The ecological role of wrack debris for macroinvertebrate assemblages on atlantic sandy beaches: spatial and temporal patterns

Accumulations of seagrass, macroalgae and other organic materials, collectively known as wrack, commonly occur on sandy beaches worldwide, and are critical to the functioning of these ecosystems. On the upper beach, the deposition of wrack debris provides habitat and food for rich supralittoral macrofauna of crustaceans and insects, which cannot be found in any other environment and play key roles in the ecosystem functioning. Despite this, the removal of wrack debris is a widespread practice on sandy beaches, and there is little information regarding the effects of such wrack removal on associated macroinvertebrates. The general aim of this thesis is to evaluate the spatio-temporal variation in the distribution and utilization of wrack debris by supralittoral arthropods and its biological responses to wrack removal, which, in turn, is relevant to the management of this key resource (i.e. wrack debris) on sandy beaches. We investigated the influence of the spatial distribution and the relative ageing of upper wrack deposits on the composition and structure of macroinvertebrate assemblages on six sandy beaches located at Brazilian and Spanish Atlantic coasts (Chapter 1). We found that, independently of the wrack composition, the spatial distribution of wrack deposits at different tidal levels (i.e. bands) on the upper beach influence the structure of supralittoral assemblages. Bands wrack play a role shaping the density of dominant taxa (i.e. Talitridae, Tenebrionidae and Staphylinidae) on Atlantic sandy upper shores. In the Chapter 2, the abiotic factors that could influence the distribution of supralittoral arthropods was analysed in relation to the deposition and location of wrack debris at two Spanish Atlantic beaches. We found differences in the community structure and composition between microhabitats (bare and wrack-covered areas), due to the influence of wrack deposits on the habitat selection by arthropod species. The labile organic matter (BPC) combined with temperature or moisture mostly explained the distribution pattern of assemblages. Changes of these habitat features in relation to the ageing of wrack debris play an undeniable role in the utilization of wrack by supralittoral arthropods, and therefore, in the spatial distribution of species between wrack bands. These results provided evidences about the role of algal wrack structuring assemblages (diversity and composition) of supralittoral arthropods. The colonization process by supralittoral arthropods at naturally strandlines was investigated in the Chapter 3. For this, we manipulated the presence of stranded wrack and then monitored the wrack-associated fauna after the stranding of new wrack debris on the Atlantic upper beaches (southern Brazil and south-western Spain) for a 47-day period. Our results showed that naturally stranded wrack debris can be promptly invaded by a wide range of colonizer species (i.e. talitrid, dipterans, tenebrionids, staphylinids, and spiders) that indicate high heterogeneity of habitat and/or food sources available for supralittoral arthropods in natural driftlines. Moreover, the results demonstrated that the colonization of strandlines was slower in southern Brazil (16-day period) than in south-western Spain (3-day period). This process was driven by changes in the densities of Talitridae, Staphylinidae, and Tenebrionidae species in both regions. Moreover, beetles of Cleridae and Nitidulidae also contributed to colonizer community in southern Brazil (Paraná region). The dynamics of wrack supply, the species-specific adaptations (i.e. mobility) and strategies to use wrack debris (as refuge and/or feeding site) could explain the different colonization patterns of assemblages in both Atlantic regions. The direct effect of wrack removal on the community structure and composition was analysed using a manipulative experiment, where we evaluated the biological responses before and after to remove all organic debris detached at the Atlantic upper shores (Chapter 4 and 5). We found that a short-term wrack removal resulted in detectable changes in the density of some species, mainly talitrid amphipods, and therefore in community structure, but these was rapidly recovered, although the timing of recovery was more protracted for supralittoral assemblages of southern Brazil than did of south-western Spain. The temporal variability of wrack biomass and the specie-specific adaptations of invertebrates inhabiting strandlines might determine the response of the supralittoral assemblages to wrack removal on beaches located at both sides of Atlantic Ocean. This thesis increases our knowledge concerning the interaction between wrack debris and supralittoral macroinvertebrates, highlighting the role of stranded organic materials in shaping communities as shelter, breeding and/or feeding sites. Moreover, the results of this study demonstrate that the removal of wrack debris affect the structure and composition of supralittoral assemblages. Therefore, wrack debris should be considered a key component on sandy beaches because it provides habitat and food for supralittoral macroinvertebrates, forming islands of biodiversity which support coastal food webs. The conservation of wrack debris and the reduction of human impact on associated fauna are crucial to maintain the functional integrity of beach ecosystems.Universidad Pablo de Olavide; Universidad de Cádiz226 páginas; inglé