A Comparative Study of Water Quality and Trophic State Contribution on Natural and Articial Lakes Global Warming Potential

9 Natural lakes and reservoirs are emitters of GHGs in the atmosphere 10 contributing to 31% of the annual CO 2 emissions of those from fossil fuel 11 combustion. Measurements of GHGs emissions in reservoirs demonstrate 12 that hydropower may actually not be as “green” as once thought. It is 13 estimated that emissions from reservoirs may be equivalent to 7% of the 14 global warming potential (GWP) of other documented anthropogenic 15 emissions. Aim of this work is to assess the impact of water quality 16 deterioration and the subsequent increment of biological productivity of a 17 waterbody on GHGs emissions. Therefore, the trophic state, the carbonic 18 GHGs emissions and the GWP of one natural lake domestic wastewater 19 receiver and two different age hydroelectric reservoirs, located in North 20 West Greece, were studied. Gross emissions of CO 2 and CH 4 were in-situ 21 measured using a static floating chamber and specific emissions as well as 22 GWP were calculated. Furthermore, water quality and trophic state were 23 evaluated based on the application of ΥΔΩΡ (hydõr) Water Quality Index 24 and Florida Trophic State Index using physicochemical characteristics 25 measurements. Data statistical interpretation revealed that CH 4 has strong positive correlation with GWP, temperature, water quality and trophic state. 27 There is a seasonal variation of GWP that follows the seasonal variation of 28 CH 4 emissions induced by water temperature. Specific CH 4 emission rate 29 presents the most reliable indicator for assessing the impact of a waterbody 30 in terms of GWP, especially of a hypertrophic one. Water quality and trophic 31 state indices can be used for a rough comparison of GWP between 32 waterbodies with the same climatic conditions. 33

measurements. Data statistical interpretation revealed that CH4 has strong 26 positive correlation with GWP, temperature, water quality and trophic state. 27 There is a seasonal variation of GWP that follows the seasonal variation of 28 CH4 emissions induced by water temperature. Specific CH4 emission rate 29 presents the most reliable indicator for assessing the impact of a waterbody 30 in terms of GWP, especially of a hypertrophic one. Water quality and trophic 31 state indices can be used for a rough comparison of GWP between 32 waterbodies with the same climatic conditions.

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Anthropogenic activities, such as fossil fuel combustion, livestock,

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GHGs emissions from aquatic systems are highly influenced by 50 waterbody's trophic state, which in turn is a function of nutrients availability, 51 usually phosphorous and less often nitrogen, as well as other parameters 52 such as seasonal variations, grazing of phytoplankton by zooplankton, 53 mixing depth of water, etc. 7-11 . According to European Environment Agency 54 12 the trophic status of a waterbody is classified into 5 categories, 55 Oligotrophic, Oligo/Mesotrophic, Mesotrophic, Meso/Eutrophic and 56 Eutrophic. The term Hypereutrophic has been also used for characterizing 57 highly eutrophic waterbodies including artificial lakes mainly the first years of 58 their formation 13,14 . 59 In freshwater reservoirs, carbon dioxide and methane are formed due 60 to organic matter imported from the catchment area or produced due to in the sediments, a portion of which is oxidized to CO2 by methanotrophic 72 bacteria in both water column and sediments 17 . The fraction of CH4, which 73 is oxidized before being emitted to the atmosphere, varies across freshwater 74 aquatic systems depending on oxygen levels and temperature 18 . 75 Since nutrient availability is directly related to organic matter 76 decomposition, monitoring and assessing the trophic state (status) of an 77 aquatic system is essential to evaluate the environmental impact of 78 reservoirs. The trophic status of an aquatic system indicates its 79 environmental health and is expressed by a basic classification scale 80 showing rather its biological productivity than its water quality. Trophic 81 status can be calculated using Trophic State Indices (TSI), combining 82 quality parameters, usually water clarity, algal activity, phosphorus and 83 nitrogen availability 19 .

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On the other hand, water quality assessment of an aquatic system is 85 a more complicated procedure due to the numerous physicochemical and 86 biological parameters that may affect it 20 . The plethora of factors needed to 87 yield a single result of the overall water quality, such as (i) the large number 88 of data necessary for the qualitative evaluation, (ii) the special knowledge 89 and expertise required, as well as the (iii) difficulties arising in combining is the calculation of a Water Quality Index (WQI). A WQI is a number, a 95 scale, a word, a symbol or a color that expresses the water quality of an 96 aquatic system at a specific area in a specific period 22, 23 . 97 According to literature review, there are limited data regarding the 98 correlation of a waterbody's water quality and trophic state with GHGs 99 emissions and subsequently GWP. Aim of this work is to correlate water 100 quality and trophic state indices as well as GHGs emissions with GWP in 101 order to obtain indicators for easily assessing GWP of waterbodies. For this 102 purpose, one natural lake and two hydroelectric reservoirs were studied.

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Physicochemical characteristics and gross emissions of CO2 and CH4 were 104 measured. Water quality and trophic state indices, as well as specific 105 emissions and GWP were calculated and statistically interpreted.

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The research methodology of this work includes the selection of sampling 108 stations and the measurement of water physicochemical parameters, CO2 109 and CH4 emissions in the collected samples. Thereafter, calculation of 110 specific emission rates and GWP was carried out, whereas water quality 111 and trophic state indices were also calculated. The obtained data were 112 statistically processed using SPSS software and interpreted accordingly. respectively, for the production of hydroelectric energy. Both reservoirs are 128 fed with water from Aliakmon River, which is the longest river in Greece 129 (approximately 300 km).

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Ilarion Reservoir is a relatively young and small reservoir downstream 131 Aliakmon River, created in stiff rocky hills, forming a deeper canyon-like lake 132 (terrain slopes between 15° to 45°). It covers a total surface of 22 km 2 with a 133 maximum depth of 65 m. Prior to its impoundment, 23.1% was river, 53.03% 134 was non-forest soil (grassland, rocks, and agricultural land), and 23.96%   For the collection of GHGs emissions, specifically CO2 and CH4, a

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The specific GHGs emission rate was calculated according to 186 Equation (1), using CO2 and CH4 concentration measurements with the 187 static floating chamber previously described.
where: C is the specific gas emission rate (mg m −2 h −1 ); ρ is the gas density .
where: qi is the Sub-index of sample for i parameter; qi,e is the Sub-index of 206 sample for i parameter exceeding permitted value; RWi is the relative weight according to ΥΔΩΡWQI is presented in Table 2.

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Based on the results of overall FTSI, the waterbodies are classified 271 according to their trophic state as "Oligotrophic", "Mid-Eutrophic",

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"Eutrophic" and "Hypereutrophic" and corresponds to "Good", "Fair" and 273 "Poor" water quality respectively (Table 3). A "Good" lake water quality is 274 one that meets all lake-use criteria (swimmable, fishable and supports 275 healthy habitat); a "Fair" lake water quality can be considered highly 276 productive and a reasonable lake for fishing and most water sports, while a 277 "Poor" lake water quality means that probably the lake use criteria are not 278 meet 13 . attributed to water stratification of the lake, due to phototrophic organisms' 320 growth. The relatively high turbidity and color in Zazari Lake as well as its  the differences between the three waterbodies.

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As it can be seen in Table 4, all sampling stations of Aliakmon River 363 basin except Zazari Lake present similar water quality characteristics.

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Statistical analysis of all physicochemical parameters between all sampling 365 stations showed significant differences (t-test, p < 0.05) only in Zazari Lake.

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The similar water quality of sampling stations at Aliakmon River basin is 367 attributed to their common feed. The water quality of the older and considered stabilized Polyfytos

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Reservoir and the younger Ilarion Reservoir is characterized as "Fair" and 385 "Good" respectively, according to ΥΔΩΡWQI (Table 5). On the contrary, in Reservoir presenting better trophic state than Polyfytos (Table 5). This is in  Table 5), as well 419 as the limited biomass impoundment (canyon-like rocky area) indicate that 420 this reservoir has reached its stabilization period, which is in accordance GHGs emission 45 . Domestic pollution of Zazari Lake ceased five years ago.

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Considering the significantly higher CH4 emissions of Zazari Lake, as well 442 as the deteriorated water quality and trophic state compared to that of 443 Polyfytos and Ilarion Reservoirs, it is concluded that Zazari Lake can be 444 characterized as a young artificial reservoir that has not yet been stabilized.

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According to the results of this study, Ilarion Reservoir has entered its 446 stabilization period (lower emissions) six years after its formation, while  The emissions of CO2 and CH4 from Zazari Lake follow the same 492 seasonal pattern with that of GWP, but they were quantitatively higher than 493 those of Polyfytos and Ilarion Reservoirs (

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According to trophic state index, both the six years old Ilarion Reservoir, as 519 well as the forty-three years old Polyfytos Reservoir are classified as 520 oligotrophic through mid-eutrophic reservoirs with "Good" water quality 521 characterization. On the other hand, Zazari Lake, which is considered as a 522 five years old reservoir, is classified as a hypereutrophic Lake with "Poor" 523 water quality characterization.

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The application of the water quality index revealed that water quality of 525 Ilarion Reservoir is better than of Polyfytos Reservoir, characterized as 526 "Good", whereas water quality of Polyfytos Reservoir is characterized as 527 "Fair". Zazari Lake exhibited the worst water quality, characterized as "Poor".