Flow velocity and nutrient uptake in marine canopies
Centre for Marine Ecosystems Research / School of Science
Marine canopies can reduce velocities within the canopy, promoting the settling of particles, increasing the residence time of dissolved nutrients and providing a refuge for biota. Increased velocities have also been shown to enhance nutrient uptake, which has been attributed to a decrease in the thickness of the diffusive boundary layer adjacent to plant cells. In this paper, we attempted to unify the varying effects of velocity on nutrient uptake in canopies into a simple framework. We analyzed the interaction between flow conditions and nutrient uptake in marine canopies (coral, macroalgal and seagrass) by synthesizing data from 13 previously published studies conducted over the past 20 yr. We present a conceptual framework for the relationship between canopy uptake of nutrient pulses and flow velocities using a canopy control volume approach combined with analysis of non-dimensional Damköhler and Reynolds numbers, identify 4 possible regimes and explore velocity thresholds that trigger regime transitions. All data showed increasing nutrient uptake with increasing flow velocities, though many data sets showed saturation of uptake rates. The relationship between flow velocity and nutrient uptake varied with canopy species, nutrient species and canopy density. We found evidence for 2 of the proposed regimes; however, the remaining 2 regimes, though suggested in the literature, were not evidenced by the data. The analysis highlighted that our understanding of canopy function under disturbed conditions is currently based on experimental data from ecosystems under a relatively narrow range of flow and canopy conditions that are unlikely to be stressful to the organism. We are therefore unlikely to fully understand the response of canopies to strong perturbations of the external hydrodynamics.