The Ebro delta is a complex ecosystem in which natural dynamics coexist with production activities, such as fishing, aquaculture and agriculture. To preserve that delicate balance, it is essential to know the effects of such activities on 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 that can harm biodiversity. With that in mind, the Agrifood Research and Technology Institute (IRTA) and the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) have developed a model to determine the dispersion pattern of discharges in the El Fangar bay and minimise their impact on bivalve aquaculture. Using WT rhodamine, a red, water-soluble and harmless dye, researchers traced the effluents from the discharges and identified the areas most affected by them. The experiment will provide further knowledge of how freshwater 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 will also pave the way for a definitive reorganisation of aquaculture activities in the bay, based on the establishment of an approximately 80-hectare buffer zone in which no bivalve cultivation will be allowed.

The map of affected areas was produced using a complex hydrodynamic model developed by UPC and IRTA researchers. “Before designing solutions, it was essential to understand the bay’s physical behaviour and to anticipate different eventualities. Combining on-site observations, numerical modelling and the results of our experiment with rhodamine enabled us to broaden our hydrodynamic knowledge of the bay,” remarks Manel Grifoll, a researcher from the Maritime Engineering Laboratory (LIM), who is linked to the Barcelona School of Civil Engineering (ETSECCPB) and the Barcelona School of Nautical Studies (FNB). To that end, on both the day of the experiment (9 June) and the following day, the researchers monitored the dispersion of the rhodamine stain with fluorimeters and measured surface water speed and direction using Lagrangian drifters. Bringing this information together with previous knowledge of water circulation in the Ebro Delta area made it possible to establish a high-resolution three-dimensional numerical model capable of simulating the behaviour of irrigation water currents in the bay in different wind conditions. The study also involves LIM-IPC researcher Manuel Espino.

Mollusc protection protocol
In addition to studying the situation in the Ebro Delta, one of the aims of the researchers was to establish a protocol for applying the same protective measure in other areas in which 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. The results were presented on 29 September 2021 at a transfer session attended by representatives of Spain’s Ministry of Agriculture, Fisheries and Food; Catalonia’s Ministry of Climate Action, Food and Rural Agenda; and aquaculturists and shellfish farmers from the Ebro region and elsewhere.

The experiment with rhodamine and its continuation with the bivalve mortality risk assessment project have been co-funded by the European Maritime and Fisheries Fund and the Catalan Directorate-General for Maritime Policy and Sustainable Fisheries, and carried out jointly 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.

Determining the mortality origin
The project began in 2020, following several episodes of extraordinary mortality among adult oysters of marketable size in some of the El Fangar bay’s farms in the previous years. Those losses, which were as high as 50% in some cases, were not associated with the most usual causes, such as the herpes virus or high temperatures. Some of the aquaculturists affected saw a connection with the discharge cycles of irrigation water from rice fields, which can contain pollutants especially in May and June. Subsequent observations by IRTA and UPC researchers also confirmed a spatial correlation between the areas with the highest mortality and those with the most direct contact with the effluents discharged.

However, according to scientists, 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, the IRTA’s team will continue working to determine what caused the deaths. As part of a study scheduled to run until 2023, different pathogens will be isolated and inoculated into healthy oysters to assess risks of chemical and microbial origin and take subsequent measures to mitigate their impact.