Algae and herbivores combat eutrophication
Aquaculture is an increasingly important part of Europe's prominent fishing industry. Fish farm effluent, however, can negatively impact local water quality and often causes eutrophication. Marine scientists from Europe, the Middle East and China combined their talents to design an ecologically sound solution. Funded by the INCO 2 Programme, the ZAFIRA project (Zero discharge aquaculture by farming in integrated recirculating systems in Asia) attempted to fit the IMTA concept (Integrated Multitrophic Aquaculture) to East Asian aquaculture. This involved collaboration between the University of Wageningen (AFI-WU), the University of Kent (IFM), Israel Oceanographic and LImnological Research (NCM), Shanghai Fisheries University (SFU) and the University of Dalian (DFU). Participants introduced algae, specifically the diatom Navicula lenzii, to the effluents of an intensive fish farm to remove nitrate and phosphate. The performance of this process was optimised by applying a sequential-batch culture system in which the harvest cycle was adjusted according to the time of year. Diatom production was further enhanced by adding silicate and iron chloride to the effluent treatment tank. Measurements indicated significant uptake of nitrate and phosphate by the algae, though not all sequestered nutrients were subsequently retained. In the next phase, brine shrimp (Artemia salina) consume the diatoms. The National Centre for Mariculture (NCM) obtained the best results, as indicated by specific growth rates, nutrient retention and food conversion rates, in indoor and small scale outdoor tanks. The experience acquired during ZAFIRA highlighted specific challenges that must be addressed in order for such systems to become operationally feasible outdoors. For instance, sufficiently large volumes are required to produce the necessary amounts of algae. In addition, other filter feeders, such as oysters and abalone, could be the key to boosting nutrient removal and need to be investigated.
