Depth and vertical hydrodynamics constrain the size structure of a lowland streambed community

Abundance–body mass (N–M) relationships are prominent macroecological patterns and provide an integrated measurement of the structure and energy flow through natural communities. However, little is known about how N–M relationships are constrained by local environmental conditions. Here, we quantify how sediment depth and direction of surface–groundwater exchange (vertical hydrodynamics), two major drivers of the streambed ecology, determine N–M scaling in a sandy lowland European stream. Streambed assemblages included flagellates, ciliates, meiofauna and macroinvertebrates, and spanned five orders of magnitude in body mass. We detected a significant interaction of body mass with depth and vertical hydrodynamics with a sharp reduction in N–M slopes in the hyporheic zone and under upwelling conditions. These results revealed that streambed assemblages become more size-structured as environmental constraints increase with direct implications for the metabolic capacity and functioning of the system.