Spatio-temporal variability of strandings of the southern bull kelp Durvillaea antarctica (Fucales, Phaeophyceae) on beaches along the coast of Chile—linked to local storms

Abstract

Availability of floating seaweeds may depend on the seasonal cycles of benthic populations, but our ability to predict temporal patterns of stranded biomasses is still limited. Season-dependent, local storms favor detachment of seaweeds from the substratum, which can be reflected in the stranded biomasses on adjacent beaches after these events. Hence, we hypothesized that seaweed strandings are positively correlated with storm intensity. Using as a model the southern bull kelp Durvillaea antarctica (Chamisso) Hariot, a species frequently found in seaweed strandings in Chile, bimonthly surveys were carried out on three beaches: Pichicuy (32° S), Itata Norte (36° S), and Curinanco (39° S) for 3 years (2014 to 2017). Stranded biomass, total length, and wet weight of specimens were quantified and related to local storms (using the Douglas sea scale). Stranded biomasses decreased in the spring months of each year, being higher in Pichicuy and Curinanco than Itata Norte. Regression models showed a better fit with recent storms in Curinanco compared to other beaches. An interannual decrease of beach-cast raft size was observed, showing smaller specimens in Itata Norte than those in Pichicuy and Curinanco. Reduced habitat availability and the exploitation of natural beds in the central zone (34°–37° S) might explain the decrease of biomasses and sizes of stranded bull kelps. Also, oceanographic features at intermediate (i.e., local winds and currents) and large scales (i.e., El Nino) can help to explain the temporal variability, particularly in spring and summer. Our results suggest that harvesting of stranded bull kelps might be most favorable in summer and autumn.