Evaluation of phenolic content of wrack debris on estuarine beaches: The effect on upper beach macrofauna

Wrack supply represents the main organic input in sandy beaches ecosystems and is a common feature of beaches around the world. The chemical composition of the wrack is important in determining the extent and rate of the decay; in particular, phenolic compounds may affect consumption rates by wrack-associated macrofauna and, consequently, modify the nutrient processing and recycling in sandy beaches. The aim of this research was to investigate the possible links between phenolic content of wrack supplies and the macrofaunal community inhabiting them. To achieve this, patches of wrack in three beaches of the NW coast of Spain were analysed and the phenolic contents and macrofaunal distribution, both at spatial and temporal scales, were assessed. Wrack and macrofaunal samples were collected along two shore-parallel transects, coinciding with the wrack lines of previous tidal deposition. The results showed a general spatial distribution in the phenolic content of the beach-cast according to their tidal position on the beach. Thus, two distinct zones were identified: the “spring-tide deposited” zone located in the upper beach close to the base of the dunes and composed by wrack material with the lowest phenolic concentrations; and a “freshly deposited” zone with wrack deposits containing the highest concentrations of deterrent compounds. Nevertheless, only supratidal grazers, mostly linked to fresh deposits, have shown a slightly negative association between their abundance and the increasing phenolic contents of the stranded macroalgae. The lack of a consistent pattern in macrofaunal occupation of fresh and decayed drift lines detected suggests the species could be selecting habitats for reasons other than phenolic content.