A field scale evaluation of Gracilaria lemaneiformis co-cultured with Crassostrea gigas as a nutrient bioextraction strategy in Yantian Bay, China.

Abstract Nutrient bioextraction using Gracilaria lemaneiformis co-cultured with Crassostrea gigas in a eutrophic, intensive mariculture area was evaluated over 49 days in Yantian Bay, which is an enclosed bay within Sansha Bay, China. The amount of nitrogen removed by G. lemaneiformis was 4.07 ± 0.25 kg and 4.49 ± 0.57 kg per ton (FW) at depths of 0.4–0.8 m and 1.0–1.2 m, respectively. The ranges of ammonium-nitrogen (NH 4 -N), nitrite-nitrogen (NO 2 -N), nitrate-nitrogen (NO 3 -N), and inorganic phosphate (PO 4 -P) reduction efficiencies in the co-cultured area were 39.22–80.34%, 4.70–26.14%, 13.42–48.84%, and 25.33–41.36%, respectively. The total wet weight of harvested G. lemaneiformis was 5.44 and 3.92 times greater than the initial mass (500 g FW/m), with highest specific growth rates (SGR) of 4.67 ± 0.21%/d and 3.44 ± 0.18%/d at 0.4–0.8 m and 1.0–1.2 m, respectively. A total of 3.11 kg of nitrogen could be removed from the co-cultured area per ton of C. gigas harvested with a total length (TL) of 80 ± 12 mm. Based on the nitrogen balance in this co-cultured system, the largest proportion of nitrogen removal from the co-cultured system was the harvest production of G. lemaneiformis (25.44%), followed by C. gigas (8.27%). The quantity of nitrogen excreted and metabolized by C. gigas was 33.40 kg, which could be entirely assimilated by the G. lemaneiformis during the experimental period. Based on these results, at least 6.76 tons of G. lemaneiformis should be co-cultured per ton of C. gigas (77–80 mm TL) to remove the nitrogen produced by C. gigas in this mariculture area. The results of this study indicate that the co-culture G. lemaneiformis with C. gigas is an environmentally-friendly mariculture method which could effectively remove surplus nutrients and reduce the environmental impact of mariculture areas. Highlights • It is environmentally advantageous to co-culture Gracilaria lemaneiformis with Crassostrea gigas in an enclosed bay. • Nutrients flux of the co-cultured system was determined. • A field scale evaluation of G. lemaneiformis in the co-cultured system as a bioextraction strategy was given. [ABSTRACT FROM AUTHOR]