The influence of hydrodynamics on the growth, erosion and biomechanical properties of Laminaria digitata
Kelp forests are described as one of the most productive marine habitats globally which cover approximately 25% of the world’s coastline supporting a diverse range of flora and fauna. There is also now a growing awareness that macroalgae may be significant contributors to blue carbon as it is estimated that ~90% of carbon fixed by macroalgae is transported by water motion and sequestered in coastal or offshore sediments. Macroalgae such as the kelp Laminaria digitata are found in temperate waters growing in the low intertidal region along energetic coastlines exposed to a range of hydrodynamic environments. The influence of water motion on fundamental processes such as growth, erosion and the mechanical properties of kelp has received some research, however, teasing apart the effect of waves and currents on the biology of kelp is unknown. Here a combination of field and laboratory experiments were conducted to establish the influence of wave and current motion on L. digitata (i) relative growth rate of the meristematic region and the entire blade; (ii) erosion rate from the distal end; (iii) mechanical properties of blade tissue (breaking stress and strain) (iv) and mechanism underlying the associated mechanical properties of L. digitata at three hydrodynamically different regimes: low current and low wave (LCLW), high current and low wave (HCLW) and high wave and low current (HWLC). Results suggest that differences in L. digitata relative growth rates were not attributed to the seawater nutrient concentrations or temperature but to the hydrodynamic environment. 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 ii rate observed between the wave and current motion could be related to the frequency and magnitude at which the hydrodynamic forces act. However, the hydrodynamics appeared to have a lesser effect on the erosion rate (~1 – 1.25 RER d-1 ) observed which was attributed to seasonal changes in seawater nitrate concentrations. Yet estimates of carbon loss per day of L. digitata were driven by the hydrodynamic environment with a significantly higher loss of carbon observed at the high wave condition (~0.17 g C d -1 ). Counteracting for the similarity in erosion rate between hydrodynamic regimes, mechanical properties (breaking stress and strain) were greatest in the high wave and high current condition (~3 MPa and ~0.6 mm respectively) than those from the LCLW condition (~2 MPa and ~0.3 mm). These results suggest that in a benign hydrodynamic environment (LCLW) kelp are not as strong or as extensible as the kelp from more exposed environments and so they do not need to allocate as much energy into structural integrity. In the meristematic region blade thickness was composed of ~50% medulla and ~50% cortex cells however, in the distal end of the blade as the tissue got stronger the cortex cells constituted ~60% of the blade thickness. However, neither medullary or cortical thickness could be attributed to the breaking stress of blade tissue amongst the hydrodynamic regimes. While there was a difference in growth rate of L. digitata between waves or currents, the differences of these two types of hydrodynamics is less clear on erosion, cellular structure and the biomechanics. Results highlight the complexity between the biology of the kelp, hydrodynamic environment and surrounding environmental variables and when considered together they show the adaptability of L. digitata to the environment in which it lives in. This plasticity could be favourable under future predicted climate conditions depending on transition time/transgenerational ability.
Macroalgae, Waves, Currents, Kelp, Hydrodynamics, Erosion
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