Monitoring of shellfish banks sediments in the Ría of Arousa (Galicia, Spain)

The main drivers of the continuous decline in shellfish productivity along the intertidal sandbanks in the Ría de Arousa (Galicia, Spain) since the last decade are unknown. While the impact of increasing temperatures and abrupt decreases in salinity has been extensively studied, there is a lack of information about the physicochemical quality of the sediments, which is essential for marine resources protection guidelines. Moreover, the shellfish community suggests that a decrease in the sediment grain size and the presence of contaminants (e.g., heavy metals) may also have contributed to the reduction in shellfish productivity. 

To test this hypothesis, we monitored the environmental status of the sediments along four shellfish sandbanks in the southern margin of the Ria de Arousa which currently differ in bivalve productivity. Since 2022, we conducted a seasonal sediment sampling strategy obtaining an extensive dataset of grain size and heavy metal concentrations (i.e., Cr, Zn, Cu, Ni , and Pb). Our analysis shows that harvest sediments mainly consist of sand with a low percentage of mud which tends to increase in the two banks near river mouths. As expected, coarser grain sizes prevent the accumulation of heavy metals. Statistical analysis reveals that heavy metal concentrations vary according to natural processes, suggesting that their concentration in the sediments is not likely contributing to the decline in the abundance of shellfish individuals.  Our findings provide valuable information that might help to improve the management of such traditionally productive environments.