Seafloor Ecosystem Function Relationships: In Situ Patterns of Change Across Gradients of Increasing Hypoxic Stress

Abstract

Seafloor ecosystems play pivotal roles in biogeochemical cycling, but hypoxia (O2 < 2 mg l−1) is changing the way they contribute to ecosystem function on a global scale. A major issue for mitigation of eutrophication-driven hypoxia is the continued release of phosphorus and nitrogen from sediments. Nutrient cycling is affected by sediment characteristics, benthic communities, and oxygen conditions, but the context dependency of these processes in natural ecosystems is poorly known. The Baltic Sea is naturally a low-diversity system, where hypoxia has further decimated the benthic communities. To investigate how oxygen conditions affect the relationship between benthic fauna and nutrient fluxes across the sediment–water interface, we conducted macrofaunal sampling and measurements of benthic oxygen and nutrient fluxes (NO3 −, NO2 −, NH4 +, PO4 3−, SiO4) at 26 sites across the entire north–south salinity gradient of the Baltic Sea (>1200 km). This broad-scale sampling confirmed the strong salinity-driven diversity gradient and large spatial variations in oxygen conditions, which affected the status of the benthic communities. Benthic nutrient fluxes varied several orders of magnitude, both between sea areas and along gradients of hypoxia within areas. DistLM modeling indicated that benthic fauna can affect nutrient cycling, also under hypoxic conditions, with the invasive polychaete Marenzelleria spp. being particularly important. However, as the oxygen content decreases—even slightly—the subsequent changes in the faunal abundance and functional diversity may influence the nature and the rate of fluxes. These results imply that management targets for oxygen concentrations need to be raised to sustain healthy ecosystem functioning and to facilitate the recovery of large degraded sea areas.