Deposition patterns of algal wrack species on estuarine beaches

Abstract

Wrack supply is a common feature of beaches around the world. The species composition and spatial distribution of wrack deposits have been widely recognized as two significant factors in the ecological role of macroalgae within beach ecosystems. These accumulation processes may be intensified by bloom events of opportunistic algae, which frequently occur in estuarine environments. For this study, wrack supply was assessed over 13 months in two estuarine beaches of the Galician coastline, NW Spain. Algal biomass and wrack species distribution were characterized, paying special attention to possible relationships between beach slope, tidal height and algal morphology. The results show that the average values in total monthly biomass of stranded macroalgae and seagrasses were similar for both beaches, but certain differences in temporal and spatial variability throughout the year's cycle were detected. Despite this, the distribution of algal species along the beach face was similar in both beaches: large amounts of sheet-like and thin branched species (e.g. Ulva spp., Gracilaria gracilis) dominated the wrack deposits on the low-tide terraces, whereas thick leathery species with positive flotation properties (e.g. Fucus spp., Ascophyllum nodosum) showed larger amounts in the upper foreshore zones. The relative contribution of the major morphological groups of algal wrack shifted throughout the year, suggesting an inverse relation between thick leathery group and sheet-like species, which could be associated with factors such as differences in macroalgae's life cycles and nutrient requirements of fast- and slow-growing species. This study demonstrates that beach topography, together with the morphology of macroalgae species influence the distribution of wrack deposits in estuarine beaches. Further research on the implications of wrack dynamics for sedimentary characteristics and faunal distribution is needed.