Characteristics of nutrients and eutrophication assessment in water quality of Gracilaria tenuistipitata and Gracilaria asiatica growing sea area

ObjectiveIn order to provide reference for Gracilaria sellection for construction of marine ranching and artificial restoration of eutrophic sea area,the present experiment was conducted to investigate the characteristics of nu-trients and the level of eutrophication in Gracilaria growing sea area.MethodG. tenuistipitata and G. asiatica growing in sea area of Wuli Village and Xiacun Village,Daguansha,Beihai,Guangxi were research objectives. Water quality indica-tors including ammoniacal nitrogen(NH4+-N),nitrate nitrogen(NO3--N),nitrite nitrogen(NO2--N),labile phosphate(DIP) and chemical oxygen demand(COD)were monitored annually. Single factor pollution index method and eutrophication evaluation method were applied to investigate characteristics of nutrients and the level of eutrophication of water quality for G. tenuistipitata and G. asiatica growing sea area combining nitrogen-phosphorus ratio(N/P).Result67%propor-tion of inorganic nitrogen(DIN)in month basis and 83%proportion of DIP in month basis monitoring data exceeded the fourth grade seawater quality in G. tenuistipitata growing sea area,75%proportion of COD monitoring data was in accor-dance with the third and fourth grade seawater quality. Annual variation of N/P ranged from 3.46 to 18.14,and the mean value was 9.05,92%proportion of N/P in month basis were lower than the Redfield value. The main characteristics of nu-trients was nitrogen-limited. Eutrophication index(Ei)ranged from 11.06 to 54.42 with average of 35.04,presented severe eutrophication state. The rank of aveareg Ei for various seasons was:autumn(54.42)>summer(41.95)>winter(32.75)>spring(11.06). In G. asiatica growing sea area,100%proportion of DIN and DIP in month basis monitoring data matched the first and second grade seawater quality,100%proportion of COD in month basis monitoring data was in accordance with the third and fourth grade seawater quality. Annual variation of N/P was 1.22-85.45 with average of 26.24,58%proportion of N/P in month basis was lower than the Redfield value. The main characteristics of nutrients was phosphrous-limited. Annual change of Ei ranged from 1.82 to 12.75,and the eutrophication states were various(poor,mild,moderate, serious and severe). The rank of average Ei for various seasons was:summer(12.75)>autumn(4.91)>spring(2.13)>winter (1.82).ConclusionDIN and DIP are the main nutrients for wild G. tenuistipitata and G. asiatica populations growing in Beihai,Guangxi. G. tenuistipitata and G. asiatica can grow in sea water with several eutrophication conditions. They are both suitable speices for construction of marine ranching and artificial restoration of eutrophic sea area,but resistance of G. tenuistipitata to eutrophic water is stronger than G. asiatica.