Efficiency of Daphnia magna in removal of green microalgae cultivated in Nordic recirculating aquaculture system wastewater

The increase of global aquaculture production has boosted the development of recirculating aquaculture systems not only because they reduce water use but also provide opportunities for waste management and the use of released nutrients. The dissolved nutrients can be efficiently removed from recirculating aquaculture system wastewater by microalgae, and microalgae can be harvested from the wastewater with low costs by zooplankton such as Daphnia. The purpose of this study was to evaluate the feasibility of using microalgae and Daphnia for bioremediation of recirculating aquaculture system wastewater in Nordic conditions. We evaluated the growth and filtration efficiency of the waterflea Daphnia magna on four green microalgae species cultured at 17 ± 0.3 °C in recirculating aquaculture system wastewater as compared to microalgae medium. There was no difference in the growth of Daphnia fed with Monoraphidium griffithii and Selenastrum sp. cultured in either media, while with Chlamydomonas reinhardtii and Haematococcus pluvialis growth of Daphnia was slower when cultured in wastewater than in algae medium. Higher filtration efficiency was achieved with the Daphnia density of 100 individuals L−1, than with higher tested densities (200 and 300 ind. L−1). After 48 h, Daphnia had removed 80% of M. griffithii, 70% of H. pluvialis, but only 20% of Selenastrum sp. from the wastewater. Phosphate was re-released to water after 72 h feeding on Selenastrum sp., but not when feeding on M. griffithii. Taken together, of the four tested green microalgae, M. griffithii was found to be the most suitable microalga species for the microalgae-Daphnia bioremediation system. The results support the potential of using microalgae and Daphnia for bioremediation and for producing biomass in Nordic recirculating aquaculture system wastewater.