Filtration pressure by bivalves affects the trophic conditions in Mediterranean shallow ecosystems

Bivalve filtration may control the amount of seston in coastal waters, reducing local euthrophication and keeping degrading phenomena like hypoxia and anthropogenic pollution under control. Two Sicilian brackish-marine ponds (Ganzirri and Faro) present us with the opportunity to gain data on the effect of bivalve filtration on the amount of particulate organic matter in the field. The cultivation of bivalves has been carried out in both of the ponds since the early 1990s but stopped in Ganzirri in 1995.We tested whether the cessation of bivalve cultivation influenced features of organic matter available to suspension feeders (total suspended matter, its inorganic and organic fractions, chlorophyll a, carbohydrates, proteins and lipids). Since the bivalve cultivation was stopped in Ganzirri in 1995, chlorophyll a sharply and significantly increased compared to Faro, where, in contrast, they remained the same as in previous decades. Recent data shows that organic matter was significantly higher in Ganzirri than Faro and that differences were maintained throughout the study period. Using clearance rate data from the literature, we determined that bivalves can filter the available volume in Ganzirri by about 540 times and in Faro by 650 times per year. Thus bivalve farmed biomass (about 300 tonnes per year of fresh biomass) can exert a high filtration pressure to both (i) control the phytoplankton biomass and trophic dynamics in ponds, and (ii) reduce a possible role of natural-with-sea exchange and polluted waters coming from the hinterlands.