Bioaccumulation of PCBs by a seaweed bloom (Ulva rigida) and transfer to higher trophic levels in an estuarine food web

Many urban estuaries worldwide contain both eutrophication-induced macroalgal blooms and persistent organic pollutants (POPs) in their sediments. Between 2007 and 2012, we studied an annually occurring green tide of Ulva rigida in the PCB-contaminated Superfund site of New Bedford Harbor, MA, USA. Ulva PCB concentrations were highest (95−99 mg kg ⁻¹ dry weight) near the contamination source and decreased to only 2−4 mg kg ⁻¹ at the site’s southern end, approximately 2.5 km away. To assess the bloom’s potential role as a PCB contributor to higher trophic levels, we performed stomach content analysis on the system’s primary mid-trophic level predator, the salt marsh mummichog Fundulus heteroclitus, and stable isotope analysis on associated potential energy sources. In addition to detritus and macroinvertebrate prey, Ulva was a major component of mummichog stomach contents, and a ¹⁵ N-labeled feeding experiment demonstrated that mummichogs can assimilate ingested Ulva. Among invertebrate prey, stable isotope mixing models showed Ulva as the main diet source for amphipods Gammarus spp., while grass shrimp Palaemonetes spp. and sandworms Nereis spp. relied on a mix of Ulva and Spartina or particulate organic matter, respectively. Sandworms and grass shrimp were the main mummichog energy sources, suggesting that this estuarine keystone species is linked to the Ulva bloom mainly through predation on Ulva-consuming grazers rather than direct ingestion. Our data provide evidence for a potentially overlooked impact of macroalgal blooms—namely, their potential role in the trophic transfer of PCBs and other bioaccumulated pollutants.