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.