Storm resilience of subtidal soft-bottom mussel beds: mechanistic insights, threshold quantification and management implications.
Abstract
With the projected escalation of extreme storm events, coastal ecosystems risk undergoing catastrophic shifts and losing essential ecosystem services. Subtidal soft-bottom mussel beds, vital components of these ecosystems, are particularly vulnerable to hydrodynamically-induced dislodgement (i.e., detachment of mussel clumps from the bed), especially during storms. However, the mechanisms underlying the resilience-comprising both resistance and recovery-of these beds to storms remain unclear, despite being essential for informed management. This study addresses this knowledge gap regarding subtidal soft-bottom mussel beds by: (i) quantifying their dislodgement threshold (i.e., the hydrodynamics causing widespread dislodgement of mussel clumps) using novel in situ monitoring methodologies in a representative region, namely the Dutch Wadden Sea; (ii) unveiling the influence of prior life history (here, wave exposure extent) and storm durations on their dislodgement thresholds through a flume study; and (iii) assessing the impacts of repeated storms and prior life histories (here, wave exposure extent and substrate types) on their recovery (i.e. mussel re-aggregation) through mesocosm experiments. Integrated experimental evidence indicates that: (i) hydrodynamic-induced dislodgement is a sudden process characterized by distinct near-bed orbital velocity thresholds, which were identified at our study site to be between 0.45 and 0.50 m s-1; (ii) peak storm intensity, rather than storm duration, primarily drives the dislodgement of subtidal soft-bottom mussel beds, and prior wave exposure extent regulates the dislodgement threshold; (iii) repeated storms do not seem to affect the recovery of these beds following storm-related disturbances when the conditions between storms are conducive to mussel re-aggregation, whereas substrate type significantly impacts recovery.