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Dyeing irrigation water to study the impact of fresh water on bivalve cultivation in the Ebro Delta

Dyed waters by the rhodamine. Source: IRTA (CC BY-NC 4.0)

Researchers from IRTA and the UPC have used the tracer dye rhodamine to create a dispersion model for the fresh water that reaches Fangar Bay

The methodology used to define buffer zones could be applied elsewhere in Catalonia to study any chemical or microbiological contamination possibly affecting molluscs and to comply with European food hygiene and animal health recommendations

There have been episodes of bivalve mortality in the Ebro Delta’s bays, and the researchers’ methodology can be used to trace the dispersion of possible chemical or microbiological contaminants to study phenomena such as oyster death

The Ebro Delta is a complex ecosystem where natural dynamics coexist with production activities, such as fishing, aquaculture and agriculture. To preserve that delicate balance, it is vital to know how the activities in question affect their environment. Rice farming is the region’s principal agricultural activity, and one of its main environmental impacts is caused by the discharge of irrigation water into the sea. Such discharges have a positive and a negative side, in that they provide the ecosystem with nutrients but also contain toxic components of pesticides which can harm biodiversity. With that in mind, the Institute of Agrifood Research and Technology (IRTA) and the Technical University of Catalunya · BarcelonaTech (UPC) have developed a model to determine how discharged irrigation water circulates in Fangar Bay and minimize its impact on bivalve mollusc aquaculture. Using rhodamine WT, a red, water-soluble dye, researchers traced the effluents in the discharges and identified the areas most affected by them. The experiment will further knowledge of how fresh water is dispersed in the bay, making it possible to study and check whether chemical or microbiological contaminants could be involved in the death of bivalves. It could also pave the way for a definitive reorganization of aquaculture activities in the bay, based on the establishment of an approximately 80-hectare buffer zone in which, to avoid animal and human health risks, no bivalve cultivation will take place.

In addition to studying the situation in the Ebro Delta, one of the aims of the research was to establish a protocol for applying the same protective measure in other areas where bivalves are cultivated along the Catalan coast. “In Catalonia, we have succeeded in defining and implementing the methodology for establishing buffer zones,” says Margarita Fernández, a researcher from IRTA’s centre in Sant Carles de la Ràpita, with some satisfaction. All the relevant knowledge was presented on 29 September at a transfer event attended by representatives of Spain’s Ministry of Agriculture, Fisheries and Food and the Government of Catalonia’s Ministry for Climate Action, Food and Rural Affairs, as well as aquaculturists and shellfish gatherers from the Ebro region and elsewhere.

The project began in 2020, following various episodes of extraordinary mortality among adult oysters of marketable size in some of Fangar Bay’s farms in the previous years. Those losses, which were as high as 50% in certain cases, were not associated with the most usual causes, such as the herpes virus or high temperatures. Some of the aquaculturists affected thought there might be a link with the discharge cycles of irrigation water from rice fields, which, particularly in the months of May and June, can contain residues of contaminating substances. Subsequent observations made by IRTA and the UPC confirm a spatial correlation between the areas with the highest mortality and those with the most direct contact with the effluents in the discharged water.

According to the scientists, however, the episodes of death were not necessarily directly caused by the chemical agents in pesticides, which are already covered by specific environmental regulations. The first diagnostic tests on bivalves suggest that such agents are more likely to have an indirect effect. “It could be a combination of factors, such as pesticides making the bivalves more vulnerable to pathogens in the marine ecosystem,” explains Fernández. Although the project is now at an end, IRTA’s team will continue working to determine what caused the deaths. As part of research scheduled to run until 2023, different pathogens will be isolated and inoculated into healthy oysters in order to assess risks of chemical and microbial origin and take subsequent measures to mitigate their impact.

In Fangar Bay, in the meantime, a first step can be taken by implementing the buffer zone, the first to be applied to bivalve mollusc cultivation in Catalonia. Buffer zones are a preventative public health measure advocated by the European guide to good practice. The Government of Catalonia’s Directorate-General for Maritime Policy and Sustainable Fisheries has already received a map of the area unsuitable for bivalves, which it can take into account when issuing new licences or reorganizing aquaculture infrastructures in the bay.

Determining how the water behaves

The map in question was produced using a complex hydrodynamic model developed by scientists from the UPC and IRTA. “Before designing solutions, it was vital to understand the bay’s physical behaviour and anticipate the different contingencies,” remarks Manel Grifoll, a researcher from the UPC’s Maritime Engineering Laboratory, who has links with the Barcelona School of Civil Engineering (ETSECCPB) and the Barcelona School of Nautical Studies (FNB). “Combining in situ observations, numerical modelling and the results of our experiment with rhodamine enabled us to broaden our hydrodynamic knowledge of the bay,” he continues. To that end, on both the day of the experiment and the following day, the researchers monitored the dispersion of the rhodamine stain with fluorometers and measured surface speeds and directions using Lagrangian drifters. Bringing the information obtained together with previous knowledge of water circulation in the delta made it possible to establish a high-resolution three-dimensional numerical model capable of simulating the behaviour of irrigation water currents in different wind conditions.

The researchers’ efforts have resulted in the systematization of a methodology that had already been tested in the nearby Alfacs Bay in 2019, when, as part of the European project SEAFOODTOMORROW, in which IRTA participated, rhodamine was used to trace the dispersion and diffusion of waste water discharged from the treatment plant in Sant Carles de la Ràpita. “In Fangar Bay, we have taken a methodology originally proposed for defining buffer zones for bacteriological contamination and applied it to chemical contamination,” says Margarita Fernández in summary.

The experiment with rhodamine and its continuation with the bivalve mortality risk assessment project have been jointly funded by the European Maritime and Fisheries Fund and the Government of Catalonia’s Directorate-General for Maritime Policy and Sustainable Fisheries, and carried out with the collaboration of the Federation of Mollusc Producers of the Ebro Delta (Fepromodel) and the irrigation user community Comunitat de Regants – Sindicat Agrícola de l’Ebre.

Cultivation area of bivalve. Source: IRTA