| Eutrophication is the main eco-environmental problem in many aquaculture waters worldwide.The growth and metabolism of shellfish which affects the carbon and nitrogen budge in semi-enclosed bay through ingestion,calcification,excretion and biological deposition is a hot research topic in the field of environmental geology and marine environmental science.This paper focused on Crassostrea hongkongensis,Qinzhou Bay as the typical study area where eutrophication is relatively serious.Through controlled laboratory condition and the method of Alkalinity anomaly technique,the salinity of seawater and three age groups were distinguished,and the different ages were quantified.The carbon and nitrogen distribution in the growth and metabolic process of Crassostrea hongkongensis with different seawater salinity was analyzed,and the carbon and nitrogen budget model of Crassostrea hongkongensis was constructed.The budget model was applied to the Maowei sea(inner bay of Qinzhou Bay)in the intensive distribution area of oyster aquaculture activities in the near river,to calculate the high-density subsidence and drainage of the Maowei sea/floating drainage to preserve the seedlings(corresponding to the first age group and low salinity of the indoor experiment),floating The net flux of nitrogen budget in one year of row-growing(corresponding to the second instar and medium salinity in the indoor experiment)and floating row and fattening(corresponding to the third instar and high salinity in the indoor experiment),and the terrestrial nitrogen input flux.Comparing the net fluxes of nitrogen in the tidal waters of the bay and the nitrogen diffusion fluxes in the bottom sediments of the inner bays,the contribution of oyster aquaculture in the Maowei sea to the improvement of eutrophication in the inner bays was estimated.The main research conclusions are as follows:(1)The results of controlled laboratory condition show that the feeding rate per unit mass of Crassostrea hongkongensis gradually decreases with the increase of shellfish age.The feeding rate of a single individual increased with the increase of the shellfish age.The single individual feeding rate of the first-age oyster was0.47 times that of the third-age Crassostrea hongkongensis.The unit mass biodeposition rate of Crassostrea hongkongensis gradually decreased,but the individual biological deposition rate of Crassostrea hongkongensis increased gradually with the increase of shellfish age.(2)Using the method of Alkalinity anomaly technique,the diurnal changes in the physiological and metabolic activities of Crassostrea hongkongensis of three ages under three seawater salinity gradients of 17‰,21‰ and 25‰ were studied.The results show that the calcification rate of Crassostrea hongkongensis has a certain rhythm,and the calcification rate in the afternoon is lower than that in the morning and evening.With the increase of salinity,the oxygen consumption rate,ammonotelic rate,calcification rate and respiration rate of Crassostrea hongkongensis increased first and then decreased.The physiological metabolic activity of oyster of all ages reached the peak when the salinity was 21‰.The physiological metabolic activity per unit mass of Crassostrea hongkongensis decreased with the increase of age.(3)According to the growth and metabolism results of the first two parts,the results of calculating the carbon and nitrogen budgets of Crassostrea hongkongensis cultivation activities in Qinzhou Bay show that: in the one-year cultivation cycle,Crassostrea hongkongensis absorbed and utilized21524.02 t C in the water body,forming three oysters.Carbon burial exports – 3259.82 t of shell carbon sequestration,374 t of soft-body carbon sequestration and 4077.69 t of biological sediment,respectively,35.83% of the carbon was fixed by oyster physiological metabolic activities;calcification released 1892.71 t of carbon dioxide back to the environment.At the same time,during the one-year aquaculture cycle,Crassostrea hongkongensis absorbed and utilized 1776.73 t of nitrogen in the water body,and finally fixed 570.97 t of nitrogen(including 76.72 t of soft nitrogen fixation and 494.25 t of biological sediment),and produced967.89 t of excretion.Inorganic nitrogen returns to the environment.(4)For the Maowei sea in the inner bay of Qinzhou Bay,where nitrogen exceeds the standard currently,the terrigenous input to the Maowei sea is 8581.44t/year,and the nitrogen input flux generated by the diffusion of sediments from the bottom of the Maowei sea to the water interface 44.14 t/year,the total net input nitrogen flux is 8625.58 t/year;the net output nitrogen flux of Crassostrea hongkongensis growth and metabolism is 391.20t/year,and the Qinzhou Bay hydrodynamic-water quality model simulation calculation obtains the tidal effect of nitrogen in the Maowei sea The net output flux was 722.19 t/year.The nitrogen budget model indicated that the total nitrogen fixation rate of shellfish to terrigenous input and bottom sediment diffusion to the water interface through growth and metabolic activities was 4.56%. |
