Field Studies

Because of their morphological differences, the mechanical behavior in situ of D. antarctica and D. willana can be expected to differ. The effect of the buoyancy of the blade can be gauged by examining the simultaneous response of D. antarctica and D. willana to waves. Field experiments studying D. antarctica and D. willana under natural conditions were conducted at St. Clair, a suburban beach near Dunedin, during the period January 18 to 28, 2000 [26]. The sampling all took place at St. Clair seawall. This site is characterized by a rocky shoaling platform backed by a seawall. The beach boulders were in the range of 0.2 to 0.6 m in diameter. It is not directly exposed to open ocean surf, and waves occasionally broke directly in this region; more often, the waves broke slightly offshore and then would rush in as a bore. A local D. antarctica population was located some 10 m offshore from the site of the experiments, whereas D. willana did not occur there.

Samples of D. antarctica and D. willana of intermediate morphology were taken from Lawyers Head, a rocky outcrop about 3 km away, using a chisel to remove the thalli from the substratum. The harvested individuals were then mounted in small concrete blocks, which were then attached to a region of flat substratum using eight self-fastening metal bolts (dynabolts) and four webbing belts with ratchet locks. Equipment used included three-dimensional accelerometry (Figure 3.5) and wave gauges (see [26] for methodological details). The tidal range during the experiments was 2 m.

The accelerometers were calibrated before and after each experiment. This was necessary because the long cables (greater than 40 m) affected nominal factory calibration. The wave gauge data can only be considered representative of wave height, and the arrival time of the waves depended on the relative position to the plants. The wave gauge was guyed to dynabolts to hold it securely in position. The wave gauge data were logged using a TattletaleĀ® logger (Onset Computer Corporation) running at 32 Hz.

FIGURE 3.5 A three-dimensional accelerometer was mounted within a cut section in the palm of the Durvillaea blade. A second accelerometer was attached at the distal end of the lamina.
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