Blades of bull kelp balance drag reduction and sunlight exposure in different flow environments via changes in width and flatness.

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Bull kelp (Nereocystis luetkeana) is a marine macroalga that grows in coastal waters between Alaska and central California. It resembles a vine-like plant with a long, thin stipe (stem-like structure) up to 30 meters long, anchored into the sea floor by a holdfast (root-like structure). At the stipe’s other end is a gas-filled float that holds 30-60 photosynthetic blades, each reaching up to 4 meters in length, near the water’s surface. Many marine macroalgae must withstand significant hydrodynamic forces imposed by tidal currents, waves and surface chop. If the mechanical force exerted by fluid flow exceeds the breaking strength of a kelp stipe or holdfast, the macroalga can break away and potentially die. Thus adaptations managing drag exerted on the kelp can be advantageous.

One example is the blade shape of bull kelp, which varies between habitats with differing degrees of flow. Bull kelp exposed to rapidly moving water grow flat, narrow blades compared to those found in calmer, protected sites that grow undulate (ruffled), wide blades. In faster flowing waters, narrow, flat blades flutter at smaller amplitudes allowing them to clump into more streamlined bundles. Experiments demonstrate that both the narrowness and flatness of a blade contributes to how well the blades can clump together, and that blade shape changes in response to mechanical stress (the changes are plastic). Both the cabbage-like kelp Saccharina sessile and the giant kelp Macrocystis pyrifera exhibit similar differences in blade shape between fast and slow flow environments, suggesting this difference in blade shape is a common way to manage drag.

If narrowness and flatness reduce drag, why would any kelp have wide, ruffled blades? Streamlining of bundles comes at the cost of self-shading, which leads to a reduction in photosynthetic rate. By moving at greater and more varied amplitudes, wide and ruffled blades prevent self-shading by spreading themselves apart and increasing their exposure to sunlight. Bull kelp experience a trade off between minimizing drag by streamlining and maximizing photosynthesis by spreading out. Since drag increases with flow speed, kelp in habitats exposed to powerful waves and tides invest in minimizing drag at the cost of photosynthesis while kelp in more protected habitats need not invest so heavily in drag reduction and can increase solar exposure.

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“The undulate blades of N. luetkeana from sites with low flow remain spread out and flutter erratically in moving water, thereby not only enhancing interception of light, but also increasing drag. In contrast, strap-like blades of kelp from habitats with rapid flow collapse into streamlined bundles and flutter at low amplitude in flowing water, thus reducing both drag and interception of light. Transplant experiments in the field revealed that shape of the blade in N. luetkeana is a plastic trait. Laboratory experiments in which growing blades from different sites were subjected to tensile forces that mimicked the hydrodynamic drag experienced by blades in different flow regimes showed that change in shape is induced by mechanical stress.” (Koehl et al. 2008:834)

“Field measurements showed that when the strap-like blades of a N. luetkeana at an exposed site collapsed into a streamlined bundle and fluttered at low amplitude, the flux of photosynthetically active radiation (PAR) reaching blades within the clump was reduced by about 70% compared with the PAR flux encountered by the blade at the top of the bundle…In contrast, field measurements of light under the ruffled blades of N. luetkeana at a protected site showed that the PAR flux to the lower blades in a clump was reduced by only about 16% as the undulate blades flapped erratically and remained spread out in flowing water. Thus, N. luetkeana illustrate that a ruffled blade shape can reduce self-shading for macroalgae with multiple blades in flowing water.” (Koehl et al. 2008:837)

Journal article
How kelp produce blade shapes suited to different flow regimes: A new wrinkleIntegrative and Comparative BiologyJuly 10, 2008
Koehl MAR; Silk WK; Liang H; Mahadevan L

“Those [Saccharina sessile] from sheltered localities, which are broad, bullate blades, experience greater drag at a given water velocity than ones from localities more exposed to the action of waves, which have smooth, deeply dissected blades. All specimens rearranged their blades as water velocity increased, resulting in a decrease in effective drag at higher water speeds, but individuals with smooth, dissected blades assumed a more compact shape at high current speeds and thus reduced their effective drag over that of broad-bladed individuals at the same speed.” (Armstrong 1989:115)

Journal article
The behavior in flow of the morphologically variable seaweed Hedophyllum sessile (C. Ag.) SetchellHydrobiologiaJanuary 1, 1989
Armstrong SL

“Measurements of up-and-down motion of individual blades in the flow tank indicate that wide, undulate blades flap with greater amplitude and larger variation in amplitude than narrow, flat blades (Fig. 5).” (Koehl and Alberte 1988:441)

Journal article
Flow, flapping, and photosynthesis of Nereocystis luetkeana: a functional consequences of undulate and flat blade morphologyMarine BiologyJanuary 1, 1988
Koehl MAR; Alberte RS

Journal article
Morphological variations of Macrocystis pyrifera in the Falkland Islands in relation to environment and seasonMarine BiologyJanuary 1, 1989
van Tussenbroek BI

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Bull kelpNereocystis luetkeanaSpecies

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