Monterey Bay Aquarium Research Institute
Marine Botany

Mazzaella flaccida

Another Unfinished Chapter:

Forces of Drag and Acceleration on Mazzaella or Iridaea flaccida

In order to understand how the mechanical properties of the thallus are affected by wave forces, it is necessary to determine how much force a thallus will experience. The thallus experiences drag, acceleration, and lift, which all exert forces on the plant which may rip it of the rocks or tear its thallus. Using a flow tank I measured drag on the thallus, but recent research by the Denny lab here at Hopkins indicates that this is not the most important force the algae experiences. The acceleration reaction has more influence on species distributions, and only by looking at both drag and acceleration can one form an accurate picture of the forces experienced by an algae. Unfortunately, I could only measure drag, not acceleration, with the equipment available.

The drag coefficient,, can be calculated from the measured force of drag, the algae's projected area A, the water velocity u, and the density of the medium (for seawater = 1024 kg/ m2).

This equation works for a nondeforming, uniform body. Of course, Iridaea is too smart to keep displaying so much area for force to act on, and deforms in flow. The Vogel number (E) is a good measure of how well an alga has reconfigured its shape to reduce drag.

A smart and reconforming alga like Iridaea has a negative Vogel number. The force of drag on Iridaea increases with less than the square of velocity.

Using a drag coefficient we can determine what sort of force an alga will suffer when the water around it is moving at a certain speed. Tests of the thallus material and holdfast strength will indicate what sort of force an alga can survive. When we combine this knowledge with an idea of the size (and thus speed) and frequency of the largest waves arriving at a certain coastline, we can predict how often our species will likely be ripped off the rocks or torn to pieces by the surf. This information about survivorship will help us predict community composition and structure in the surf zone, as well as optimal thallus size. An alga with a high drag coefficient (velocities produce large forces on the plant) and high Vogel number (a very inflexible alga) will not do very well in the intertidal and may not occur in the surf zone unless it is made of unusually tough material and has an unusually strong holdfast.

The force of lift due to differential pressure in front of, above, and behind the alga also acts to pull an alga off the rocks, as does the acceleration reaction from the constant accelerations and decelerations from waves passing over algae living in the surf zone. We did not measure these forces, but the acceleration reaction seems to play an important role in defining the force an alga experiences (Gaylord et al., 1994). An algal thallus is often intricate and traps much water against its surface. This water must all be accelerated when the force of a wave passes over the thallus, and this large volume of added mass gives algae high inertia coefficients. This ensures that the plant experiences large accelerational forces despite a small thallus. Acceleration is much harder to measure than drag, so the impact of acceleration on algae is still being investigated.

Research here at Hopkins Marine Station by the Denny Lab (Gaylord et al., 1994) showed that the optimal size predicted for Iridaea flaccida matches the mean observed size only when both drag and acceleration are taken into account. Looking only at drag, predicted size is 7-11 times that actually observed on wave exposed shore (A optimal = 0.0595 - 0.0915 m2 and mean A = .0083) and 40-70 times the observed size on protected shores so Òdrag alone is not a primary factor in determining algal sizeÓ. Taking acceleration into account, A optimum for I. flaccida is .0072-.0078 m2, close to the mean size observed of A = .0083 m2. This assumed a = 250m/s2, a value hopefully appropriate for an exposed site; acceleration has not yet been measured. They found Vogel numbers of Iridaea flaccida, E = -.76 at .5-4 m/s and -1.01 at .5-3 m/s. Their average measured CD was .22 in 15 m/s water.

For flow speeds of 80 cm/ s, my data on I flaccida, I found a drag coefficient for the nonreproductive blade of .138. The reproductive blade had a coefficient of .191, not surprising considering that it is tougher because it is a winter blade while theirs was a summer blade, so it deforms less easily in flow to present minimum surface area to the force). In addition, it has surface inconsistencies (cystocarps) which would disturb flow. These values are both somewhat smaller than that measured by the Denny lab. Although it would make sense that the toughening of the blade in the winter would increase CD, my lower values of CD are not hugely different from the values they measured, so the difference may not be significant. My Vogel number, E, for the nonreproductive plant was -.135, while the reproductive specimen had an E of -.413. These values are much less negative than the values found for the Denny lab specimens. Once again some of this worsening of index could be due to reduced ability to conform to flow because of winter blade toughening; however, the degree of decrease is very surprising. Just compare these figures with those of other materials. A plastic sheet, which conforms very well to flow and would probably have properties similar to Iridaea's thallus, has an E of -1.05, while a nonconforming whiffle ball has an E of -.02 (Gaylord et al, 1994). Thus the thallus would appear in my tests to have properties closer to that of a whiffle ball than a plastic sheet, which does not make sense. Moreover, in this test I had a more negative (better) E for the reproductive specimen, while we would expect it to be less conforming due to winter toughening of the blade.

Although my data proved inconclusive, the Denny lab's work would indicates that drag is not the controlling factor in species distribution anyways. I would be better served in further investigations by looking at the acceleration reaction.

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