Tidal range and recovery from the impacts of mechanical beach grooming

Mechanical grooming to remove litter and wrack from sandy beaches reduces strandline biodiversity. The impact of tidal range on recovery rates of strandline ecosystems after grooming has not been examined to date, even though tidal range is known to affect the spatial and temporal patterns of seaweed. We compared taxon richness of macroinvertebrates that occur all year round at 104 sites on two coastlines at similar latitudes in Northern Europe that have pronounced differences in tidal range. Macroinvertebrate taxon richness was positively correlated with algae depth on both groomed and ungroomed beaches but was lower on groomed beaches. This was the case even in the off season despite wrack depths returning to similar levels found on ungroomed beaches. These impacts of grooming which extend into the winter offseason where found to be higher on beaches with a lower tidal range. We suggest this is likely to be because in areas with little tidal variation, irregular and unpredictable storm events are likely to be the predominant source of new wrack deposits. Our results suggest it is particularly important that management strategies to mitigate the impacts of grooming are adopted in areas with low tidal range.