title: Biological vs. physical mixing effects on benthic food web dynamics.
abstract: Biological particle mixing (bioturbation) and solute transfer (bio-irrigation) contribute extensively to ecosystem functioning in sediments where physical mixing is low. Macrobenthos transports oxygen and organic matter deeper into the sediment, thereby likely providing favourable niches to lower trophic levels (i.e., smaller benthic animals such as meiofauna and bacteria) and thus stimulating mineralisation. Whether this biological transport facilitates fresh organic matter assimilation by the metazoan lower part of the food web through niche establishment (i.e., ecosystem engineering) or rather deprives them from food sources, is so far unclear. We investigated the effects of the ecosystem engineers Lanice conchilega (bio-irrigator) and Abra alba (bioturbator) compared to abiotic physical mixing events on survival and food uptake of nematodes after a simulated phytoplankton bloom. The (13)C labelled diatom Skeletonema costatum was added to 4 treatments: (1) microcosms containing the bioturbator, (2) microcosms containing the bio-irrigator, (3) control microcosms and (4) microcosms with abiotic manual surface mixing. Nematode survival and subsurface peaks in nematode density profiles were most pronounced in the bio-irrigator treatment. However, nematode specific uptake (Δδ(13)C) of the added diatoms was highest in the physical mixing treatment, where macrobenthos was absent and the diatom (13)C was homogenised. Overall, nematodes fed preferentially on bulk sedimentary organic material rather than the added diatoms. The total C budget (µg C m(-2)), which included TO(13)C remaining in the sediment, respiration, nematode and macrobenthic uptake, highlighted the limited assimilation by the metazoan benthos and the major role of bacterial respiration. In summary, bioturbation and especially bio-irrigation facilitated the lower trophic levels mainly over the long-term through niche establishment. Since the freshly added diatoms represented only a limited food source for nematodes, the macrobenthic effect was more pronounced in niche establishment than the negative structuring effects such as competition.
authors: Department of Biology, Marine Biology Section, Ghent University, Ghent, Belgium. Ulrike.Braeckman@UGent.be
authors: Ulrike
authors: U
authors: Braeckman
authors: Pieter
authors: P
authors: Provoost
authors: Tom
authors: T
authors: Moens
authors: Karline
authors: K
authors: Soetaert
authors: Jack J
authors: JJ
authors: Middelburg
authors: Magda
authors: M
authors: Vincx
authors: Jan
authors: J
authors: Vanaverbeke
mesh_terms: D000818:Animals; D001419:Bacteria / Q000254:growth & development; D002735:Chlorophyllides / Q000378:metabolism; D017753:Ecosystem; D020387:Food Chain; D019015:Geologic Sediments / Q000032:analysis* / Q000382:microbiology; D007448:Invertebrates / Q000254:growth & development; D009348:Nematoda / Q000254:growth & development; D010100:Oxygen / Q000378:metabolism
chemical_list: D002735:Chlorophyllides; C034388:chlorophyllide a; D010100:Oxygen