| In the present study, the basic issues i.e., the isotopic turnover and fractionationof the sea cucumber Apostichopus japonicus were primarily determined.Subsequently, the source and cycle of the main nutrients in the sea cucumberfarming pond, the reuse and recycling processes of polluting nutrients in the seacucumber polyculture systems with abalone, the microalgae fixation of the expiratorycarbon nutrient (CO2) by the sea cucumber, as well as the C, N, P nutrient budgetsof the sea cucumber under different food quality and body weight were studied.As a result, the biological remediation of the polluting nutrients during the seacucumber farming activities was analyzed and quantified. The main results arelisted as follows:1. Study on the carbon isotopic turnover of the sea cucumber: The carbonisotopic turnover rates of body wall and whole intestine tissues of sea cucumber, A.japonicus with different size were investigated. The result showed that carbonisotopic turnover rate of intestine was faster than that of body wall. The turnoverrate of carbon stable isotope was decreased with the increasing body size of seacucumbers owing to the decrease in the metabolic activities of the largerindividuals. Compared to the role of growth in the carbon turnover, metabolismwas more principal for driving the turnover of carbon stable isotope with thecontribution ranging from80%to90%for intestine and60%to75%for body wall.Analysis on the relationship of carbon replacement to the metabolism of seacucumbers showed that the half-lives of carbon isotope replacement for bothintestine and body wall tissue were significantly related to the oxygen consumption rates of the sea cucumbers.2. Determination of the tissue-diet isotopic fractionation by the seacucumber: The trophic shift (fractionation) for stable isotopes was the theoreticfoundation of the stable isotope ecological studies. In the present study, thefractionation between sea cucumber A. japonicus and its diet was determined.The results showed that the stable isotope enrichment for carbon and nitrogen insea cucumber was1.20‰±0.39‰and1.90‰±0.29‰, respectively, which wassimilar with that of aquatic ammonia excretion organisms.3. Study on the C, N, P nutrient budgets of the sea cucumber under differentdietary proportions of macroalgae and sea mud: Formulated diets containingmacroalgae Sargassum thunbergii and sea mud with5different proportions, i.e.,20:80,40:60,60:40,80:20and100:0were fed to sea cucumber A. japonicus for50days. The effects of macroalgae contents in the diet on C, N, P nutrient budgets of seacucumber were studied. The results showed that the C, N, P nutrient budgets of seacucumber were significantly related to the contents of S. thunbergii in the diet. Thefeed intake rate and faecal production rate of sea cucumber decreased significantlywith the increasing proportion of S. thunbergii in the diet. Metabolic rates i.e., oxygenconsumption rate, ammonium excretion rate, phosphorus excretion rate, the ingestionrates, scope for growth and absorption efficiency of C, N, P nutrients significantlyincreased with increasing proportion of S. thunbergii level in the diet. No significantdifference in the scope for growth and absorption rate of nutrients between differentdiets was observed when the content of S. thunbergii in the diet exceeded60%~100%. Hence, for the adult sea cucumber, the maximum proportion of S. thunbergii tosea mud was60:40, which might not only meet the need of nutrient requirements ofsea cucumber for growth and metabolism, but also reduced the waste release resultedfrom sea cucumber farming activities.4. Study on the C, N, P nutrient budgets of the sea cucumber under different body weight: The C, N, P nutrient budgets of5groups of sea cucumber A. japonicuswith different dry body weight i.e.,0.63g±0.08g,1.22g±0.06g,1.55g±0.16g,2.88g±0.23g and7.66g±0.65g were studied. The results showed that the C, N, Pnutrient budgets were significantly related to the body weight of the sea cucumber.The oxygen consumption rate, ammonium excretion rate and phosphorus excretionrate increased with increasing body weight of sea cucumber while the oxygenconsumption rate, ammonium excretion rate and phosphorus excretion rate per unit ofbody weight changed in the opposite trend. The feed intake rate and faecal productionrate per unit of body weight, ingestion rate per unit of body weight, the scope forgrowth per unit of body weight of C, N, P nutrients decreased with increasing bodyweight of sea cucumber. However, there was no obvious correlation betweenabsorption efficiency of C, N, P nutrients per unit of body weight and the body weightof sea cucumber. Furthermore, no significant difference in the absorption rate per unitof body weight of nutrients between different body weights was observed.5. Study on the seasonal changes in the food uptake by the sea cucumber:The present study investigated the seasonal changes in the C and N stable isotopevalues of the typical food sources of sea cucumber A. japonicus, including particulateorganic matter (POM), macroalgae, benthic microalgae and animals such as nematodeand copepod and quantified the relative contributions of the various items to the fooduptake of sea cucumbers by means of stable isotope technique. The results showed thesignificant changes in the stable isotope values of the principal food sources due to theseasonality of the micro-or macroalgae prosperity and to the fluctuation in theenvironmental conditions. As a result, sea cucumber exhibited correspondingalteration in feeding strategy in response to the changes in food conditions.Calculation with stable isotope mixing model revealed that macroalgae was theprincipal food source for sea cucumber throughout the1–yr investigation with thecontribution proportion ranging from28.1%to63.2%on average while the averagecontributions of nematode and copepod, POM, benthic microalgae to the total fooduptake of sea cucumber seasonally ranged from6.3~22.2%,22.6~39.1%,2.8~6.5 %and2.8~4.2%, respectively.6. Study on the optimization of co-culture system of sea cucumber withabalone: Juvenile sea cucumber A. japonicus (1.84g±0.01g) and juvenile charmabalone Haliotis discus hannai (2.58g±0.11g) were co-cultured in a static culturesystem in laboratory with different initial sea cucumber and abalone biomass ratiosi.e.,0:0,0:100,30:70,50:50,70:30and100:0for12weeks. Growth and survival ofsea cucumber and abalone, C, N and P nutrient levels in water as well as in thesediment, and the removal efficiency of culture waste such as uneaten feed and faecesby sea cucumber were observed. The results showed the significant differences in thegrowth, survival and final total biomass of animals, changes of nutrient levels eitherin the water body or in the sediment, and the removal efficiency of culture waste bysea cucumber, due to the different initial sea cucumber and abalone biomass ratios.The final total biomass of sea cucumber and abalone was283.16g±9.94g at theco-culture model in which the initial sea cucumber and charm abalone biomass ratiowas30:70. Furthermore, compared with other co-culture models, the sea cucumberremoved the culture wastes, transferring the organic wastes into sea cucumber productat highest efficiency, and subsequently significantly alleviating the environmentalimpact on the waters as well as bottoms in the farms. Thus, it was suggested that seacucumber co-cultured with abalone at initial biomass ratio of30:70was a promisingco-culture model which might provide an environmental and economic win-winresolution scheme that both reduced pollution and enhanced aquatic production.7. Study on the microalgae fixation of the expiratory carbon nutrient by thesea cucumber: The microalgae fixation of the expiratory carbon nutrient (CO2) bythe sea cucumber A. japonicus was quantified with carbon stable isotope techniques.The results showed that the respiration activities by A. japonicus altered theconcentration as well as carbon isotopic values of dissolved inorganic carbon (DIC) inthe natural sea water significantly under the current experimental conditions. The δ13Cvalue of expiratory carbon by the sea cucumber was-12.92‰±0.04‰, which was obviously lower than that in the natural sea water. As a result, the photosynthesis ofmicroalgae exhibited strong preferential selectivity of expiratory carbon by the seacucumber in the sea water during the48h microalgal incubation. Calculation withstable isotope mixing model revealed that the proportional contribution of expiratorycarbon by the sea cucumber to the total fixed carbon by microalgae was80.67%,while that of DIC in the natural sea water was only29.37%, indicating that mutuallybeneficial mechanisms, which were similar with the “micro-loop” among planktons,might occur between sea cucumber and microalgae. |
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