Aquaculture production has been gaining an increased worldwide importance over the last decades, equalling worldwide fisheries production and is expected to surpass it rapidly. While intensive aquaculture productions attain high productivity, they require the feeding of the species being produced and often involve the addition of antibiotics and other medicaments. On the other hand, extensive aquacultures rarely involve the addition of pharmaceutics and feeding of the farmed species, as they rely on the food provided by the natural environment. Although most worldwide aquaculture production is focused on fish farming there is an increasing demand for multicultures where bivalves, such as mussels and oysters, are farmed along with fish and other organisms (e.g. sea cucumbers). Bivalves can reduce waste dispersal of fishes as they are filter feeders. Moreover, they also have a high gastronomic value and can reach high prices for reduced quantities, as is the case with oyster, especially if they are farmed in a sustainable fashion. This study was focused on the assessment of the environmental conditions in an extensive oyster aquaculture in the Sado estuary in Portugal, with a production tank of approximately 1.5 ha. The main purpose of this assessment was to determine how the local aquacultures affect the water quality of the Sado estuary and vice-versa. More concretely, this study will determine how the oyster aquaculture affects nutrient availability, dissolved oxygen and the composition of suspended particles, in particular, the dynamics of phytoplankton communities and other suspended particles in the water column.
Weekly water samples were collected, from June 2019 onwards, with an expected duration of one year. Samples were taken from the aquaculture tank as well as from the estuarine adjacent channel, which can exchange water with the production tank through a manual gate. During sample collection several water parameters (e.g. temperature, salinity, dissolved oxygen, pH) were recorded using a multiparameter sonde. In the laboratory, water samples were filtered for pigment analysis through HPLC (High Performance Liquid Chromatography), SPM (Suspended Particulate Matter) allowing to obtain SOM (Suspended Organic Matter) and SIM (Suspended Inorganic Matter). Dissolved nutrient concentrations (Nitrogen, Phosphorus and Silicate) in the water were also measured.
Preliminary results show no significant differences between the water quality observed in the estuary and in the aquaculture, for most parameters. In the production tank, temperature and salinity observations show seasonal variation, ranging from 24ºC and 44ºC, in summer, to 9.6ºC and 22.5ºC, in winter, respectively. Chlorophyll a peaked in July and October. Ammonium concentrations reached almost 12.5 μM in August and dissolved Oxygen sat% reached a minimum (69%) both in September and October. The main difference observed between the aquaculture and the estuary is that the water characteristics of the tank are much more constant and have lower variability than those from the estuary. This suggests that regular opening, every two weeks during the spring tide, seems adequate, promoting water renewal that has no apparent negative effects on the estuary as well as on the aquaculture. However, it may be worth monitoring oxygen and ammonium levels to avoid toxicity to oysters.