Benthic respiration and stoichiometry of regenerated nutrients in lake sediments with Dreissena polymorpha

Abstract

The effect of patchy colonies of the invasive zebra mussel (Dreissena polymorpha) on sedimentary processes was investigated in a mesotrophic lake (Plateliai Lake, Lithuania). Benthic fluxes of O2, TCO2, CH4, Mn2+, Fe2+, N2, the inorganic forms of N, Si and P and dissolved organic C and N were quantified by dark incubations of sediments cores, with and without D. polymorpha. Individual mussels also were incubated for metabolic measurements. Sediments with D. polymorpha had significantly higher O2 and TCO2 fluxes and displayed higher rates of denitrification. The presence of mussels also resulted in higher regeneration of P and N (mostly as ammonium) while the effect on Si was not significant. However, likely due to the low zebra mussel biomass (57.2 ± 25.3 gSFDW m−2), biodeposition has not changed the ratio between anaerobic and total respiration. Methane and reduced metals fluxes were in fact similar in the presence and absence of D. polymorpha. Incubations of mussels without sediments confirmed that bivalve metabolism was the main driver of benthic respiration and nutrient recycling. Nitrate production suggested the presence of nitrifiers associated with the molluscs. The main outcome of this study was that zebra mussels alter the quantity and the stoichiometry of nutrients regenerated by the benthic compartment. The enhancement of nitrogen loss via denitrification, by a factor of 1.5, was much less pronounced than the increase in ammonium recycling rate, stimulated by a factor of 33. Negligible PO4 3− fluxes in bare sediments (−3.4 ± 6.8 μmol m−2 h−1) increased in the presence of mussels and considerable amounts of this nutrient (69.6 ± 29.4 μmol m−2 h−1) were mobilized to the water column. Further research should address other nutrient sources to the lake to verify whether altered rates and stoichiometry of benthic regeneration can affect primary producer community composition and activity.