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

The kelp Laminaria digitata grows in the low intertidal region along energetic coastlines exposed to a range of hydrodynamic environments. Macroalgae in the intertidal zone can experience both waves and currents independently, but it is unknown how they influence growth rate. Relative growth rate of the meristematic region and the entire blade of L. digitata were measured to assess the influence of wave and current motion from three hydrodynamic environments; low current and low wave (LCLW), high current and low wave (HCLW) and high wave and low current (HWLC). Alongside hydrodynamic data, seawater nutrient concentrations and temperature were also collected and analysed. Results suggest that differences in L. digitata relative growth rates were not attributed to the seawater nutrient concentrations or temperature but to the hydrodynamic environments. At the high current condition, kelp growth rate of the meristematic region was enhanced by 45% compared to the high wave condition. When including the entire blade growth rate, an average increase of 25% was observed between the high current and high wave condition. Potentially, the division in growth rate observed between the wave and current motion is related to the frequency and magnitude at which the hydrodynamic forces act. These findings highlight the complexity of the hydrodynamic environment and that forces associated with waves and currents may have a significant role on the productivity of kelp.