| Summary: | Sandy beaches are dynamic ecosystems occurring between temperate to tropical latitudes. In-situ primary productivity of wave-exposed sandy beaches is often low, but the food web of such shores may be supported by marine subsidies in the form of stranded macrophyte material. The input of washed-up seaweed, also termed wrack, varies seasonally and between zones. Durvillaea antarctica and D. willana are two species of kelp that dominate the exposed shores of New Zealand. Research on the relationship between stranded seaweed and macrofaunal assemblages on New Zealand sandy shores is limited, hence this study aimed to produce a quantitative list of the macrofaunal species associated with stranded D. antarctica on an Otago exposed sandy beach, and to investigate its decomposition rate over a 30-day period in autumn and winter. In autumn and winter 2008, six mesh bags containing 1 kg samples of D. antarctica were placed at four sites along Victory Beach. On days 1, 3, 6, 10, 20 and 30, kelp samples were randomly selected from each site and a 10 x 10 cm core for macrofauna was taken directly under the kelp (0 m), at 0.5 and 1 m distance away. Core and kelp samples were sieved on a 1 mm mesh to extract macrofauna, which were identified and counted. Amphipods were separated into adult and juvenile stages. Macrofaunal biomass was also determined. The water, carbon and nitrogen content of the kelp samples were measured. Twenty macroinvertebrate taxa were found to be associated with the samples of D. antarctica during both seasons. Amphipoda, Staphylinidae, other Coleoptera and Diptera made up 99.9% of total macrofauna abundance and biomass. All four taxa were present on day 1 of sampling. Amphipoda was the dominant coloniser in terms of abundance and biomass for both seasons. On average, it comprised 74-96% of total abundance and 53-98% of total biomass. Amphipoda and Staphylinidae differed significantly in abundance between days of exposure, where amphipods were most abundant during early stages of colonisation and staphylinids were most abundant on day 20 of exposure. All main taxonomic groups, except other Coleoptera, differed in biomass between days of exposure and between seasons, with autumn having the highest biomass. The presence of D. antarctica significantly impacted the abundance of macroinvertebrate species, where abundance was highest directly under the kelp. The abundance and biomass of adult and juvenile amphipods in kelp and sediment samples were influenced by different factors. Core abundance indicated that adults are more dependent on the presence of stranded D. antarctica. Decomposition rate and moisture content of D. antarctica differed significantly between days of exposure and between seasons. D. antarctica dried and decomposed at a faster rate in autumn than in winter. Carbon content ranged between 22-35% and differed significantly over time. The nitrogen content ranged between 0.78-1.26%. Correlations between macrofaunal abundance and the decomposition rate, and total macrofaunal abundance and the C:N ratio, suggest that the macrofaunal community on Victory Beach prefers old wrack over fresh wrack and a carbon-rich food source. Beach grooming and/or seaweed harvesting has been implicated as one of the main management concerns for the future of the ecology of sandy shores. To determine the effects of removing beach-cast seaweed on associated fauna it is essential to investigate its ecological significance in sandy beach food webs. The present study contributes to our understanding of the significant role of stranded seaweed in sandy shore foodwebs.
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