Effects of a small-bladed macroalgal canopy on benthic boundary layer dynamics: implications for nutrient transport

Abstract

Field and laboratory velocity profiles were used to quantify boundary layer dynamics within communities of a small (< 0.2 m tall), dense canopy-forming seaweed Adamsiella chauvinii (Rhodophyta) in a soft-sediment habitat and to examine the role of hydrodynamics in modulating nutrient supply. At the 'canopy scale' there was a mixing layer at the fluid−canopy interface where turbulent kinetic energy was greatest, potentially enhancing nutrient uptake in this region. In the lower half of the canopy, a drag-dominated area of very low water velocity (< 0.01 m s −1 ) occurred. Spectral analysis revealed a reduction in energy within the canopy of around 1 Hz. The hydrodynamic parameters obtained from flume measurements were in good agreement with those recorded in the field. To understand the implications of the hydrodynamic environment on nutrient uptake, a flushing ratio was developed that compares the time for macroalgae to remove all nutrients from the canopy volume relative to the timescale for flushing. Results suggest that when nutrient demand is low, the canopy is well flushed and not mass-transfer limited. However when nutrient demand is high, the canopy can deplete nutrients more quickly than they can be replenished by ambient flows and are potentially mass-transfer limited.

Seasonal differences in the effects of oscillatory and uni‐directional flow on the growth and nitrate‐uptake rates of juvenile Laminaria digitata (Phaeophyceae)

“…Water velocities within the culture tanks could not be measured by standard methods such as Acoustic Doppler Velocimeters (Kregting et al. ). Thus, flow speeds were calculated by adjusting the voltage of the power supplies and measuring the distance the arms moved over a period of time.…”

“…In such current‐dominated locations it is not uncommon for macroalgae to experience flow rates <6 cm · s −1 for a defined period of time during a tidal cycle (Kregting et al. , ). Low flow rates or stagnant conditions may also occur in rock pools during slack tides or within dense canopy‐forming macroalgae (Kregting et al.…”

“…Low flow rates or stagnant conditions may also occur in rock pools during slack tides or within dense canopy‐forming macroalgae (Kregting et al. ) resulting in macroalgae frequently experiencing mass‐transfer limitation on timescales of minutes to hours.…”

Effects of intertidal seagrass habitat fragmentation on turbulent diffusion and retention time of solutes

“…If t R has larger values than t Up , then physical limitation on nutrient uptake is expected (Kregting et al, 2011). The t Up was estimated using averaged values of Z. noltii leaf biomass, assuming that both ammonium and phosphate follows first order uptake kinetics (Pérez-Lloréns and Niell, 1995;Alexandre et al, 2010 ) and phosphate (10 min) were higher than t R within Z. noltii patch edges (2.6-5.6 min), suggesting that nutrient uptake is not physically limited by solutes renewal at the leading edge of these patches.…”

Slow‐flow habitats as refugia for coastal calcifiers from ocean acidification

“…), and under a canopy of the small red seaweed A. chauvinii , flow is reduced by 95% to 1 cm · s −1 (Kregting et al. ). At the surface of a canopy of the articulated coralline alga Arthrocardia corymbosa , velocities of ~3 cm · s −1 were recorded which were ~60% slower than that of the mainstream seawater velocity (8 cm · s −1 ); when a canopy of brown seaweeds was added above the A. corymbosa canopy, the velocity at the coralline canopy surface was reduced to 1.2 cm · s −1 (i.e., by 85%).…”