Methane as a carbon source for the food web in raised bog pools

Abstract

Raised bog pools are extremely nutrient poor and rich in humic substances, and these features limit primary production. To assess the base of the invertebrate food web in bog pools we measured the stable-isotopic signatures of primary producers, dead organic matter, and invertebrates, and the composition and stable-C-isotope ratio of their phospholipid-derived fatty acids (PLFAs). The stableisotopic signatures showed the presence of multiple trophic levels and differential use of basal food sources by the invertebrates among and within species, individuals, and size classes. Carnivorous and omnivorous invertebrates assimilated polyunsaturated fatty acids (PUFAs) derived from algae, and possibly macrophytes, and fatty acids that are specific for methane-oxidizing bacteria (MOB). Part of the bacterial biomass conveyed to higher trophic levels in the bog pools originated from MOB. Pelagic zooplankton appeared to rely more on bacteria, whereas insects relied more on algae. Periphyton, a primary algal food source, was the basal food source most depleted in 13C and was inferred to sustain §½ the invertebrate food web. The relatively depleted d13C values of PUFAs in invertebrates suggest a role for methane-derived C. We argue that the CO2 assimilated by the algae could be derived from MOB. Therefore, depleted d13C values of invertebrates do not necessarily indicate a direct pathway between MOB and these invertebrates because algae may form an intermediate level.