The effect of thallus size, life stage, aggregation, wave exposure and substratum conditions on the forces required to break or dislodge the small kelp Ecklonia radiata

Abstract

Canopy removal by storms is a primary cause of mortality for the small kelp Ecklonia radiata in temperate Australasia. We simulated hydrodynamic drag from storms with in situ pull-tests to determine whether thallus size, life stage, aggregation, wave exposure and substratum affect the canopy removal process. A total of 466 individuals were pulled off 20 rocky reefs at 8–10 m depth in southwestern Australia. The majority (71%) of thalli dislodged at the rock implying that the canopy removal process in southwestern Australia is a substratum-controlled process. Dislodgment bared clean substratum where re-invasion by propagules or encroachment would be necessary to fill up the gaps. Maximum break forces (150–250 N) were found for large late stage kelps and kelp aggregates from wave exposed sandstone and granite reefs, and minimum values (25–100 N) for small juveniles and solitary kelps from protected limestone reefs. By applying size and break force data to the drag equation, water velocities required to break or dislodge E. radiata were calculated to 2–5 ms-1 for large kelps. These velocities are frequently encountered in waveexposed shallow subtidal habitats, suggesting that thallus size is limited by the hydrodynamic environment.