Recent reviews by researchers from academia, industry, and government have revealed that the criteria used by the Stockholm Convention on persistent organic pollutants under the United Nations Environment Programme are not always able to identify the actual bioaccumulative capacity of some substances, by use of chemical properties such as the octanol-water partitioning coefficient. Trophic magnification factors (TMFs) were suggested as a more reliable tool for bioaccumulation assessment of chemicals that have been in commerce long enough to be quantitatively measured in environmental samples. TMFs are increasingly used to quantify biomagnification and represent the average diet-to-consumer transfer of a chemical through food webs. They differ from biomagnification factors, which apply to individual species and can be highly variable between predator-prey combinations. The TMF is calculated from the slope of a regression between the chemical concentration and trophic level of organisms in the food web. The trophic level can be determined from stable N isotope ratios (δ(15) N). In this article, we give the background for the development of TMFs, identify and discuss impacts of ecosystem and ecological variables on their values, and discuss challenges and uncertainties associated with contaminant measurements and the use of δ(15) N for trophic level estimations. Recommendations are provided for experimental design, data treatment, and statistical analyses, including advice for users on reporting and interpreting TMF data. Interspecies intrinsic ecological and organismal properties such as thermoregulation, reproductive status, migration, and age, particularly among species at higher trophic levels with high contaminant concentrations, can influence the TMF (i.e., regression slope). Following recommendations herein for study design, empirical TMFs are likely to be useful for understanding the food web biomagnification potential of chemicals, where the target is to definitively identify if chemicals biomagnify (i.e., TMF > or < 1). TMFs may be less useful in species- and site-specific risk assessments, where the goal is to predict absolute contaminant concentrations in organisms in relation to threshold levels.

The elemental composition of marine cyanobacteria is an important determinant of the ecological stoichiometry in low-latitude marine biomes. We analyzed the cellular carbon (C), nitrogen (N), and phosphorus (P) contents of Prochlorococcus (MED4) and Synechococcus (WH8103 and WH8012) under nutrient-replete and P-starved conditions. Under nutrient-replete conditions, C, N, and P quotas (femtogram cell Ϫ1 ) of the three strains were 46 Ϯ 4, 9.4 Ϯ 0.9, and 1.0 Ϯ 0.2 for MED4; 92 Ϯ 13, 20 Ϯ 3, and 1.8 Ϯ 0.1 for WH8012; and 213 Ϯ 7, 50 Ϯ 2, 3.3 Ϯ 0.5 for WH8103. In P-limited cultures, they were 61 Ϯ 2, 9.6 Ϯ 0.1, and 0.3 Ϯ 0.1 for MED4; 132 Ϯ 6, 21 Ϯ 2, and 0.5 Ϯ 0.2 for WH8012; and 244 Ϯ 21, 40 Ϯ 4, and 0.8 Ϯ 0.01 for WH8103. P limitation had no effect on the N cell quota of MED4 and WH8012 but reduced the N content of WH8103. The cellular C quota was consistently higher in P-limited than in nutrient-replete cultures. All three strains had higher C : P and N : P ratios than the Redfield ratio under both nutrient-replete and P-limited conditions. The C : N molar ratios ranged 5-5.7 in replete cultures and 7.1-7.5 in P-limited cultures; C : P ranged 121-165 in the replete cultures and 464-779 under P limitation; N : P ranged 21-33 in the replete cultures and 59-109 under P limitation. Our results suggest that Prochlorococcus and Synechococcus may have relatively low P requirements in the field, and thus the particulate organic matter they produce would differ from the Redfield ratio (106C : 16N : 1P) often assumed for the production of new particulate organic matter in the sea.The marine cyanobacterium Prochlorococcus is a small photosynthetic prokaryote (diameter, 0.5-0.8 m) that is 1 Corresponding author (Stebe@ebc.uu.se). Present address: Department

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Stable isotopes have been used to analyse food webs and (or) trace movements of animals for about 30 years. There has been some debate on the use of different tissues and treatments before isotope analysis, as well as on seasonal effects. We found different crayfish (Pacifastacus leniusculus) tissues (muscle, hepatopancreas, exoskeleton, gill, and whole body) to have different isotope values. Lipid extraction made whole-body carbon isotope values higher but had no effect on nitrogen isotope values. Acidification made whole-body isotope values lower. For crayfish, there was no seasonal or interannual variation in isotope values. In contrast to studies based on gut content analysis, we found adult crayfish to be at least as carnivorous as young-of-the-year crayfish. Earlier studies often have assumed that each food source contributes both nitrogen and carbon in equal proportions. Omnivores do not fit easily into this view. We suggest that nitrogen and carbon in an organism could come from different sources. Adopting this view for a pond food web could render crayfish both predators and detritivores as crayfish prey on nitrogen sources (other invertebrates) and consume large amounts of detritus to satisfy their carbon demand.

1. It is often assumed that lakes highly influenced by terrestrial organic matter (TOM) have low zooplankton food quality because of elemental and/or biochemical deficiencies of the major particulate organic carbon pools. We used the biochemical [polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA) -20:5x3] and elemental (C : P ratio) composition of particulate matter (PM) as qualitative measures of potential zooplankton food in two categories of lakes of similar primary productivity, but with contrasting TOM influence (clear water versus humic lakes). 2. C : P ratios (atomic ratio) in PM were similar between lake categories and were above 400. The concentration (lg L )1 ) and relative content (lg mg C )1 ) of EPA, as well as the particulate organic carbon concentration, were higher in the humic lakes than in the clearwater lakes. 3. Our results show high fatty acid quality of PM in the humic lakes. The differences in the biochemical quality of the potential zooplankton food between lake categories can be attributed to the differences in their phytoplankton communities. 4. High biochemical quality of the food can result in high efficiency of energy transfer in the food chain and stimulate production at higher trophic levels, assuming that zooplankton are able to ingest and digest the resource available.