Enhanced benthic biodiversity and primary productivity with reduced turbidity in a coastal receiving environment

Abstract Sediment loading from land and turbidity in coastal waters has increased because of human activities in coastal catchments and through climate‐related increases in storm intensity and frequency. Here, we investigated changes in coastal seafloor biodiversity and ecosystem function across a suspended sediment concentration gradient in a New Zealand nearshore receiving environment. Photosynthetically active sunlight radiation reaching the seafloor (PAR S ) increased from shoreward to seaward, providing evidence of increasing water clarity with increasing distance away from the major source of sediment to the system, the Wairoa River. Using the aquatic eddy covariance technique, we observed the lowest and highest benthic gross primary productivity, respectively, at sites closest to and furthest from the Wairoa River mouth. The seafloor at the furthest site had the highest sediment chlorophyll a content, highest chlorophyll : phaeophytin ratio, and richest, most abundant, and most diverse infauna and epifauna. Sediment and faunal data from 12 additional ancillary sites provided context for the four focal sites where we measured functions. Net ecosystem metabolism represents the balance of daily gross primary production and community respiration (R), with negative values indicating net heterotrophy (i.e., gross primary production < |R|). The most diverse and productive site away from the Wairoa River mouth had the most heterotrophic net ecosystem metabolism value, indicating that it was the largest net emitter of CO 2 . Our study highlights the importance of considering biodiversity and ecosystem function co‐benefits when evaluating the carbon capture potential of coastal ecosystems.