Sea wrack (dead, shore-cast macrophytes) may affect the productivity and diversity of small islands by providing a marine nutrient subsidy. We explored this potential nutrient transfer from a productive marine environment to small islands (124m2 – 2880440m2) along the Central Coast of British Columbia by asking the following questions: i) how much wrack is accumulating along shorelines and in nearshore environments?; ii) what biophysical environmental variables are responsible for affecting patterns of wrack accrual?; iii) what are the temporal differences in patterns of wrack accumulation and species composition? We surveyed 459 shoreline sites across 100 islands over the course of three summers (2015-2017). At each site, we recorded biomass, species composition, and biophysical variables including wind, swell, wave period, tide, wave exposure, shoreline slope, shoreline width, shoreline substrate, and the shorelines proximity to a kelp or seagrass bed.
Zostera spp., Fucus spp., Pterygophora californica, Macrocystis pyrifera, and Nereocystis leutkeana had the highest gross biomass measured across all sites. Preliminary analysis suggests the total standing volume of wrack differed significantly between rock, sand, and gravel beaches. Sandy beaches had the highest median accumulation of wrack followed by gravel beaches and rock shorelines, respectively. Beaches of different slopes differed significantly in wrack accumulation. Inclined shorelines (5-20 degrees) accumulated the most median biomass followed by flat (0-5 degrees) and steep sites (>20 degrees). The standing volume of wrack did not differ between sites of varying degrees of width and sites of varying levels of wave exposure. For further analysis we will use generalized mixed-effect models to examine the biophysical factors that lead to wrack accrual and NMDS modeling to distinguish for patterns of species composition. This study provides a quantitative examination of how sea wrack links marine and terrestrial environments, an important cross-boundary subsidy.