Effects of algal enrichment and salinity on sediment particle reworking activity and associated nutrient generation mediated by the intertidal polychaete Hediste diversicolor

The activities of marine benthic invertebrates link to many ecosystem properties and are often used as an alternative to the direct measurement of ecosystem functioning, such as nutrient cycling. However, links between bioturbation metrics and functional classifications of organisms based on bioturbation mode are rarely tested under a range of circumstances and are commonly assumed to be universally applicable. In this study we examine the effects of particle reworking by the polychaete Hediste diversicolor on the mediation of nutrient concentrations for a range of salinity and sediment algal enrichment levels. Sediment particle reworking increased in the presence of moderate algal enrichment but was reduced at higher levels of algal enrichment and was not affected by salinity. Nutrient concentrations in the water column, however, responded to the independent effects of salinity and algal enrichment. This study demonstrates that variation in particle reworking and mixing activity of H. diversicolor that relate to changes in environmental condition can be sufficient to alter species contributions to ecosystem functioning without altering the mechanistic basis of mediation. Our findings indicate that faunal processes that lead to changes in ecosystem functioning can be differentially expressed in a dynamic environment, and call for more careful consideration in the design and analysis of experiments that attempt to link the functional role of species to ecosystem properties.