Spatial Distributions of Grazing Activity and Microphytobenthos Reveal Scale-Dependent Relationships Across a Sedimentary Gradient

The density, spatial structure and functional roles of macrofaunal and microphytobenthic (MPB) communities change across sedimentary gradients. Grazing by macrofauna can impose considerable top-down control on MPB biomass at the scale of the animal's feeding ambit (cm scale), yet how relationships between deposit feeders and MPB scale up across such transitional environments (10's m scale) is poorly understood. We determined the relationship between sediment chlorophyll-a concentration (a proxy of MPB biomass), distance to feeding traces (a proxy of recent deposit feeding activity) made by the tellinid bivalve Macomona liliana (at cm scale) and macrofaunal densities (at 10's m scale) across a sediment mud content gradient. Correlative relationships, estimated by generalised least-squares regression, between recent deposit feeding activity and MPB biomass were scale dependent and significant only at the site (10's m) scale. MPB biomass declined by 28 % as coverage of feeding traces increased from 2 to 28 %, with feeding trace area contributing significantly to variation in chlorophyll-a (std. coefficient = -0.24, p = 0.01). However, the interaction term between the density of the suspension-feeding clam Austrovenus stutchburyi and sediment mud content explained a larger amount of the variability (std. coefficient = 0.72, p < 0.001). Our results demonstrate that significant effects on MPB biomass can emerge across large, spatially heterogeneous areas of tidal flat, despite appearing stochastic at small scales. They also highlight the need to consider interactions between MPB and macrofauna across abiotic gradients and the potential roles of non-deposit feeding taxa.