Quantifying temporal trends in anthropogenic litter in a rocky intertidal habitat

Most monitoring studies of marine anthropogenic debris have focused on sandy beaches, so little is known about litter on rocky shorelines. We surveyed litter trapped on a rocky intertidal shore in False Bay, South Africa, between May 2015 and March 2018. An exceptional upwelling of seabed litter occurred in November 2017 (70 items∙m^-1). Excluding this event, monthly clean-ups at spring low tide collected 2 (1.3-3.1) items∙m^-1∙month^-1 and 31 (19.4-49.4) g∙m^-1∙month^-1 of which 74% was plastic (31% by mass). Litter loads peaked in autumn when seasonal rains washed litter into False Bay, suggesting that most litter comes from local land-based sources. Litter composition differed from that on a nearby sandy beach, with more glass and other dense items on the rocky shore, but 60% of plastic items floated in water. Sand inundation and biotic interactions helped to trap buoyant plastics in the intertidal zone.