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

Abstract

1. In this field study, diatom‐specific and bacteria‐specific fatty acids were used as biomarkers to evaluate the differences in diet betweenChironomus plumosus (a spring‐emerging cohort) andC. anthracinus(an autumn‐emerging cohort), andProcladiusspp. Furthermore, total lipid content ofChironomuslarvae was analysed.2. Individual dry mass of the spring cohort ofC. plumosusrapidly increased during spring and early summer. Surprisingly, the autumn‐emerging cohort showed remarkably little growth over the same time interval. The individual dry mass ofC. anthracinusinitially 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 inC. plumosus(> 3‐fold increase) than inC. anthracinus(1.5‐fold). Conversely, the bacterial indicatingisoform of septadecanoic acid (17 : 0i) was higher inC. anthracinusthan in the spring‐emerging cohort ofC. plumosus. This shows thatC. plumosusassimilates energy from the spring diatom bloom to a greater degree, whereasC. anthracinusfeeds more exclusively on detritus in the sediment.4. Concentrations of 17 : 0iinProcladiuslarvae were 0.54 ± 0.13 mg g–1, i.e. consistently higher than for bothChironomustaxa, 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 relatedChironomusspecies and between different cohorts.6. The different feeding modes of bothChironomusspecies may be important for among‐lake distribution patterns, with filter‐feedingC. plumosusbeing dominant in relatively shallow lakes and deposit‐feedingC. anthracinusmore common in deeper lakes with stable stratifications. This conjecture was supported by a compilation of data from Lake Mälaren.