The effect of adult cockles, Austrovenus stutchburyi, on sediment transport

Sediment transport and resuspension are essential processes in soft-sediment environments that mediate shellfish population dynamics, water column turbidity, and estuary geomorphology at multiple scales. Adult cockles, Austrovenus stutchburyi (in New Zealand), are key organisms on intertidal sand flats that influence sediment properties, and their removal could impact the stability of the seabed and affect crucial feedback processes in this system. We conducted a field experiment to explore our hypothesis that the loss of adult cockles will increase sediment transport and resuspension non-linearly. Changes in sediment transport regimes will then influence the properties of sediment such as grain size, organic content, and chlorophyll a content. We also expected that the direction and magnitude of this effect would depend on the varying environmental characteristics across the sites, such as hydrodynamic activity and sediment grain size. We excluded adult cockles from 3 x 3 m intertidal plots at 15 sites across a natural adult cockle density gradient (300-3500 ind/m2). After seven months, sediment traps were deployed in exclusion and control plots to measure the sediment transported over two tidal cycles associated with the bedload and water column with differing densities of adult cockles. Adult cockle density was a significant predictor in all models to explain sediment transport and accounted for the largest proportion of variability explained. Relationships between cockle density and sediment transport changed between sites and over time. In these situations, simple linear relationships were not always apparent due to the interacting effects of cockle density, hydrodynamic process, and sediment characteristics. The influence of adult cockles on sediment transport was also affected by the standing stock of microphytobenthos and interacted with the mud content in their surrounding environment. Our study demonstrates the existence of an important feedback between adult cockle density, sediment stability and sediment transport that is mediated by hydrodynamic processes and sediment characteristics.