Introduction
Oyster farming represents 33% of global bivalve production. The Pacific cupped oyster Crassostrea gigas (Thunberg, 1793) is the most reared species (~653,000 tons in 2019), while the production of the European flat oyster Ostrea edulis (Linnaeus, 1758) is less substantial (~2,200 tons in 2019) (FAO, 2019). Oyster farming is associated to some important ecosystem services (Clements and Corneau, 2019), but simultaneously it could negatively impact on the sea bottom health, through the release of faeces and pseudofaeces, rich in inorganic and organic matter (Newell, 2004), hereafter collectively referred as bio-deposits. High amounts of bio-deposit production can cause structural changes in benthonic communities, leading to hypoxic events, loss of biodiversity and predominance of detritivorous species (Newell, 2004). Eutrophic environments such as coastal lagoons are characterised by large amount of phytoplankton and suspended particulate matter available for filter-feeders, thus representing suitable sites for oyster farming but also resulting in a large production of bio-deposits (Newell, 2004). The aim of this study was to compare the daily bio-deposition rate of two oyster species, C. gigas and O. edulis, in two coastal lagoons potentially suitable for farming and characterised by different environmental and trophic conditions.
Materials and methods
This study was conducted in two shallow eutrophic lagoons (Magni et al., 2008) located in the central-western coast of Sardinia Island (Western Mediterranean, Italy), S’Ena Arrubia and Corru S’Ittiri. In September 2021 and over a period of 24 hrs, we evaluated the bio-deposit production of the two oyster species and the natural deposition rate of the lagoons: Total Particulate Matter (TPM), Particulate Inorganic Matter (PIM) and Particulate Organic Matter (POM). Seven replicates per treatment were performed. Individuals had a mean wet weight of 43.4 ± 0.8 g (C. gigas) and 46.3 ± 0.6 g (O. edulis), they were of hatchery origin (Marinove, France) and they were reared in the same lagoons. Prior to the trials, individuals were brushed cleaned to remove epibionts and acclimated for a week to the natural conditions of the lagoons. The bio-deposit production and the natural deposition were evaluated by using 21 (7 per treatment) cylindrical PVC tubes with a close-end fund (200 mm, height; 100 mm, diameter), suspended on a floating rope at 15 cm depth. Environmental parameters (temperature, salinity and dissolved oxygen) were measured with data loggers placed at 15 cm depth. Trophic conditions were assessed on surficial water samples collected in triplicates for measurement of chlorophyll a (Chl-a), TPM, PIM and POM, following the methods described in Parson, Maita & Lalli (1984). Data were analyzed by Statistica 6.1 StatSoft, Inc. (2004).
Results
In S’Ena Arrubia, the bio-deposit production was significantly different between species, C. gigas showed higher production of TPM (p < 0.01), PIM (p < 0.01) and POM (p < 0.001) than O. edulis. In Corru S’Ittiri, although we registered high values of TPM, PIM and POM produced by C. gigas, no significant differences were highlighted in comparison with O. edulis. Natural sedimentation resulted significantly different between lagoons, with a higher production of TPM, PIM and POM in S’Ena Arrubia than Corru S’Ittiri (p < 0.01) (Fig. 1). Environmental and trophic parameters were significantly different between S’Ena Arrubia and Corru S’Ittiri. In particular, Corru S’Ittiri registered a higher salinity (p < 0.05) and Chl-a (p < 0.001), and a lower amount of TPM (p < 0.001) and PIM (p < 0.001) than S’Ena Arrubia.
Discussion and conclusions
According to our results, the bio-deposit production of oysters was different between species and varied on the basis of the farming site characteristics. In the two lagoons the amount of bio-deposit produced by C. gigas was higher than those produced by O. edulis, this probably due to a higher filtration activity and metabolic rate of C. gigas, as previously observed by Nielsen et al. (2017) under laboratory conditions. Regardless of the oyster species we recorded a higher bio-deposit production in S’Ena Arrubia, probably due to the higher amount of TPM and PIM in the water column in comparison with Corru S’Ittiri, rather than the amount of Chl-a, which was higher in Corru S’Ittiri. In conclusion, the two oyster species produce different amount of bio-deposit, the production is site-specific and vary on the basis of the farming site characteristics, in particular the amount of available particulate matter. This indicates that the effects of oyster aquaculture on the sediments of the farming site must be evaluated specifically in each location.
References
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