Fatty acid (FA) concentrations and their seasonal variations were quantified for profundal benthic invertebrates, surficial sediment, and sedimenting matter from Lake Erken, Sweden. Food quality for profundal zoobenthos, as indicated by the concentrations of long-chain polyunsaturated FA, ω3 FA, or eicosapentaenoic acid (EPA) in sediment and sedimenting matter, was highest in spring and autumn and markedly lower in summer. Surficial sediment was consistently lower in all FA than was sedimenting matter. Palmitoleic acid (16:1ω7) was the dominating FA in both sedimenting matter and sediment. In fauna samples, EPA, palmitic acid (16:0), palmitoleic acid, and vaccenic acid (18:1ω7) were among the dominant FA. Docosahexaenoic acid was found only in the predators Chaoborus and Procladius. Differences between functional feeding guilds were found for the diatom-indicating FA palmitoleic acid and the bacteria-specific FA isoseptadecanoic acid (17:0iso). Furthermore, principal components analysis showed marked differences in FA composition among taxa. These differences reflect the relative contribution of food from autotrophic (phytoplankton production) and heterotrophic sources (detrital food web) in profundal invertebrate taxa.
Abstract– Various sizes of roach, perch, pike, zander (pike‐perch) and crucian carp were collected from lakes of different trophic levels in order to study the variation of lipids and fatty acids (FA) within and between species. Freeze‐dried samples of the dorsal muscle were analysed quantitatively for total lipid content and FA content. The results indicate that total lipid and FA contents can vary considerably, both within and between species. In contrast to herbivorous fish, carnivorous‐piscivorous fish FA patterns were more constant and independent of size ‐ fry excepted. Lipid and FA contents of roach from two oligotrophic lakes were significantly higher than in roach from a eutrophic lake. Differences in basic food webs may be responsible for these results. In the oligotrophic lakes, the algal flora was dominated by species classified as high quality food for grazers, e. g. flagellates and diatoms. It was shown in earlier papers that these algal groups contain two long‐chained FA of φ 3 type (eicosapentaenoic acid and docosahexaenoic acid) which are used as criteria indicating high nutritional value. Conversely, the eutrophic lake was dominated by blue‐greens, a group of autotrophs lacking these long‐chained φ 3 FA. Blue‐greens have generally been classified as poor food for grazers. (The fatty acids are described by three numbers, x:yφz, where x=number of carbon atoms, y=number of double bonds, and z=position of the first double bond counted from the methyl end of the molecule.)
The variability of mercury (Hg) levels in Swedish freshwater fish during almost 50 years was assessed based on a compilation of 44 927 observations from 2881 waters. To obtain comparable values, individual Hg concentrations of fish from any species and of any size were normalized to correspond to a standard 1-kg pike [median: 0.69 mg kg−1 wet weight (ww), mean ± SD: 0.84 ± 0.67 mg kg−1 ww]. The EU Environmental Quality Standard of 0.02 mg kg−1 was exceeded in all waters, while the guideline set by FAO/WHO for Hg levels in fish used for human consumption (0.5–1.0 mg kg−1) was exceeded in 52.5 % of Swedish waters after 2000. Different trend analysis approaches indicated an overall long-term decline of at least 20 % during 1965–2012 but trends did not follow any consistent regional pattern. During the latest decade (2003–2012), however, a spatial gradient has emerged with decreasing trends predominating in southwestern Sweden.
Methylmercury (MeHg) transfer from water into the base of the food web (bioconcentration) and subsequent biomagnification in the aquatic food web leads to most of the MeHg in fish. But how important is bioconcentration compared to biomagnification in predicting MeHg in fish? To answer this question we reviewed articles in which MeHg concentrations in water, plankton (seston and/or zooplankton), as well as fish (planktivorous and small omnivorous fish) were reported. This yielded 32 journal articles with data from 59 aquatic ecosystems at 22 sites around the world. Although there are many case studies of particular aquatic habitats and specific geographic areas that have examined MeHg bioconcentration and biomagnification, we performed a meta-analysis of such studies. Aqueous MeHg was not a significant predictor of MeHg in fish, but MeHg in seston i.e., the base of the aquatic food web, predicted 63% of the variability in fish MeHg. The MeHg bioconcentration factors (i.e., transfer of MeHg from water to seston; BCF) varied from 3 to 7 orders of magnitude across sites and correlated significantly with MeHg in fish. The MeHg biomagnification factors from zooplankton to fish varied much less (logBMF, 0.75 ± 0.31), and did not significantly correlate with fish MeHg, suggesting that zooplanktivory is not as important as bioconcentration in the biomagnification of fish MeHg across the range of ecosystems represented in our meta-analysis. Partial least square (PLS) and linear regression analyses identified several environmental factors associated with increased BCF, including low dissolved organic carbon, low pH, and oligotrophy. Our study reveals the widespread importance of MeHg bioconcentration into the base of the aquatic food web for MeHg at higher trophic levels in aquatic food webs, as well as the major influences on the variability in this bioconcentration.
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