Impacts of mussel (Mytilus galloprovincialis) farming on oxygen consumption and nutrient recycling in a eutrophic coastal lagoon

Abstract

Fluxes of oxygen, nitrogen and phosphorus were determined in two areas of the Sacca di Goro lagoon, at a site influenced by the farming of the mussel Mytilus galloprovincialis and a control site. Mussel farming induced intense biodeposition of organic matter to the underlying sediments, which stimulated sediment oxygen demand, and inorganic nitrogen and phosphorus regeneration rates compared to the nearby control station. Overall benthic fluxes (-11.4 ± 6.5 mmol O 2 m )2 h )1 ; 1.59 ± 0.47 mmol NH 4 + m )2 h )1 and 94 ± 42 lmol PO 4 3) m )2 h )1 ) at the mussel farm are amongst the highest ever recorded for an aquaculture impacted area and question the belief that farming of filter-feeding bivalves has inherently lower impacts than finfish farming. In situ incubations of intact mussel ropes demonstrated that the mussel rope community was an enormous sink for oxygen and particulate organic matter, and an equally large source of dissolved inorganic nitrogen and phosphate to the water column. Overall, a one meter square area of mussel farm (mussel ropes and underlying sediment) was estimated to have an oxygen demand of 46.8 mmol m 2 h )1 and to regenerate inorganic nitrogen and phosphorus at rates of 8.5 and 0.3 mmol m 2 h )1 , with the mussel ropes accounting for between 70 and more than 90% of the overall oxygen and nutrient fluxes. Even taking into account that within the farmed area of the Sacca di Goro lagoon, there are 15-20 m )2 of open water for each one covered with mussel ropes, the mussel ropes would account for a large and often dominant part of overall oxygen and nutrient fluxes. These results demonstrate that it is essential to take into account the activity of the cultivated organisms and their epiphytic community when assessing the impacts of shellfish farming. Overall, whilst grazing by the mussel rope community could act as a top-down control on the phytoplankton, most of the ingested organic matter is rapidly recycled to the water column as inorganic nutrients, which would be expected to stimulate phytoplankton growth. Consequently, the net effect of the mussel farming on phytoplankton dynamics, may be to increase phytoplankton turnover and overall production, rather than to limit phytoplankton biomass.

Spatial heterogeneity in the food web of a heavily modified Mediterranean coastal lagoon: stable isotope evidence

“…Alternatively, their δ 15 N values could result from the assimilation of biodeposits produced by oysters and associated suspension-feeders. This hypothesis is consistent with the fact that oyster and mussel lines are known to act as traps for POM and biodeposits (Nizzoli et al 2005). Further research should compare the δ 13 C and δ 15 N values of selected suspension-feeders and their biodeposits.…”

Functional bioturbator diversity enhances benthic–pelagic processes and properties in experimental microcosms

“…The influence of biodeposition on sedimentary biogeochemical processes has been reported from environmental studies designed to determine the effect of mussel and oyster cultures on marine benthic habitats (e.g., Chamberlain et al 2001, Callier et al 2006. Most of these studies showed that organically rich feces and pseudofeces accumulate at the water-sediment interface and create reducing conditions in the sediment (Nizzoli et al 2005, Lindqvist et al 2009). This organic-matter enrichment stimulates O 2 uptake by microorganisms at the sediment surface (Heilskov and Holmer 2001) and often increases the flux of nutrients at the watersediment interface (Christensen et al 2003, Gibbs et al 2005, Giles and Pilditch 2006.…”

Influence of farmed pearl oysters and associated biofouling communities on nutrient regeneration in lagoons of French Polynesia

“…The intensive development of bivalve aquaculture results in major alterations of ecosystem processes, including an impact on benthic-pelagic coupling (sedimentation and remineralization) (Crawford et al 2003, Giles et al 2006, Mitchell 2006 and nutrient regeneration in the water column through bivalve excretion (Leblanc et al 2003, Mazouni 2004, Nizzoli et al 2005. Under nutrient-limiting conditions, it is argued that such enhancement of nutrient cycling may stimulate primary production (Asmus & Asmus 1991, Dame & Libes 1993.…”