A method for calculating the net carbon emissions of the entire life cycle of lake algae

Abstract

The present invention discloses a method for calculating the net carbon emission of the whole life cycle of lake algae. First, determine the decomposition rate of organic carbon during the algal decay period in the target lake x , the proportion of methane produced by decomposition x 1 , and substitute x and x 1 into Equation #imgabs0# to calculate the change in warming potential caused by the fixation of unit mass of carbon by algae; determine the concentration of water chlorophyll ( Chla ), and use an empirical model to estimate the primary productivity (PP) of the lake: #imgabs1#. Finally, based on the primary productivity of the lake, calculate the net carbon emission of lake algae #imgabs2#. Through the comprehensive calculation of carbon fixation and carbon release in the whole life cycle of algae, the present invention can obtain the net carbon amount in the whole life cycle of algae, providing more objective and reasonable technical support for evaluating the "carbon source" or "carbon sink" function of the lake ecosystem.

Description

Technical Field

[0001] The present invention relates to the field of water ecological environment, and particularly to a method for calculating the net carbon emission of the whole life cycle of lake algae. Background Art

[0002] Climate change is one of the most severe challenges faced by the world in the 21st century. In the past 100 years, the global average temperature has increased by 1.1 °C, and it is expected to continue to rise by 1.0 - 5.7 °C by the end of the 21st century. The excessive emissions of greenhouse gases such as carbon dioxide (CO 2 ), methane (CH 4 ) are the main reasons for climate change. As a hot spot of the carbon cycle, the lake ecosystem plays an important role in regulating the atmospheric greenhouse gas concentration level. Algae are an important part of the lake ecosystem, and the carbon budget of its whole life cycle affects the carbon sink / source function of the lake.

[0003] During the growth stage, algae photosynthetically fix a large amount of CO 2 , which is considered a "carbon sink" process, and eutrophic lakes often experience carbon limitation due to this. According to statistics, the carbon fixation capacity of algae in inland water ecosystems is as high as 4.4×10 9 tons per year. However, only a small amount of organic carbon is permanently deposited and buried in the lake ecosystem, and most of it is eventually decomposed into CO 2 and CH with a higher warming potential 4 , and then returns to the atmosphere again. Some studies have shown that 80% of the organic carbon in lakes is eventually decomposed into CO 2 and CH 4 and released into the atmosphere. The organic carbon fixed by algae belongs to endogenous organic carbon, which is more easily decomposed than terrestrial organic carbon. The photosynthetic fixation of CO 2 during the growth stage of algae and the emission of CO 2 and CH 4 during the decay stage, due to the differences in the decomposition rates of algal-derived organic carbon and the CO 2 / CH 4 ratios in each lake, there is uncertainty in the net carbon amount of the whole life cycle of algae. At present, there is still a lack of a method for calculating the net carbon of the whole life cycle of algae, which restricts the assessment of the carbon sink / source function of the lake ecosystem.

[0004] Object of the Invention

[0005] The object of the present invention is to analyze the carbon budget process of the whole life cycle of algae, and based on the carbon cycle path and final destination of carbon in the whole life cycle of lake algae, quantitatively calculate the net carbon emissions of the whole life cycle of lake algae, so as to provide more objective and reasonable technical support for evaluating the "carbon source" or "carbon sink" function of the lake ecosystem.

[0006] In view of the lack of means for quantifying the carbon budget of algae, the present invention proposes a method for calculating the net carbon emission of the whole life cycle of lake algae.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A method for calculating the net carbon emission of the whole life cycle of lake algae, comprising the following steps:

[0009] (1) Determine the decomposition rate of algal-derived organic carbon and the production ratio of CH 4 Ratio

[0010] Determine the decomposition rate (x) of organic carbon during the algal decay period in the target lake, and the production ratio of CH 4 Ratio (x 1 );

[0011] (2) Net carbon accounting of lake algae

[0012] Substitute x and x 1 into formula (1) to calculate the change in global warming potential caused by the fixation of unit mass of carbon by algae;

[0013] ΔGWP = 3.67x + (1.33k - 3.67)x 1 -3.67 (1)

[0014] The process of calculating the change in global warming potential (GWP) caused by the fixation of unit mass of carbon by algae in formula (1) is as follows:

[0015] ΔGWP = GWP emission -GWP fixation

[0016]

[0017] GWP fixation = 3.67

[0018] In the formula, GWP emission and GWP fixation are the GWPs of algal-fixed organic carbon and carbon emissions respectively, where 3.67 is the CO 2 equivalent conversion coefficient of unit mass of carbon. and are the GWPs of CO 2 and CH 4 respectively, and the specific calculation is as follows:

[0019]

[0020]

[0021] In the formula, 3.67 and 1.33 are the conversion coefficients for converting unit mass of carbon into CO 2 and CH 4 respectively, and the k value is CH on a certain time scale4 GWP. At 20-year, 100-year, and 500-year scales, k is 80.8, 27.2, and 7.3 respectively.

[0022] After simplification, the calculation of the change in warming potential caused by the fixation of unit mass of carbon by algae is as follows:

[0023] ΔGWP = 3.67x + (1.33k - 3.67)x 1 -3.67

[0024] (3) Determine lake chlorophyll and primary productivity

[0025] Determine the chlorophyll (Chla) concentration per unit water area, and use an empirical model to estimate the lake's primary productivity (PP).

[0026]

[0027] The chlorophyll concentration per unit area is converted based on the measured chlorophyll concentration per unit volume (Chla') and the euphotic layer depth (Z eu ) of the water body:

[0028] Chla = Chla' / Z eu (3)

[0029] The estimation of the euphotic layer thickness is based on an empirical model between it and chlorophyll:

[0030]

[0031] When the calculated value of the euphotic layer thickness Z eu is greater than the actual water depth h, then select the actual water depth h; when the calculated value of the euphotic layer Z eu is less than the actual water depth h, then select the calculated value of the euphotic layer Z eu .

[0032] (4) Calculate the net carbon emissions of lake algae

[0033] Based on the lake's primary productivity, calculate the net carbon emissions of lake algae

[0034] Net Carbon = PP * ΔGWP (5)

[0035] The decomposition rate x of organic carbon during the algal decay period described in step 1) is determined based on the sediment deposition rate and the exposure time of dissolved oxygen; the proportion x of methane produced by decomposition 1 is measured using the static chamber method or determined using the thin film boundary layer model.

[0036] Specifically, in step 1), the decomposition rate (x) of organic carbon on the surface of lake sediments during the algal decay period is determined based on the sediment deposition rate and the exposure time of dissolved oxygen:

[0037] x = 1 - (23.3 - 4.39 * log T)

[0038] Wherein, T is the dissolved oxygen exposure time, and is calculated as follows:

[0039]

[0040] Wherein, D is the sediment dissolved oxygen penetration depth, in cm; R is the sediment deposition rate, which is determined by 210 the Pb method:

[0041] R = λ * Z / ln(C0 / Cz)

[0042] Wherein: R is the deposition rate (cm·a -1 ), λ is the 210Pb decay constant (3.11×10 -2 y -1 ), Z is the depth (cm), C0 and Cz are the excess values of 210Pb in the surface layer and the layer at depth Z respectively, and Z / ln(C0 / Cz) is obtained from the slope of the linear fit between the natural logarithm of the excess 210Pb value and the depth.

[0043] CH 4 The proportion x 1 is measured by the static chamber method. The static chamber is cylindrical (bottom area: 330 cm 2 , height: 20 cm), made of plexiglass, equipped with a small fan inside, and plastic foam is pasted around the chamber to provide buoyancy support. After the static chamber is placed on the water surface, the air intake is opened to balance the internal and external pressures. The fan inside the chamber is turned on 10 s before sampling to make the air inside the chamber mix evenly and ensure stable sampling. Every 5 min, 20 mL of gas inside the chamber is extracted with a syringe and stored in a vacuum sample bottle. Sampling is taken 5 times at each sampling point, and the entire gas sampling process is 20 min. The gas samples CO 2 and CH 4 concentrations are analyzed and measured using a gas chromatograph. Based on the linear fit, the CO 2 and CH 4 concentration change rates and

[0044]

[0045] Preferably, in step (1), x 1 can also be determined using the thin film boundary layer model:

[0046]

[0047] F = K gas (C w-C eq )

[0048] In the formula, F is the gas diffusion flux, mol m -2 h -1 ; K gas is the mass transfer coefficient of the gas, cm h -1 ; C w is the gas concentration on the water surface, mol L -1 ; C eq is the theoretical concentration after the gas on the water surface reaches equilibrium with the water, mol L -1 .

[0049]

[0050] Sc, the Schmidt constant of the gas at t °C. For fresh water, it can be calculated by the following formula:

[0051] Sc(C02) = 1911.1 - 118.11t + 3.4527t 2 - 0.04132t 3

[0052] Sc(CH4) = 1897.8 - 114.28t + 3.2902t 2 - 0.03906t 3

[0053] The value of n is -2 / 3 when U 10 ≤ 3.7 m s -1 ; and -1 / 2 when > 3.7 m s -1 .

[0054]

[0055] U 10 is the wind speed at 10 m above the water surface (m s -1 ). It can be converted by monitoring the wind speed U z at z (m) above the water surface:

[0056]

[0057] z, the height (m) where the wind speed is measured; U z , the wind speed at z, m s -1 ; C d10 , the drag coefficient at 10 m, take 0.0013; k, the Von Karman constant, take 0.41.

[0058] The gas concentration above the water surface is directly measured by gas chromatography. After the surface water body reaches equilibrium, the concentration is measured by the shaking equilibrium method. The specific process is as follows: Carefully suck a certain volume of liquid into a 100 ml syringe (without air bubbles), then fill it with a certain volume of high-purity nitrogen or air with a known gas concentration, shake it vigorously for 2 minutes, and after leaving it to stand for 2 hours, analyze the gas components in the headspace by gas chromatography.

[0059] or In the formula, C h is the gas concentration in the headspace after shaking (mol L -1 ); V h is the headspace volume (mL); V w is the water body volume (mL); α is the Henry's constant (mol L -1 atm -1 ); P g is the partial pressure of the gas (atm); β is the Bunsen coefficient (C water / C air ). The conversion between α and β can be carried out through formula (11):

[0060]

[0061] The unit of α is mol L -1 atm -1 , and the value of R is taken as 0.0821 atm L mol -1 K -1 .

[0062] In step (3), the chlorophyll concentration in the water body can be measured by the hot alcohol method. Specifically, filter the water sample with a GF / C membrane, place the membrane in a glass tube, suck 5 mL of 90% hot alcohol, heat it in a water bath for about two minutes, let it stand in the dark for 4 hours and then filter, and measure the absorbance at 665 nm and 750 nm. Calculate the chlorophyll concentration according to C (μg / L) = (value before acid - value after acid) × 27.9 × 1000 × V (extraction) / V (filtration).

[0063] In step (3), the chlorophyll concentration per unit volume of the water body (Chla') can also be measured by the acetone method. Specifically, take 50 mL of water sample (adjusted according to the algae concentration level), filter it through a cellulose acetate membrane with a pore size of 0.45 μm, fully dissolve the filter membrane in 10 mL of 90% acetone solution, extract it at 4°C for 24 hours, centrifuge it at 5000 rpm for 10 minutes, and take the supernatant to measure the absorbance values at wavelengths of 630 nm, 645 nm, 663 nm, and 750 nm (A630, A645, A663, A750 respectively). Calculate from the following relationship:

[0064]

[0065] Among them, V is the volume of the water sample (L); V1 is the volume after the extract is made up to a constant volume (L); σ is the optical path of the cuvette (cm).

[0066] The determination of the chlorophyll concentration in water bodies can also be obtained by means of remote sensing satellites, sensors, etc.

[0067] Beneficial effects:

[0068] Compared with the prior art, the present invention has the following advantages:

[0069] In view of the comprehensive net carbon impact of photosynthetic carbon fixation in the growth stage and carbon emission in the decay stage of algae, in the decay stage of the entire life cycle of algae, algal-derived organic carbon is easily decomposed, often rapidly consuming dissolved oxygen and inducing anaerobiosis in water bodies, promoting the decomposition of more algal-derived organic carbon into CH 4 . Due to CH 4 having a much higher warming potential than CO 2 , the carbon budget process of first absorbing CO 2 and then releasing CH 4 in the entire life cycle of algae may cause the growth and decline of the warming potential. Therefore, algae not only act as a "carbon sink" but also as a "carbon source". When decaying, they decompose to produce CO 2 and CH 4 , and then re-enter the atmosphere.

[0070] By comprehensively calculating the carbon fixation and carbon release in the entire life cycle of algae, the present invention can obtain the net carbon amount in the entire life cycle of algae, providing more objective and reasonable technical support for evaluating the "carbon source" or "carbon sink" function of lake ecosystems. Description of the drawings

[0071] Figure 1 Schematic diagram for accounting the net carbon emission of lake algae;

[0072] Figure 2 Accounting of the net carbon emission of algae in Taihu Lake in 2020. Detailed implementation manners

[0073] The present invention will be further described below with reference to the drawings and embodiments.

[0074] Embodiment 1

[0075] Taking Taihu Lake as the object, the model is described and applied. First, using the sediment deposition rate and the dissolved oxygen exposure time, the mineralization and decomposition rate x of organic carbon in Taihu Lake is determined to be 0.75; using the static chamber method, the methane production ratio x 1 is determined to be 0.13. Substitute x and x 1 into

[0076] ΔGWP = 3.67x + (1.33k - 3.67)x1 -3.67

[0077] Select k as the 100-year scale, that is, k = 27.2, and substitute it to obtain ΔGWP = 3.31.

[0078] Through the hot ethanol method, the chlorophyll concentration per unit volume of water body in Lake Taihu from January to December 2020, Chla', was measured. According to the empirical model of lake chlorophyll and primary productivity, the primary productivity value PP in Lake Taihu from January to December 2020 was calculated by substitution.

[0079]

[0080] The chlorophyll concentration per unit area Chla is converted based on the measured chlorophyll concentration per unit volume (Chla') and the euphotic layer depth of the water body (Z eu ) as follows:

[0081] Chla = Chla' / Z eu (3)

[0082] The estimation of the euphotic layer thickness is based on the empirical model of chlorophyll:

[0083]

[0084] Among them, if Z eu > the actual water depth h, then Z eu = h. After verification, Z eu > the actual water depth h, then Z eu = h = 1.6m.

[0085] Finally, based on the lake primary productivity, the net carbon emissions of lake algae are calculated. Substitute PP and ΔGWP into

[0086] Net Carbon = PP * ΔGWP

[0087] The results are as Figure 2 shown. The net carbon emissions of Lake Taihu algae show a trend of first rising and then falling, reaching the highest in September, reaching 1.2×10 4 mg / m 2 d.

[0088] Table 1

[0089]

[0090]

Claims

1. A method for calculating the net carbon emissions of lake algae throughout their life cycle, characterized in that, it includes the following steps: (1) Determine the decomposition rate of algal organic carbon and the proportion of methane production in the target lake Determine the decomposition rate of organic carbon during the algal decay period in the target lake x , and the proportion of methane produced by decomposition x 1 ; (2) Net carbon accounting of lake algae Bring x and x 1 into Equation (1) to calculate the change in warming potential caused by the algae fixing a unit mass of carbon. Among them, k The value is the CH 4 warming potential value on a certain time scale, （1） (3) Determine the lake chlorophyll and primary productivity Determine the chlorophyll concentration per unit area Chla , and use an empirical model to estimate the primary productivity PP of the lake: （2） The chlorophyll concentration per unit area is based on the measured chlorophyll concentration per unit volume and the euphotic depth of the water body Z eu and is calculated as follows: （3） (4) Calculate the net carbon emissions of lake algae Based on the primary productivity of the lake, calculate the net carbon emissions of lake algae （4） In step (3), when the calculated value of the euphotic layer depth Z eu is greater than the actual water depth h, then the actual water depth h is selected; when the calculated value of the euphotic layer depth Z eu is less than the actual water depth h, then the calculated value of the euphotic layer depth is selected Z eu .

2. The method for calculating the net carbon emissions of lake algae throughout their life cycle according to claim 1, characterized in that, The organic carbon decomposition rate during the algal decline period described in step (1) x is determined based on the sediment deposition rate and the dissolved oxygen exposure time.

3. The method for calculating the net carbon emissions of lake algae throughout their life cycle according to claim 1, characterized in that, The proportion of methane generated by decomposition in step (1) x 1 It is determined by the static chamber method or determined by the thin film boundary layer model.

4. The method for calculating the net carbon emissions of lake algae throughout their life cycle according to claim 1, characterized in that, In step (3), the chlorophyll concentration per unit volume is obtained by the hot alcohol method, the acetone method, or by using remote sensing satellites and sensors.

5. The method for calculating the net carbon emissions of lake algae throughout their life cycle according to claim 1, characterized in that, In step (2) k The value is selected on a 100-year scale, and k = 27.2.

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