MEPS 594:175-191 (2018)  -  DOI: https://doi.org/10.3354/meps12503

ABSTRACT: Within coastal marine habitats, intense nutrient cycling and near-seabed primary production rates are strongly influenced by the transport and transformation of materials within the sediment and across the sediment-water interface. Through processes such as bioturbation and bio-irrigation, benthic infauna play a significant role in mediating this transport, and modify many chemical and physical reactions. However, coastal ecosystems are experiencing growing impacts from a number of environmental stresses, one of which is reduced levels of dissolved oxygen (DO), known as hypoxia. Hypoxic events in coastal areas are predicted to increase as global warming and human-induced eutrophication intensify, with predicted consequences for infaunal community diversity and ecosystem function. Using a mesocosm experiment, we investigated the effects of short-term, sub-lethal hypoxia (14 d, 3.59 mg O₂ l^-1) and organism density (500, 900, 1300, 1700 and 2100 ind. m^-2) on the bioturbation activity of the brittlestar Amphiura filiformis. Nutrient fluxes were measured as an important contribution to ecosystem function. Hypoxia resulted in reduced brittlestar activity (in terms of sediment surface bioturbation), increased efflux of ammonium and silicate and an increase in the ratio of NH₄⁺:NO_x when brittlestar densities were high. No significant effects of hypoxia were detected on brittlestar burrow depth. Our results illustrate that population density plays a crucial role in exacerbating the effects of hypoxia, possibly due to greater biological oxygen demands and increased waste products as organism density increases. Consequently, during moderate reductions in DO, densely populated communities may actually be more vulnerable to hypoxic stress and exhibit greater shifts in ecosystem function than sparsely populated communities.

KEY WORDS: Low oxygen · Bioturbation · Invertebrate ecology · Benthic biogeochemistry ·? Population dynamics · Climate change · Eutrophication · Ecosystem processes