Abstract
The extreme habitat of the wave-swept intertidal requires special biomechanical adaptation of macroalgae in order to survive. This study tried to elucidate some of the mechanically relevant aspects of the functional and morphological properties of Durvillaea antarctica, the largest seaweed on this position on the shore. Frequently, D. willana, which is sympatric in New Zealand, was also tested for comparison. The experimental work was focused on the stipe as connective element between the rocky substratum and the flexible blade. Bending tests of whole stipes yielded an average bending modulus of elasticity of approximately 10 MNm-2. There were clear morphological correlations between blade length and stipe dimensions in D. willana, indicating that the former is adapted to drag-induced bending. This was in contrast to D. antarctica, which seems to be optimised to a combination of bending, torsion , and tensional loads. A mathematical model was introduced for the examination of large deflection bending of stipes with tapered morphology. Experiments in a flume revealed the effects of different blade morphologies on drag. Test velocities ranged from 0.5 to 2.8 ms-1 and drag forces of up to 300 N were recorded. The ability to reconfigure and thus reduce the amount of drag was comparable to other flexible seaweeds at a Vogel number of -0.6 to -1.1. Individuals with a wave-exposed morphology had a significantly higher Vogel number than those with intermediate or wave-sheltered morphology. Due to reconfiguration, drag forces increased almost linearly with increasing velocity and correlated highly with the overall thallus length. Tension forces of up to 300 N were recorded in the field on stipes of transplanted Durvillaea. Considerably higher forces of up to 800 N were measured in the first in situ experiments on Durvillaea. Accompanying accelerations at the stipe apex were as high as 50 ms-2. An informal field survey on the mortality and recruitment of a standing population of D. antarctica supported previous work in NZ, but rejected the notion of a 'weedy' strategy attributed to this species in Chile. The predominant mode of failure for D. antarctica was the complete dislodgement of the holdfast from the rocky substratum, whereas D. willana more often failed at the stipe. The findings were related to other work on seaweeds and also to studies on terrestrial plants.