Alien reef-building polychaete drives long-term changes in invertebrate biomass and diversity in a small, urban estuary

Two of the greatest threats to native biodiversity are the construction of artificial structures in natural environments and the introduction of invasive species. As the development and urbanisation of estuaries continues at an increasing rate worldwide, these environments are being simultaneously affected by these threats. This study quantifies the spread of an invasive reef-building polychaete, Ficopomatus enigmaticus, in a small, highly manipulated urban estuary in South Africa and investigates its role as an ecosystem engineer. Anthropogenic changes to the Zandvlei Estuary, including construction of a rubble weir and canalisation near the estuary mouth, construction of an extensive marina development and hardening of the banks with concrete, have facilitated the expansion of F. enigmaticus. The standing stock of F. enigmaticus increased from 13.69 t, as measured in 1986, to 50.03 t in 2012, due both to increase in the total area colonised and standing stock per m2. Since F. enigmaticus reefs support a greater biomass of infauna than adjacent sandy areas, total invertebrate biomass in the estuary is estimated to have increased from less than 0.30 t in 1942, to over 56.80 t in 2012, due mainly to hardening of banks in parts of the main estuary with concrete and construction of a marina system. A positive correlation between reef mass and infaunal biomass, density and diversity was also found.