Effects of nutrient enrichment and grazing by an invasive filter feeder on phytoplankton biomass in a South West Atlantic coastal lagoon

Abstract

Invasive species can exert strong effects on structure and function of marine ecosystem. In estuaries, nutrient enhancement due to anthropogenic activities can increase microalgal biomass but invading filter-feeders can eventually regulate their abundance. This is what we hypothesized that is happening in a SW Atlantic coastal lagoon (Mar Chiquita, 37°40′S, 57°23′W, Argentina) invaded by the polychaete Ficopomatus enigmaticus. Here, we experimentally evaluated this hypothesis in a mesocosms experiment. Four treatments were performed: (T1) with nutrients (NO3, PO4 and NH4), (T2) with reefs and nutrients, (T3) with reefs, and (T4) without reefs or nutrients. Water samples were obtained to determine in vivo chlorophyll a (Chla) and nutrients concentration. Additionally, to evaluate the trophic position and particle selectivity by the polychaete, analysis of stable isotopes of particulate organic matter (POM) of the water and individuals were performed. Stable isotopes analysis showed that the main resource used by F. enigmaticus was the fraction of POM between 62 and 250 µm, showing particle selectivity and suggesting that the reefs have the potential to promote shifts in size, composition and biomass of local food source. Mesocosms experiments showed that nutrient supply increased the Chla concentration, but when reefs were added, the Chla decreased. Thus, nutrients increased the phytoplankton biomass but grazing by the reefs counteracted these increments. This indicates that an invader such as F. enigmaticus might modulate an anthropogenic impact via suspension feeding, and therefore its role in mitigate the consequences of eutrophication may be highly important.