K. Vermonden

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 stable-isotopic 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 &dgr;13C 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 &dgr;13C values of invertebrates do not necessarily indicate a direct pathway between MOB and these invertebrates because algae may form an intermediate level.

The influence of environmental factors and dredging on chironomid larval diversity in urban drainage systems in polders strongly influenced by seepage from large rivers

Abstract.  Interest in the biodiversity value of urban waters is growing. Understanding key ecological processes is essential for effective management of these aquatic ecosystems. Our paper focuses on identifying the key factors that structure chironomid assemblages, such as water quality and dredging, in urban waters strongly influenced by seepage of large rivers. Chironomid assemblages were studied in urban surface-water systems (man-made drainage ditches) in polder areas along lowland reaches of the rivers Rhine–Meuse in The Netherlands. Multivariate analysis was used to identify the key environmental factors. Taxon richness, Shannon index (H′), rareness of species, and life-history strategies at urban locations were compared with available data from similar man-made water bodies in rural areas, and the effectiveness of dredging for restoring chironomid diversity in urban waters was tested. Three different chironomid associations were distinguished by Two-Way Indicator Species Analysis. Variation within and among chironomid associations were significantly related to substrate (sludge layer and substrate type: sand vs clay), % cover of lemnids, submerged vegetation, filamentous algae, and water transparency. Chironomid taxon richness and H′ were similar in urban and rural waters, probably because of their similar hydrologic, morphologic, and water-quality conditions and their similar dredging and weed-control regimes. Rareness was slightly higher in urban than in rural waters. In urban water systems, chironomid taxon richness was negatively related to sludge layer and % cover of lemnids. Dredging changed chironomid species composition, and increased taxon richness and life-history strategies indicative of good O2 conditions. Therefore, dredging can be regarded as an effective measure to restore diversity of chironomid communities in urban waters affected by nutrient-rich seepage or inlet of river water.