| Balancing the global carbon budget is always a hot spot in research on global carbon cycle. According to recent studies, karst system is actively join in global carbon cycle and make nonnegligible contributions to the evaluation of global carbon dudget, but the stability of carbon sink in karst system is still uncertain. However, it will be resolved when a new concept "Terrestrial aquatic biological carbon pump" is introduced into karst carbon cycle. In this case, Reservoirs as the most important artificial water bodies provide the essential space for better operation of the concept in land water bodies. Thus, what’s the function of karst reservoir in resolving the stability problem of karst carbon sink? How much of carbon can this kind of mechanism retrieve? which factors will influence the carbon sink effect? All the questions above are waiting for being resolved, and what we have done aims to these questions. Hoping for that the results of our research can provide the scientific basis for the accurate evaluation of karstification on carbon cycle, and provide the technical support for the national policy on reduction of carbon emission and increase of carbon sink.We chose two study areas with different geological settings in Guangxi, southwest China. One is Dalongdong reservoir in Shanglin county (maximum depth:17.5m), the drainage basin of which is covered by carbonate rocks,.The other is Wulixia reservoir in Xing’an county (maximum depth:38m), where the outcrop of stratum is mainly silicate. Field measurement and sampling of water, sediments were carried out in July and August 2013, and March, July and August 2014 in different study areas under different meteorological conditions:The research time covered rainy season, dry season and heavy rainfall runoff process. Field measurement parameters include water temperature (T), pH, conductivity (EC), dissolved oxygen (DO), transparency, turbidity (Tb), chlorophyll a (chl a), HCO3-and Ca2+. Some parameters including anion-cation,compositions, TOC,δ13CDIC in water samples, and mechanical composition, TOC, TN,813COC in sediment samples have been tested in the laboratory.Experimental results show that1. Carbon exchange across water-air interface. Dalongdong reservoir is the CO2 source of atmosphere both in rainy season(CO2 efflux:12.48 mg/(m2/h)) and dry season (CO2 efflux:68.17 mg/(m2/h)), which it become weaker in rainy season. On the contrary, Wulixia reservoir is the CO2 sink in rainy season (CO2 efflux:-19.72 mg/(m2/h)) and storm event(CO2 efflux:-2.51 mg/(m2/h)), but it’s smaller in the latter.2. Dissolved inorganic carbon (DIC). Along the flow path, average DIC concentration of Dalongdong reservoir in rainy season (from 2.78 to 3.55 mmol/L, the whole reservoir average is 3.41 mmol/L) and Wulixia reservoir under the influence of heavy rainfall and runoff (0.91 mmol/L) increases from reservoir tail to dam. Inversely, DIC concentration decrease from reservoir tail to dam in Wulixia reservoir in rainy season (from 1.00 mmol/L to 0.88 mmol/L, average 0.92 mmol/L) and Dalongdong reservoir in dry season (from 3.63 to 3.33 mmol/L, average 3.43 mmol/L)In vertical profile, the average DIC concentration has been increasing from water surface to 10m depth in Dalongdong reservoir (from 2.31 to 4.45 mmol/L) and to the bottom in Wulixia reservoir (from 0.85 to 1.05 mmol/L) in rainy season. But when the water depth greater than 10m, even to the bottom, in Dalongdong reservoir, DIC shows decrease trend (from 4.45 to 3.69 mmol/L). The DIC distribution in Wulixia reservoir vertical profile under the influence of heavy rainfall show decrease trend, which is inverse with its in rainy season (from 1.01 to 0.83 mmol/L). The variation ranges of DIC in Dalongdong reservoir in dry season gets smaller, comparing with it in rainy season (from 3.60 to 3.20 mmol/L).3. Organic carbon in sediments. The average organic carbon content in surface sediment in Dalongdong and Wulixia reservoir are 2.01% and 1.34% respectively, the former is 1.5 times higher than it in the latter, which may imply that the former has a higher primary productivity. The evidences from mechanical composition, T/N and 813COC show that the sediments in Dalongdong reservoir is attributed to the autochthonous source by the phytoplankton photosynthesis. However, organic carbon is attributed to allochthonous source and there are no obvious variation regular pattern along the flow path in surface river-fed Wulixia reservoir under the great influence of heavy rainfall runoff.4. Factors controlling reservoir carbon cycle. (1) Geological settings, like lithological characters, play a fundamental role in reservoir carbon cycle. The DIC concentration of Dalongdong reservoir is about 4 times higher than that in Wulixia reservoir, which provide excess DIC to provoke CO2 degassing from surface water in Dalongdong reservoir at whole year after some DIC was utilized by phytoplankton to produce autochthonous sediment Because of the lack of DIC supply in Wulixia reservoir, it still uptakes CO2 from atmosphere even under the heavy rainfall runoff process, but it has lower productivity and the sediment is from allochthonous source. (2) Uneven heat distribution and thermal stratification in reservoir in rainy season affect the distribution of plankton, the plankton photosynthesis in rainy season and near surface part with high water temperature and strong solar radiation are stronger than that in dry season and near bottom part. (3) The distribution of DIC can be influenced by the distribution of phytoplankton which plays a vital role in sediment formation with autochthonous organic matter. (4) Recharge processes of reservoirs could influence the organic matter deposition directly. In Dalongdong reservoir, a karst subterranean river-fed reservoir, the sinking of suspended material in subterranean river path almost finishes before the groundwater discharge into reservoirs. Organic matter depositions in the bottom of reservoirs are mainly from planktons’ actions in rainy seasons. The character is more obviously in reservoir downstream areas. On the contrary, there is no clear rule in surface river-fed type reservoir, Wulixia reservoir, which is greatly influenced by precipitation and runoff.5. The carbon cycle model in subtropical reservoir. In karst reservoirs, phytoplankton concentrates in the top layers of reservoirs, photosynthesis operates effectively and completely with sufficient DIC. This process produce a lot of oxygen, and DO reach to saturation even supersaturation in the water, then a large amount of organisms are produced and deposited to the bottom. DIC is consumed in this processes, which leads the chemical reaction of carbonate systems trend to precipitation. Carbonate precipitations produced in top layers migrate to deep layers, so that the bottom water body of reservoirs also keeps a neutral or alkaline environment. Carbonate precipitation is hard to be dissolved in this environment, so it could be keep stable. However, the phytoplankton photosynthesis in silicate reservoir is limited for the lack of DIC supply, which correspondingly lead to less organic matter is produced and allochthonous organic matters dominates the reservoir sedimentation. |
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