Grazing effect of the invasive reef-forming polychaete Ficopomatus enigmaticus (Fauvel) on phytoplankton biomass in a SW Atlantic coastal lagoon

In this work we evaluate the potential grazing impact of the invasive reef-forming polychaete Ficopomatus enigmaticus in a SW Atlantic coastal lagoon (Mar Chiquita coastal lagoon; 37° 40' S, 57° 23' W; Argentina). This gregarious species feeds on suspended detritus and phytoplankton. Given the large area dominated by reefs, suspension feeding by this species might reduce the lagoon phytoplankton concentration and even hinder local eutrophication. To evaluate this hypothesis in situ replicated mesocosm experiments were performed in spring 2005 and, summer and winter 2006. Mesocosms enclosing reefs and without reefs were installed and grazing intensity was measured as the difference in chlorophyll a concentration and turbidity between the reef-treatment and the treatment without reefs. Reefs of F. enigmaticus decreased the mean chlorophyll a concentration, more during summer (56% decrease) than spring (25% decrease) and winter (19% decrease). Reefs also decreased water turbidity during summer (54% decrease) and spring (23% decrease). While previous studies indicate that the physical structure of these reefs alters water flow increasing water turbidity, our evidences show that their suspension-feeding activity can counteract this effect. Water turbidity was positively correlated with chlorophyll a concentration, which suggests that phytoplankton grazing by F. enigmaticus decreased light attenuation through the water column, with a potential for enhancement of benthic primary productivity. Therefore, our results suggest that grazing by this polychaete affects overall estuarine primary production as well as the relative importance of planktonic and benthic carbon sources to higher trophic levels.