Storage of Pacific oysters Crassostrea gigas in recirculating tank: Ammonia excretion and potential nitrification rates

Abstract

International audienceIn response to the closure of shellfish production sites due to increasing occurrence of toxic algal blooms, land-based recirculating aquaculture systems (RAS) could be used by producers to store a proportion of their stock temporarily. An RAS prototype was tested in this study to store Pacific oyster Crassostrea gigas over a range of temperatures. In such systems, the water temperature is a variable that could influence the bivalve excretion rate of ammonia, the concentration of which may become critical for the water quality. In this study, we first estimated the total ammonia nitrogen (TAN, N-NH4+ + N-NH3) excretion of the Pacific oyster and then investigated whether the bacterial population associated with the oysters could act as a natural biofilter by measuring the potential nitrification rate (PNR) of the shell. The TAN excretion rate varied significantly with temperature. The PNRs were significantly different depending on the shell surface considered with a PNR of 0.42 ± 0.11 (SD) nmol N cm−2 h−1 for the internal part, 0.96 ± 0.15 (SD) nmol N cm−2 h−1 for the external part of the shell. In addition, a 5-week experiment was conducted with a 180 kg of oyster stock stored in a 1900 L RAS to monitor TAN concentration and to establish a TAN budget at the level of an oyster population. The TAN concentration sharply declined in the RAS after 6 days and a stable concentration of 3.3 ± 0.91 (SD) μmol N L−1 was reached till the end of the experiment. This measurement, as well as the estimation of a theoretical TAN budget based on the previous experiments, showed that the PNR and some other associated processes were sufficient to counteract the ammonia fluxes excreted by the oysters. This result underlines the significant role played by the nitrifying bacterial population colonizing the bivalve shell and suggests that such an oyster stock could be stored in a recirculating tank without the addition of a biofilter, which would represent a real economic advantage

Satellite remote sensing reveals a positive impact of living oyster reefs on microalgal biofilm development

“…In some northwestern European flats, MPB maxima have been detected slightly later in spring and in September (van der Wal et al, 2010). The summer depression has been observed in some more southerly mudflats such as MarennesOléron Bay (France) (Cariou-Le Gall and Blanchard, 1995), the Tagus estuary (Portugal) (Brito et al, 2013), Cadiz Bay (Spain) (Garcia-Robledo et al, 2016), and in the Wadden Sea (The Netherlands) (van der Wal et al, 2010;Stief et al, 2013), and has been related to increased macrobenthos grazing in summer (Peletier, 1996;Weerman et al, 2011). Low NDVI values in summer may also be due to thermo-inhibition when sediment temperature exceeds 25 • C (Blanchard et al, 1997).…”

“…Via the release of organic and inorganic matter through excretion and biodeposition (Dame, 1993;Cognie and Barillé, 1999;Newell, 2004;Buzin et al, 2015), oysters can have an impact by enriching the sediment around them, increasing nutrient availability and hence development of MPB (Dame, 1993;Garcia-Robledo et al, 2016). Oyster reefs are also known to have indirect effects such as modifying the structure of the surrounding sediment and the ambient hydrodynamic conditions, facilitating MPB establishment (Colden et al, 2016).…”

Influence of OSHV-1 oyster mortality episode on dissolved inorganic fluxes: An ex situ experiment at the individual scale

“…Juvenile oyster mortality influence on fluxes (Dame 1972, Newell 1985, Bernard and Noakes 1990, Gale et al 1991, Bougrier et al 1995, Soletchnik et al 1997, Wan-soo et al 2002, Buzin et al 2015 ACCEPTED MANUSCRIPT…”

Response to the referees

“…In some northwestern European flats, MPB maxima have been detected slightly later in spring and in September (van der Wal et al, 2010). The summer depression has been observed in some more southerly mudflats such as Marennes-Oléron Bay (France) (Cariou-Le Gall and Blanchard, 1995), the Tagus estuary (Portugal) (Brito et al, 2013), Cadiz Bay (Spain) (Garcia-Robledo et al, 2016), and in the Wadden Sea (The Netherlands) (van der Wal et al, 2010;Stief et al, 2013), and has been related to increased macrobenthos grazing in summer (Peletier, 1996;Weerman et al, 2011). Low NDVI values in summer may also be due to thermo-inhibition when sediment temperature exceeds 25 • C (Blanchard et al, 1997).…”

“…While nutrient availability has long been considered to be non-limiting in intertidal mudflats (Underwood and Kromkamp, 1999), the hypothesis of nutrient inputs coming from oysters is here advanced as possibly being the main factor explaining the higher biomass of surrounding MPB. Via the release of organic and inorganic matter through excretion and biodeposition (Dame, 1993;Cognie and Barillé, 1999;Newell, 2004;Buzin et al, 2015), oysters can have an impact by enriching the sediment around them, increasing nutrient availability and hence development of MPB (Dame, 1993;Garcia-Robledo et al, 2016). Oyster reefs are also known to have indirect effects such as modifying the structure of the surrounding sediment and the ambient hydrodynamic conditions, facilitating MPB establishment (Colden et al, 2016).…”