Knots and tangles weaken kelp fronds while increasing drag forces and epifauna on the kelp

In shallow coastal areas, the fronds of long, flexible kelp can become knotted (a single frond tied around itself) and tangled (multiple fronds intertwined) as they move back and forth with ocean waves. We investigated the ecological and physical consequence of being knotted and tangled for the intertidal kelp Egregia menziesii in northern California. Knots increased the hydrodynamic forces on fronds of the kelp Egregia menziesii by 56% and weakened fronds by 18% when pulled in tension. There were more and larger epifauna (including many herbivores) on tangled fronds, which suffered greater damage by grazers than did untangled fronds. Tensile forces required to break herbivore-damaged fronds were 31% lower than forces to break undamaged fronds. Kelp with knotted and tangled fronds were more likely to break than kelp with unknotted, untangled fronds, and knots and tangles occurred most frequently in the autumn, thereby pruning the fronds and reducing the risk of whole kelp being ripped off the shore by large waves during winter storms.