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.