Influence of particle type and faunal activity on mixing of di(2-ethylhexyl)phthalate (DEHP) in natural sediments

Abstract

The influence of faunal activity and particle type on sediment-mixing processes of the particle-bound organic contaminant di(2-ethylhexyl) phthalate (DEHP) were examined over a 48 d experimental period with natural box-cosm sediments. A dual-labelling radiotracer technique using 14 C-DEHP and 51 Cr determined the fate and utilization of DEHP associated with either sediment or phytoplankton particles. Particle-mixing was estimated using a 1-dimensional transient-state biodiffusion model and expressed as Db coefficients. Sediment 51 Cr depth profiles fitted this simple biodiffusion model well, indicating randomized, vertical particle mixing. Particle reworking rates were approximately 9-fold higher in faunated treatments compared to defaunated control sediments, thus quantifying the importance of benthic fauna as agents of physical transport. Biodiffusion rates in faunated box-cosms were significantly higher in phytoplankton compared to sediment 14 C-DEHP particle treatments (p < 0.05). This was attributed to an experimental urchin effect resulting from natural variations in the density of the large macrofaunal urchin Echinocardium cordatum between particle treatments. Sediment-mixing rates were independent of particle type when this urchin effect was removed. Examination of 14 C and 51 Cr relative depth profiles showed the 2 tracers to be coupled in defaunated sediments and decoupled with animals present. This effect was also independent of particle-type, indicating no selective transport associated with the particle-type treatments. The decoupling of tracers in faunated treatments, with 14C-DEHP having consistently higher depth-weighted mean values than the 51 Cr tracer, suggested a decrease in surface 14 C-DEHP concentrations through degradation processes at the sediment-water interface. Sediment-mixing rates increased significantly with increasing total community biomass, indicating that size (biomass) may be the single most important community parameter determining sediment-mixing intensity. Downward particle transport was strongly correlated with E cordatum abundance (p < 0.01), the dominant species in the benthic community in terms of size. E. cordatum densities also correlated strongly with the number of benthic infaunal species present in the box-cosms, with maximum infaunal species numbers occurring at intermediate E. cordata densities. This is in accordance with theories on intermediate disturbance. The 2 14 C-DEHP particle-type treatments had no significant effect on E. cordata body sizes or DEHP body burden. The large urchin E. cordatum clearly dominated sediment-mixing, and did so in a 'non-selective' manner.