Effects of an invasive reef-building polychaete on the biomass and composition of estuarine macroalgal assemblages

Invasive species can transform ecological communities. Their profound effects may alter the sources and pathways of primary production. We investigated the effects of the reef forming polychaete Ficopomatus enigmaticus invasion on the biomass and distribution of estuarine macroalgae in a SW Atlantic coastal lagoon (Mar Chiquita, 37° 40′S, 57° 23′W, Argentina). Reefs built by this species serve as substrates for macroalgal development and furnish structures that modify physical and biological conditions for the surrounding benthos. We showed that (1) the red macroalga Polysiphonia subtilissima settles and grows almost exclusively on the surface of the reef, (2) the green macroalgae Cladophora sp. and Enteromorpha intestinalis are found almost exclusively in areas without reefs attached to mollusk shells and, (3) no macroalgae occur in the sediment between reefs. Manipulative experiments show that reefs provide a complex substrate for settlement and survival and therefore benefit red macroalga. These experiments also show that the invasive reef builder has negative indirect effects on green macroalgae by increasing grazing and probably by increased sedimentation between reefs. Via these direct and indirect effects, reefs change the relative biomass contribution of each macroalgal species to the overall production in the lagoon. Knowledge of these processes is important not only for predicting net effects on primary production but also because changes in macroalgal species composition may produce effects that cascade through the food web.