Buoyancy and Field Studies

In D. antarctica, the whole blade is buoyant, whereas many other large brown seaweeds have only distinct floating organs, e.g., pneumatocysts. The recorded buoyancy forces of up to 150 N are high, e.g., 15 times higher than the buoyancy forces recorded for an 8 m long individual of Nereocystis luetkeana [37]. It can therefore be expected that the buoyancy of the blade of D. antarctica has a considerable influence on the overall mechanical behavior of this species in the surf zone.

The finding that the acceleration response of D. antarctica was not particularly different on the palm compared with the frond was initially surprising as one might expect the blades to respond more strongly because buoyancy holds them up high during the passage of breaking waves. This suggests that the drag force imparted to the blades was transferred along the stipe and made itself apparent even near the holdfast. Contemporary load cell data quantifying the actual load transferred to the substrate connection supports this [26].

The frond response of the D. willana sample was probably the best track of the water elevation, clearly marking the passage of the wave crest and the gradual decay (recall that the wave gauge elevation did not exactly reflect the velocity of a passing wave). It is unclear why the D. willana structure did not pass the response in accelerometer on to the stipe. A possible explanation exists in the load driving realignment of the frond that was not constrained by buoyancy.

An alternate viewpoint for the same data is provided by examining the domain of along-blade vs. across-blade accelerations (Figure 3.16). Stevens et al. [26] studied all three axes combinations (x-y, x-z, and y-z); however, here we consider only x-y for simplicity. Clearly, the D. willana stipe palm is more constrained than the D. antarctica stipe (Figure 3.16C vs. 3.16A). The scatter in the data points of D. antarctica indicates that the blade has many degrees of freedom to attain a certain position in the water column (Figure 3.16B). In contrast, the acceleration data of D. willana shows less scatter (Figure 3.16D). This might be indicative of a higher constraint of movement of the blade in the water column. Thus, the positive buoyancy of D. antarctica increases the "movability" of the blade. This could increase rates of nutrient uptake as well as lower the fatigue strains due to repetitive bending in a preferred direction predefined by the predominant wave direction.

FIGURE 3.16 Comparison of accelerometer response (in units of g, acceleration due to gravity, where 1 g = 9.81 m s-2) for D. antarctica (A) palm and (B) frond and D. willana (C) palm and (D) frond. The results show along-frond acceleration (X) vs. across-frond acceleration (Y). Around 170 s of data subsectioned from a 2000s time series are shown.

FIGURE 3.16 Comparison of accelerometer response (in units of g, acceleration due to gravity, where 1 g = 9.81 m s-2) for D. antarctica (A) palm and (B) frond and D. willana (C) palm and (D) frond. The results show along-frond acceleration (X) vs. across-frond acceleration (Y). Around 170 s of data subsectioned from a 2000s time series are shown.

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