Macrophyte wrack on sandy beaches of the US Pacific Northwest is linked to proximity of source habitat, ocean upwelling, and beach morphology

ABSTRACT: Marine macrophyte wrack (macroalgae and seagrasses) frequently washes onto beaches but little is known about the factors controlling its biogeographic variability. We report on a large-scale study of macrophyte wrack deposition patterns on the US Pacific Northwest coast. We measured macrophyte wrack on 12 sandy beach sites from southern Washington to northern California. We found the highest wrack biomass (g m-2) occurred on southern beaches but the greatest wrack patch density (number m-2) occurred on northern beaches, resulting in some northern sites having orders of magnitude more wrack than other sites. Eelgrass (Zostera marina and Z. japonica) was present in wrack in the greatest proportions at northern sites, and kelp (e.g. Nereocystis luetkeana and Macrocystis integrifolia) was present in the greatest proportions at central and southern sites. Further analyses showed that the proximity of estuary and rocky reef habitats, ocean upwelling, and beach geomorphology and wave climate all contributed to the biogeographic patterns of beach wrack. We also found new evidence that estuarine outwelling combined with ocean upwelling can significantly contribute to these patterns.