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 13C 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 (Δδ13C) of the added diatoms was highest in the physical mixing treatment, where macrobenthos was absent and the diatom 13C 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 TO13C 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.

Ecosystem function and services provided by the deep sea

“…Seabed fluid flow can often support large complex habitats comprised of clams, mussels and tube worms (Van Dover, 2000;Cordes et al, 2010a, b). Deep-sea deposit feeders, such as thalassinid shrimps and worms (including polychaetes, echiurans and sipunculans), create extensive burrow systems that irrigate and transport organic material into subsurface sediments (Levin et al, 1997;Hughes et al, 2005;Shields and Kedra, 2009), resulting in a complex three-dimensional sedimentary matrix providing particular niches for other benthic fauna such as the microscopic meiofauna and microbes (e.g., Braeckman et al, 2011;Laverock et al, 2011). Mobile epifaunal megabenthic organisms can create and modify seabed habitats in high densities, especially urchins (echinoids; Vardaro et al, 2009) and sea cucumbers (holothurians; Billett et al, 2010), and large beds of featherstars (crinoids; Bowden et al, 2011).…”

Distribución vertical de la macroinfauna asociada a bivalvos en una planicie intermareal sedimentaria del sur de Chile

“…Resultados similares fueron registrados por Bouchet et al (2009), quienes mostraron que la distribución vertical de los foraminíferos bentónicos en sedimentos intermareales de la costa Atlántica de Francia, estuvo significativamente afectada por la bioirrigación y oxidación del sedimento producida por la macroinfauna, principalmente de bivalvos. Braeckman et al (2011) registraron, en la costa de Bélgica (Mar del Norte), que el bivalvo Abra alba (Wood 1802) desplazó la comunidad de nemátodos hacia capas más profundas del sedimento, debido a los disturbios físicos que este bivalvo genera en las capas más superficiales y/o a la explotación de recursos alimentarios compartidos. En este estudio no se evaluó el mecanismo específico a través del cual T. dombeii influye sobre la distribución vertical de la macroinfauna.…”

Surface adhesion of microphytobenthic biofilms is enhanced under Hediste diversicolor (O.F. Müller) trophic pressure

“…These results suggest that worms did not bring viable diatoms along, and therefore the provision of chlorophyll or phaeophytin by worms cannot explain the threefold increase of chlorophyll a concentrations in treatment DH. Another process to take in consideration is the bioturbation by worms, which is known to enhance the aeration of sediment, thereby allowing the diatoms to settle and photosynthesise deeper than they would without worms (Braeckman et al, 2011;Scaps, 2002). Also, bioturbation increases geochemical fluxes and therefore facilitates the recycling of nutrients in the environment (Braeckman et al, 2010;Coull, 1999;Scaps, 2002).…”