Biogeochemical buffers in a eutrophic coastal lagoon along an oxic-hypoxic transition

Abstract

The effects of bottom water oxic-hypoxic transition on estuarine benthic ecosystems have been extensively studied as oxygen (O2) depletion leads to the accumulation of toxic sulphides impacting sediment biogeochemical functioning. The role of benthic macrofauna in modulating such effects is understudied. Macrofauna may increase the resilience of sediments to critical O2 transitions via direct and indirect mechanisms acting upon organic matter (OM) and solid-phase metal pools that represent internal geochemical buffers for sediments. In this study we analysed whether and how the macrofaunal community may facilitate sediment resilience against short-term hypoxic events. Thus, net fluxes of reduced metals and inorganic nutrients were measured under oxic conditions and induced hypoxia in intact cores representing most of the sedimentary environments of the eutrophic Curonian Lagoon. Afterwards, distance-based linear models (distLM) were applied to explore the relationship between the dominant macrofauna taxa, sediment properties and the net fluxes. The results show a mosaic of sedimentary environments including OM poor (0.3%) sandy areas, OM rich (23%) muddy areas and variable combinations of these extremes. The variability of the macrofauna community was comparatively much lower, with oligochaetes and chironomids representing the most abundant taxa at most sampling sites. Along with the induced oxic-hypoxic transition metal fluxes tended to increase, together with dissolved inorganic phosphorus release, whereas dissolved inorganic nitrogen and silica tended to decrease. The distLM model evidenced that sediment properties accounted for 84.2% of the total flux variability whereas Chironomidae, Gammaridae and Gastropoda cumulatively accounted for 15.8% of the total explained variation in fluxes under oxic conditions. Under hypoxic conditions macrofauna explained a measurable but minor fraction of flux variability whereas OM and granulometry were best predictors. In the Curonian Lagoon macrofauna has a minor role in regulating fluxes and in providing biogeochemical services as compared to sediment features. As such, the benthic system appears vulnerable to short-term events of O2 shortage.