Contrasting mechanisms of dislodgement and erosion contribute to production of kelp detritus

Abstract

We quantified simultaneously dislodgement and erosion for a dominant kelp species (Ecklonia radiata) over 1 yr, and related both to potential explanatory factors (wave exposure, temperature, and kelp fecundity). Erosion was the largest contributor of detritus, accounting for 80% of annual production. Most erosion occurred as a major pulse in autumn, whereas dislodgement was a minor and constant process throughout the year. Neither erosion nor dislodgement was correlated with water velocity (as often proposed), and this finding contradicts the common assumption that high dislodgement rates during peak wave action account for the bulk of detrital production. Together with low growth, the high erosion rate led to a severe reduction of individual kelp biomass in autumn (from 600 g to 300 g fresh weight kelp−1), reducing drag forces on kelp thalli by ∼ 50%, likely reducing their susceptibility to dislodgement during peak wave action. Instead, a pulse of detrital production coincided with periods of peak kelp fecundity. We propose that sporogenesis weakens the tissue, making E. radiata more susceptible to erosion, and that the ensuing changes in kelp morphology decouple detrital production from the wave‐action forces.