FATTY-ACID BIOMARKERS SHOW DIETARY DIFFERENCES BETWEEN DOMINANT CHIRONOMID TAXA IN LAKE-ERKEN

1. In this field study, diatom-specific and bacteria-specific fatty ac ids were used as biomarkers to evaluate the differences in diet betwee n Chironomus plumosus (a spring-emerging cohort) and C. anthracinus (a n autumn-emerging cohort), and Procladius spp. Furthermore, total lipi d 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 remarkab ly little growth over the same time interval. The individual dry mass of C, anthracinus initially declined in early spring, but then increas ed during April and May. 3. Accumulation of the diatom-specific fatty acid palmitoleic acid (16 :1 omega 7) during spring was much higher in C. plumosus (> 3-fold increase) than in C. anthracinus (1.5-fold). Co nversely, 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 th e spring diatom bloom to a greater degree, whereas C. anthracinus feed s more exclusively on detritus in the sediment. 4. Concentrations of 1 7: 0i in Procladius larvae were 0.54 +/- 0.13 mg g(-1), i.e. consisten tly higher than for both Chironomus taxa, indicating that this predato r gains a relatively high fraction of its energy through detrital path ways (from bacteria). 5. These results show that fatty acid biomarkers can be an appropriate tool to detect differences in larval diet betwe en coexisting chironomid species, between two closely related Chironom us species and between different cohorts. 6. The different feeding mod es of both Chironomus species may be important for among-lake distribu tion patterns, with filter-feeding C. plumosus being dominant in relat ively shallow lakes and deposit-feeding C. anthracinus more common in deeper lakes with stable stratifications. This conjecture was supporte d by a compilation of data from Lake Malaren.