Filtration pressure by bivalves affects the trophic conditionsin Mediterranean shallow ecosystems

Bivalve filtration may control the amount of seston in coastal waters, reducing local euthrophicationand keeping degrading phenomena like hypoxia and anthropogenic pollution under control. Two Sicilianbrackish-marine ponds (Ganzirri and Faro) present us with the opportunity to gain data on the effect ofbivalve filtration on the amount of particulate organic matter in the field. The cultivation of bivalves has beencarried out in both of the ponds since the early 1990s but stopped in Ganzirri in 1995.We tested whether thecessation of bivalve cultivation influenced features of organic matter available to suspension feeders (totalsuspended 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 significantlyincreased compared to Faro, where, in contrast, they remained the same as in previous decades. Recentdata shows that organic matter was significantly higher in Ganzirri than Faro and that differences weremaintained throughout the study period. Using clearance rate data from the literature, we determined thatbivalves 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 filtrationpressure to both (i) control the phytoplankton biomass and trophic dynamics in ponds, and (ii) reduce apossible role of natural-with-sea exchange and polluted waters coming from the hinterlands.