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

Abstract

The influence of oscillatory versus unidirectional flow on the growth and nitrate‐uptake rates of juvenile kelp, Laminaria digitata, was determined seasonally in experimental treatments that simulated as closely as possible natural environmental conditions. In winter, regardless of flow condition (oscillatory and unidirectional) or water velocity, no influence of water motion was observed on the growth rate of L. digitata. In summer, when ambient nitrate concentrations were low, increased water motion enhanced macroalgal growth, which is assumed to be related to an increase in the rate of supply of nutrients to the blade surface. Nitrate‐uptake rates were significantly influenced by water motion and season. Lowest nitrate‐uptake rates were observed for velocities <5 cm · s−1 and nitrate‐uptake rates increased by 20%–50% under oscillatory motion compared to unidirectional flow at the same average speed. These data further suggested that the diffusion boundary layer played a significant role in influencing nitrate‐uptake rates. However, while increased nitrate‐uptake in oscillatory flow was clear, this was not reflected in growth rates and further work is required to understand the disconnection of nitrate‐uptake and growth by L. digitata in oscillatory flow. The data obtained support those from related field‐based studies, which suggest that in summer, when insufficient nitrogen is available in the water to saturate metabolic demand, the growth rate of kelps will be influenced by water motion restricting mass transfer of nitrogen.

Influence of waves and currents on the growth rate of the kelp Laminaria digitata (Phaeophyceae)

“…For some macroalgal species, the mass‐transfer limitation of dissolved nutrients has been noted in the subtidal environment under low unidirectional flow velocities (<0.06 m·s −1 ), as the movement of molecules to and from the blade surface is limited by flow (Kregting et al. , ). As Laminaria digitata is an intertidal species, it would be expected that mass‐transfer limitation could occur at low tides, which all sites would experience, especially during calm weather conditions.…”

Variability in iodine in temperate seaweeds and iodine accumulation kinetics of Fucus vesiculosus and Laminaria digitata (Phaeophyceae, Ochrophyta)

“…The seawater used was 0.8 lm filtered and tyndallized (heated to 89°C twice on two consecutive days) and had a salinity of 35. Over the course of the accumulation experiments, seawater and algal samples were continuously stirred using a magnetic stirrer to prevent boundary layer formation (Hurd et al 2014); a boundary layer can act as a diffusive barrier and thus may affect the acquisition of inorganic ions (Kregting et al 2015). Following an exposure of 0, 60, 120, 240, and 360 min to the external I À concentrations in seawater [I À ] sw defined, algal samples were sequentially removed, dry-blotted to remove excess water, frozen (À20°C) and freeze-dried prior to iodine quantitation.…”

“…); a boundary layer can act as a diffusive barrier and thus may affect the acquisition of inorganic ions (Kregting et al. ). Following an exposure of 0, 60, 120, 240, and 360 min to the external I − concentrations in seawater [I − ] sw defined, algal samples were sequentially removed, dry‐blotted to remove excess water, frozen (−20°C) and freeze‐dried prior to iodine quantitation.…”

Vertical mixing in coastal canopies

“…Increased nutrient uptake has been observed in seagrass meadows under oscillatory flow, relative to that under unidirectional flow (Thomas and Cornelisen 2003). It is plausible that this is related to the higher nutrient residence time in the canopy under oscillatory conditions.…”

“…For example, the in-canopy velocity (relative to the above-canopy velocity) is significantly enhanced under oscillatory flow conditions compared with the corresponding unidirectional flow (Lowe et al 2005a). Surface waves enhance the rate of nutrient uptake by submerged canopies such as seagrasses (Thomas and Cornelisen 2003) and coral (Falter et al 2004;Reidenbach et al 2007) when compared with a unidirectional current of comparable magnitude. Thus, it can be inferred that the rate of mass transfer across the top of the canopy will vary greatly between unidirectional and oscillatory flows.…”