Ferry wakes increase seaweed richness and abundance in a sheltered rocky intertidal habitat

Abstract

Because hydrodynamic regimes influence community assemblages, commercial ferry traffic can directly affect neighboring marine ecosystems by altering water movement. One of the largest ferry fleets in the world traverses the calm, protected waters of the Canadian Gulf Islands (British Columbia). To address the effects of ferry wakes on rocky marine intertidal communities, we examined community assemblages in sites impacted by ferry wakes (N = 5) relative to geographically similar control sites not directly exposed to ferry wakes (N = 6). Intertidal communities were significantly different between wake-influenced and control sites. Further analyses revealed that community level differences resulted from differences in seaweed assemblages, while invertebrate assemblages were similar. Sites exposed to ferry traffic displayed significantly greater overall seaweed abundance and seaweed species richness. Nitrate and nitrite concentrations, salinity, fetch, and tidal zonation were not significantly different between wake-impacted and control sites. However, dissolution blocks revealed that wake-impacted sites experienced increased overall water movement. Furthermore, block dissolution was negatively associated with distance from nearest ferry route and not fetch. Although dissolution block cannot disentangle effects of increased flow versus waves resulting from ferry wakes, we conclude that increased overall water movement from frequent and proximate ferry traffic stimulates primary production in rocky intertidal marine seaweeds by ameliorating mass transfer limitation.