Factors controlling macroalgae assemblages in a Southwest Atlantic coastal lagoon modified by an invading reef forming polychaete

• In the coastal lagoon, macroalgae are modified by a reef forming invader polychaete.
• We investigated consumers and nutrient effects on modified macroalgae assemblages.
• We found effects of consumers, and no effects of nutrient addition, on macroalgae.
• The consumption had different effects in areas invaded and non-invaded.
• The effect of invader could depend on other structuring force interactions.

In the Mar Chiquita coastal lagoon (37° 40′S, 57° 23°W, Argentina), the invasive reef building polychaete Ficopomatus enigmaticus, change the composition and abundance of macroalgae by providing habitat to some species and by excluding others. The magnitude of these effects may be regulated by consumers and nutrients availability. Here we evaluate the relative importance of consumers and nutrients in controlling macroalgae development on the reefs, and on the sediment in areas without reefs. Experiments showed that: (1) large consumers (birds, fishes and crabs) had no effect on red (Polysiphonia subtilissima) and green (Cladophora sp.) macroalgae on reefs; (2) large consumers had positive effect on green macroalgae development on the sediment of areas without reefs, probably by enhancing the availability of bivalve shells for macroalgae attachment; (3) small consumers (amphipods, hydrobiid snails and crabs) had negative effect on green macroalgae development, probably by direct consumption; (4) small consumers had positive effect on the biomass of the red macroalga P. subtilissima; and (5) nutrient had no effect on macroalgae. These interactions would determine the abundance and the spatial distribution of macroalgae (e.g., patchiness). Consequently, the effect of an invader ecosystem engineer in the composition of species assemblages may be mediated by other community structuring forces, which in turn may have different effects according to their interaction with the invader.