Density and size-dependent bioturbation effects of the infaunal polychaete Nephtys incisa on sediment biogeochemistry and solute exchange

The impact of bioturbation on the geochemistry of aquatic sediments is known to depend on the benthic infauna species that are present. However, burrowing and activity patterns of each species may also change during the different stages of a life cycle. In this study, we examined the effects of four size classes of the polychaete Nephtys incisa on burrow networks and sediment biogeochemistry. In our experimental aquaria, the total biovolume (∼biomass) of Nephtys was kept constant, but different age classes were introduced, so the size and abundance varied between treatments. Despite differences in the geometry of burrow networks (due to varying density and size of burrows as revealed by X-radiography), the transport of nonreactive solutes (Br⁻) showed little difference between treatments. In contrast, the depth distribution of reactive solutes (Fe²⁺, Mn²⁺, TPO³⁻ 4^{, TCO}2, O₂, pH) depended on oxidized sediment volumes and on spatial micro-heterogeneity related to burrowing patterns. Net fluxes of O₂, TCO₂, and NO⁻3 fluxes were strongly affected by age-dependent burrowing patterns. Carbonate dissolution and remineralization rates (reflected by TCO₂ fluxes) were enhanced as the size of individuals increased. NO⁻3 fluxes showed progressive change from dominance of nitrification (release) to denitrification (uptake) as burrow densities decreased with larger individuals. We conclude that different age-size classes of a single species at identical biovolume affect biogeochemical cycling differently, due to changes in burrow sizes and burrow densities. Because of redox reaction coupling associated with burrow geometries (Fe²⁺, Mn²⁺ oxidation patterns), similar magnitudes of nonlocal transport may be a misleading indicator of biogenic impacts. Our observations demonstrate that biogeochemical impacts must be evaluated in the context of size (age-) specific traits and population densities rather than biomass or biovolume alone.