Ecological impacts of two non-indigenous macroalgae on an urban rocky intertidal shore

Abstract

Non-indigenous marine species often change the abundance and diversity of native species in coastal ecosystems. On the SW Atlantic coast, the macroalgae Ahnfeltiopsis sp. (Rhodophyta, Phyllophoraceae) and Schizymenia dubyi (Rhodophyta, Schizymeniaceae) have invaded the intertidal rocky shore of Mar del Plata, Argentina (38°S, 57°W). To study the spread and ecological associations of these invasive species, algal abundance, biomass and biodiversity of benthic assemblages at three different tidal levels were examined during five years. Sparse Ahnfeltiopsis sp. thalli (3 % cover) were detected in February 2007 at the three tidal levels. By January 2011, its cover had increased to 11 %, while its biomass showed a 27-fold increase. S. dubyi was detected at the lower intertidal level in January 2010 with a cover of 2 %. By January 2011, it had increased to 5 % and spread to the other intertidal levels. The presence of these two non-indigenous algae modified the substrate and the structure and composition of the benthic assemblage. The constant increase in the algal biomass and presence along the intertidal suggest that the effect will be greater in the future. Moreover, the effects of these exotic algae could potentially displace Brachidontes rodriguezii—an important ecosystem engineer that creates microhabitat for a large number of organisms on these shores.