The role of patch size in ecosystem engineering capacity: a case study of aquatic vegetation

Submerged aquatic plants are ecosystem engineers that are able to modify their habitat. However, the role of patch size in the engineering capacity of aquatic plants has not yet been fully investigated, while it could be essential for elucidating the consequences of plant presence. Our objectives were to investigate the effects of patch size on plant-flow-sediment interactions in lotic ecosystems and to determine whether these effects differed according to environmental characteristics. We performed in situ measurements of velocity and grain size along natural patches of increasing length (L) at two sites presenting different flow and sediment characteristics. Our results indicated that a minimum patch size was needed to induce in-patch reduction of the time averaged velocity component in the flow direction (i.e. streamwise velocity) and fine sediment accumulation. Streamwise velocity decreased linearly with L independently of the site conditions. The sediment texture was instead dependent on site conditions: for the site characterized by higher velocity and coarser sediment, the sediment grain size exponentially decreased with L, reaching a minimum value at L ≥ 1.0 m, while for the site characterized by lower velocity and finer sediment, it reached a minimum value already at L > 0.3 m. This study demonstrated that a minimal patch size is required to trigger the ecosystem engineering capacity of aquatic plant patches in lotic environments and that this capacity increases with patch length. Small patches induce little to no modification of the physical habitat, with possible negative feedbacks for plants. With increasing patch size, the habitat modifications induced by plants become more important, potentially triggering positive feedbacks for plants.