Interactions between microbial activity and bioturbation modes of benthic invertebrates determine nutrient releases from reservoir sediments

Bioturbating invertebrates play key roles in nutrient recycling from sediments to the water column in most lentic ecosystems. Nevertheless, the role of bioturbating fauna on nutrient cycling depends on complex interactions between their bioturbation modes and both the sedimentary conditions and the microbial activity occurring inside sediments. This study aims to evaluate the effect of different bioturbation modes by two invertebrate species (tubificid worm Tubifex tubifex and chironomid larvae of Chironomus plumosus), on the release of nutrients (NH4+, NO3-, and PO43-) from three different types of sediments characterised by distinct microbial activities and nutrient (N and P) contents. We expected that the influence of a bioturbation mode on nutrient fluxes was dependent on the microbial activity and organic matter processes occurring in sediments. Aquatic mesocosms were setup in the laboratory with three fauna treatments (i.e. without fauna, with tubificid worms, and with chironomid larvae) and three sediment types (collected in three reservoirs). The presence of bioturbating invertebrates increased PO43- concentrations into the water column (from 1.3- to 17-fold higher concentrations) depending on the sediments tested. PO43- releases in the water column were positively linked to total P concentration and microbial activity in sediments. The influences of tubificid worms and chironomid larvae on PO43- releases into the water were not only due to their bioturbation modes but also depended on the interactions with P dynamics (e.g. adsorption of PO43- by iron oxides) at the water-sediment interface. The influence of fauna on the release of dissolved inorganic nitrogen (DIN) varied strongly among sediments due to the control of N cycling by microbial processes. In sediments characterised by the lowest microbial activity, DIN release from sediments to the water column was significantly stimulated by invertebrates (from 1.3- to 3.7-fold higher). In contrast, no fauna effect was detected compared to controls in sediments characterised by the highest microbial activity. This study demonstrated that the influence of the bioturbation mode on nutrient (i.e. PO43- and DIN) dynamics at the water sediment interface was largely modulated by the intensity of microbial activity and biogeochemical processes occurring in sediments.