Effect of filter-feeding mollusks on growth of green macroalgae and nutrient cycling in a heavily exploited coastal lagoon

Abstract

An increasing number of coastal lagoons are being exploited for intensive farming of bivalve filter-feeders. One of the most important farming sites in Europe for the Manila clam (Ruditapes philippinarum) is the Sacca di Goro lagoon (Po River Delta, Italy). Here, while on the one hand river runoff enhances high clam productivity, on the other hand it stimulates extensive blooms of green seaweeds of the genus Ulva. The latter can affect clams when macroalgal biomass collapses and decomposes. We hypothesized that the filtration of suspended particulate matter by farmed clams could enhance inorganic nitrogen and phosphorus recycling and simultaneously reduce phytoplankton biomass, thus facilitating macroalgal growth. An experiment simulating the effects of clams on early Ulva development was conducted over 7 days in outdoor flow-through mesocosms containing lagoon sediments with and without clams. On day 1, 3 and 7, macroalgal biomass and thallus elemental composition, water-dissolved nutrients and chlorophyll-a concentrations were recorded. On day 7, sediment-water nutrient fluxes were also measured on sediment cores sampled from each mesocosm. The presence of clams clearly stimulated Ulva growth and enhanced nutrient recycling rates. Furthermore, Ulva grown with clams had higher chlorophyll-a, nitrogen and phosphorus content. While sediments showed a net release of both soluble reactive phosphorus and ammonium when clams were present, in mesocosms without clams sediments constituted a sink for both dissolved inorganic phosphorus and nitrogen. Based on this evidence we can conclude that intensive clam farming over wide lagoon areas could significantly affect nutrient cycling, thus facilitating macroalgal blooms, with serious consequences for farming sustainability and for the functioning of the whole lagoon ecosystem.