assimilated over a period of time and is of nutritional importance, rather than that which is ingested at one point in time (Thomas & Cahoon 1993). The basis of the chem...