Water velocity regulates macro‐consumer herbivory on the benthic macrophyte Podostemum ceratophyllum Michx.

Abstract

Macrophytes influence aquatic ecosystems by increasing habitat complexity and providing trophic resources for aquatic fauna. While herbivory on freshwater macrophytes is widely documented in lakes and low‐velocity riverine habitats, the influence of herbivory on macrophytes in higher‐velocity habitats has rarely been examined. We investigated the hypothesis that water velocity can influence consumption rates of the submerged macrophyte, Podostemum ceratophyllum, an angiosperm that grows attached to stable substrates in high water velocity riverine habitats throughout eastern North America. Known macro‐consumers of Podostemum include crayfishes, turtles, and waterfowl. We estimated grazing pressure by conducting short‐term (up to 77‐day) transplant and consumer‐exclosure experiments, and quantified changes in Podostemum stem length when the plant was exposed to low (<0.5\xa0m/s) and higher velocities in two Piedmont rivers in Georgia (USA). Podostemum transplanted into low‐velocity habitats rapidly lost stem length unless macro‐consumers were excluded from accessing the plant. In contrast, Podostemum transplanted into high‐velocity habitats showed little change or gained stem length. We estimated that 85% (67–98%; 95% credible interval) of the daily stem growth (0.026\xa0cm cm⁻¹\xa0day⁻¹) in Podostemum was consumed during a 77‐day paired consumer‐access versus exclosure experiment conducted in mean water velocities of 0.35–0.5\xa0m/s. We also found a positive relation (R²\xa0=\xa00.58) between Podostemum biomass and water velocity (ranging between c. 0.1 and 1.3\xa0m/s) in benthic samples collected over a 2‐month period. We conclude that high water velocity reduces herbivory on Podostemum, and that water velocity can thus control the accrual of benthic plant biomass and the movement of plant‐derived materials through benthic food webs. Our research has implications for estimating resource storage and flux in lotic food webs and illuminates a mechanism by which flow regulation and management may affect basal resources in rivers.