Fatty acid biomarkers show dietary differences between dominant chironomid taxa in Lake Erken

1. In this field study, diatom-specific and bacteria-specific fatty acids were used as biomarkers to evaluate the differences in diet between Chironomus plumosus (a spring-emerging cohort) and C. anthracinus (an autumn-emerging cohort), and Procladius spp. Furthermore, total lipid content of Chironomus larvae was analysed. 2. Individual dry mass of the spring cohort of C. plumosus rapidly increased during spring and early summer. Surprisingly, the autumn-emerging cohort showed remarkably little growth over the same time interval. The individual dry mass of C. anthracinus initially declined in early spring, but then increased during April and May. 3. Accumulation of the diatom-specific fatty acid palmitoleic acid (16 : 1ω7) during spring was much higher in C. plumosus (> 3-fold increase) than in C. anthracinus (1.5-fold). Conversely, the bacterial indicating iso form of septadecanoic acid (17 : 0i) was higher in C. anthracinus than in the spring-emerging cohort of C. plumosus. This shows that C. plumosus assimilates energy from the spring diatom bloom to a greater degree, whereas C. anthracinus feeds more exclusively on detritus in the sediment. 4. Concentrations of 17 : 0i in Procladius larvae were 0.54 ± 0.13 mg g^–1, i.e. consistently higher than for both Chironomus taxa, indicating that this predator gains a relatively high fraction of its energy through detrital pathways (from bacteria). 5. These results show that fatty acid biomarkers can be an appropriate tool to detect differences in larval diet between coexisting chironomid species, between two closely related Chironomus species and between different cohorts. 6. The different feeding modes of both Chironomus species may be important for among-lake distribution patterns, with filter-feeding C. plumosus being dominant in relatively shallow lakes and deposit-feeding C. anthracinus more common in deeper lakes with stable stratifications. This conjecture was supported by a compilation of data from Lake Mälaren.     

Effects of algal diets and temperature on the growth and fatty acid content of the cichlid fish Oreochromis niloticus L. – A laboratory study

The effects of different algal foods and water temperatures on the growth and fatty acid content of the Nile Tilapia, Oreochromis niloticus L., were studied. Four types of algae, given in the same amounts as the control diet, were used as food: Microcystis aeruginosa, colonial and single-celled forms; Arthrospira fusiformis; and Scenedesmus quadricauda. The control group was fed a commercial diet of cichlid pellets, while another group was left unfed. The feeding experiment was run at 25 °C. The condition factor decreased in all algal fed fish groups, except the one fed on Microcystis colonies, whereas the control group showed no significant change. Both food quantity and quality were responsible for this result. Some short-chained fatty acids in the diets could be traced in the long-chained counter-parts in the fish tissue. Both saturated fatty acids and monounsaturated fatty acids were higher in the control vs. treatment groups, whereas the polyunsaturated fatty acids displayed no significant differences amongst any of the treatment groups studied, including the unfed group. Direct quantitative comparison of individual fatty acid in the diet vs. tissue lipids in the fish proved to be difficult due to the great capacity of these tilapias to elongate and desaturate 18 carbon acids into long-chained homologues. The effect of temperature was studied by growing the fish at 16, 20 and 25 °C. All groups were fed commercial cichlid pellets. The level of saturated and monounsaturated fatty acids increased at 20 °C, whereas polyunsaturated fatty acids showed little variation. Docosahexaenoic acid, belonging to the important omega 3 group where the first double bond starts at carbon number three, was highest at 16 °C, resulting in a markedly elevated omega-3/omega-6 ratio at that temperature.