Sediment mixed layer as a proxy for benthic ecosystem process and function

Faunal mediated particle and porewater mixing (bioturbation) alters the structure of the surface sediment layer, forming a distinct mixed layer, where the majority of organic matter degradation takes place. Current methods of assessing benthic habitat quality often reference this mixed layer as an indicator of benthic activity. Whilst a great deal of effort has been devoted to linking macro-invertebrate activity to the mixing depth, less attention has been given to defining what the mixing depth represents in terms of ecosystem process and function. Here, in situ sediment profile images are analysed using grey scale intensity analysis to distinguish the mixed zone and relate it to the physicochemical environment in order to determine the biological, chemical and physical variables most influential in its formation. Significant differences were found between biogeochemical conditions within the mixed layer relative to the underlying historic sediment layer. These were attributed to a combination of environmental variables (Fe, Mn, Si, chlorophyll a and NO3-) rather than a single dominant driver of change. Although these findings are consistent across multiple locations, the driver(s) that influence the depth of the mixed layer are site- and season-specific. The mixing depth thus provides a reasonable approximation of benthic ecosystem functioning, but when considering ecosystem process the link between the mixing depth and its driving factors (faunal mixing, food input, environmental conditions) is highly context-dependent. Conclusions on benthic community dynamics and ecosystem process, including assessments of habitat quality, cannot therefore be drawn from estimates of the mixing depth alone.