Pneumatocysts provide buoyancy with minimal effect on drag for kelp in wave-driven flow

Abstract

Many seaweeds have buoyant gas-filled bladders (pneumatocysts) that hold fronds upright in the water column and enhance their access to light for photosynthesis. However, ambient water currents bend flexible seaweeds, pushing fronds closer to the substratum where light is lower, so the hydrodynamic drag on pneumatocysts may counteract their buoyancy in flowing water. The effects of pneumatocysts on frond hydrodynamic drag were investigated in this study, as well as how the positions of pneumatocysts along fronds affect their motion and height in wave-driven water flow. The kelp Egregia menziesii was used as a model organism because it is abundant on wave-swept rocky shores and because our field surveys revealed that this species shows great variation in pneumatocyst size, number, and location on fronds. In laboratory towing-tank studies, it was found that drag on pneumatocysts was reduced when they were bent over by flowing water. The drag due to pneumatocysts was small compared to the drag on a whole frond. At flow speeds up to 0.58ms−1, the buoyant force exerted by a pneumatocyst was greater than the drag it experienced. In wave-tank experiments using models of fronds with pneumatocysts at different positions, the pneumatocysts were most effective at lifting fronds high in the water column when they were located at the distal tips of the fronds, both in small and large waves. However, if fronds had pneumatocysts that were not at the tip, an increase in the peak velocities of waves led to an increase in the heights of the fronds in the water column. In the field, pneumatocysts did not affect the back-and-forth horizontal motion of E. menziesii exposed to waves, but fronds with pneumatocysts were higher in the water column than fronds with no pneumatocysts, even when the number of pneumatocysts on a frond was low. Our results indicate that pneumatocysts can exhibit great variability in size, number, and location with only a small effect on hydrodynamic forces on a kelp, that pneumatocysts at frond tips are most effective at holding kelp high in the water column, but that only a few pneumatocysts at any location along a frond can enhance the frond's height in waves.