Scaling and structural properties of juvenile bull kelp (Nereocystis luetkeana) Data

Bull kelp (Nereocystis luetkeana), the only canopy-forming kelp in the
Salish Sea, provides primary production in the nearshore subtidal
environment and serves as important habitat for economically and
ecologically important species. An annual species, each year juvenile bull
kelp sporophytes must grow from the hydrodynamically more benign benthos
to the water column, where they experience substantial drag at the
surface. Because of the differences in morphology and ecology across life
stages, and the fact that previous work has focused mainly on adult bull
kelp, we tested whether morphology and structural properties change with
stipe length, investigating scaling of both juvenile (stipe length
<40 cm) and mature (stipe length > 40cm) kelp, and testing
how juvenile stipes fail. Juvenile bull kelp grow proportionally
(isometric growth) when young, but lengthen more quickly than would be
predicted by bulb size (negative allometry) at maturity. Based on our
data, the predicted breakpoint between isometric and allometric growth
occurred at about 33 cm, likely ~ one to two weeks of growth. Cross
sectional area of the stipe, Force to failure, Work to failure, and
stiffness (Young’s Modulus) all grow more slowly than would be predicted
based on length, while Maximum Stress and Toughness increase more quickly
than predicted. There is no change in extensibility over the size range we
tested, suggesting that this material property does not change with stipe
length. The differences in biomechanics between juvenile and adult kelp
are likely a response to the varied hydrodynamic environments experienced
during the annual lifecycle, which highlights the importance of studying
organisms across life stages.