Bioblitzes are rapid assessments of local biodiversity that provide opportunities for science, outreach, and management. Here, we present results from a bioblitz motivated by the National Park Service All Taxa Biodiversity Inventory to catalog species diversity in the rocky intertidal zones of the Boston Harbor Islands National Recreation Area. Through a combination of structured, quantitative surveys and opportunistic sampling, public participants, experienced scientists, and naturalists worked together to identify more than 130 species in the rocky intertidal zones of 8 of the Boston Harbor Islands. Sampling by experts alone yielded greater diversity than sampling conducted in collaboration with public participants. However, quantitative, structured sampling by expert-only and collaborative expert–public teams both revealed the expected negative relationship between algal species richness and intertidal elevation, suggesting that, with structure, public bioblitzes can feasibly document ecological patterns. We discuss these results in the context of rocky intertidal ecology, island biogeography, and the role of technology in the growing popularity of bioblitzes as a tool for collaborative engagement of scientists and the public in the study and management of natural systems.