ANALYSIS OF THE SEASONAL IMPACT ON ISOTOPIC BASELINES OF DISSOLVED INORGANIC CARBON ( DIC ) IN COASTAL WATERS SPERMONDE , SOUTH SULAWESI

Stable carbon isotopes have been commonly used as indicators for assessing environmental changes in aquatic ecosystems. They can be used to study the dynamics of organic matter as for understanding the overall functioning of the ecosystem, the connectivity of estuaries with terrestrial and marine coastal habitats. The objective of this study is determining the seasonal natural effects over isotopic (13C/12C) baselines in monitoring CO2 storage in dissolved inorganic materials in Spermonde waters on the west coast of South Sulawesi to some outermost island. The results show that the stable carbon isotopic of DIC (d13C-DIC) in the wet season varied between -5.36 ‰ and -7.74 ‰. These value are higher than on dry season (-4.34 ‰ to -6.82 ‰). Likewise, DIC concentration in the rainy season ranged between 9.5 mg C/L and 11.7 mg C/L, while in the dry season it varied from 8.5 mg C/L to 9.3 mg C/L. The d13C-DIC and DIC concentrations decreased towards offshore, up to some of the outer islands. Increasing in the d13C-DIC in Spermonde waters indicate that the DIC is most likely enriched by atmospheric CO2(g), which is outnumbered those of aquatic photosynthesis. This study shows that different levels and composition of d13C-DIC stretch along different rivers are attributable to the varying landscapes and quality of organic matters.


I. INTRODUCTION
River estuaries are generally considered to be the most sustainable-derived carbon to the atmosphere as CO2.To date, CO2 has received greater attention worldwide due to its involvement in the biogeochemical cycles of open coastal and marine areas.Biogeochemical cycles that occur in coastal ecosystems can affect the quality of the waters which causes biogeochemical functions and biological community structure of coastal waters to change (Borges, 2005;Borges et al., 2005;Carpenter and Brock, 2006;Chang, 2009;Robson, 2008).
Oceans have a very important role in reducing global warming or increasing atmospheric CO2 concentrations.Organic materials in the ocean are transported by physical and biological processes (Karim, 2008;Brunet et al., 2009;Schlitzer, 2000) and are decomposed by non-photosynthetic organisms (heterotropic respiration) and eventually lifted and returned to the atmosphere (Atkins et al., 2013;Cai, 2011;Hancke et al., 2008).Some of these carbon deposits as CaCO3 in the sediment and the rest is dissolved in sea water, then joins DIC (McCarthy, 2006;Mook, 1974).The stable carbon isotope composition of DIC [( 13 C/ 12 C) DIC] depends on the dissolution of minerals by CO2 biogenic origin (DIC formation) and photosynthesis (DIC consumption) and carbon isotope exchange between atmospheric CO2 and DIC (Brunet et al., 2009;Herzschuh et al., 2010;Hill and Middleton, 2006).
In measuring the isotope ratio of 13 C/ 12 C, the GDP reference standard (Pee Dee Belemnite) is used in various types of natural matrices.If the positive value shows, the sample has a larger isotope composition than standard (more enriched), whereas if the negative value indicates that the sample isotope composition is smaller than the standard.Carbon sources in the estuary region have varying isotope ratios due to the mixing of dissolved organics from the land such as plankton, terrestrial plants, algae, and marine plants (Peterson and Fry, 1987;Mladenov et al., 2005;Bouillon, 2008;Cai, 2011).
Variations in stable carbon isotope ratios in coastal and marine waters become one particular (spatial) location character that affects the level of sources in the waters.The purpose in this study is determining spatial and seasonal natural effects over isotopic ( 13 C/ 12 C) baselines in monitoring CO2 storage on the west coast of South Sulawesi to some of the outer islands in inorganic materials dissolved in Spermonde waters.

Study Site
Seawater samples for Dissolved Inorganic Carbon (DIC) and δ 13 C-DIC were taken at ten stations.The study locations were in the estuary waters of the major rivers namely the Tallo and Pangkep coasts to some of the outermost islands (estuary of the Tallo river, Barrang Lompo island, Bone Tambung island, Langkai island and Kapoposang island) and Pangkep coast (Pangkep river mouth, Laiya island, the island of Sarappo Keke, the island of Kondong Bali and the island of Kapoposang) (Fig. 1).

Measurements and Analyses
Water quality is an important indicator in knowing the fertility level of waters and is also used as an indicator of water pollution.Measurement of water quality includes pH and water temperature data that is carried out for two times sea voyages, representing the dry season (June 2017) and the rainy season (Feb 2018).The partial pressure of CO2 (pCO2) of the water column was calculated using the OCMIP Model (Ocean Carbon Cycle Model Intercomparison Project) developed by Orr et al. (1999).The atmospheric pCO2 is measured at the time of sampling using CO2 meters.To measure the value of δ 13 C-DIC, the pre-weighed GF/F 0.7 µm using the vacuum pump (200 mmHg) and pre-weighed with at least 2 L of water collected per sample.Shortly after sampling, 0.02 ml of HgCl2 was added, to stop biological activity (Dickson et al., 2007).Samples were kept in the dark and shipped on ice, immediately transported to the laboratory until the analysis.Preparation and isotope analysis δ 13 C-DIC were done by taking 1 liter of sea water, to add BaCl2 10%, the reaction results are then left to form carbonate deposits.The deposits obtained are then filtered and dried in an oven at a temperature of 50-60 o C. The dried BaCO3 deposits were then reacted with 100% H3PO4 in a tube under vacuum.The CO2 gas released in the test tube is then captured using liquid N2 with a temperature of -195oC at the condition of the vacuumed tube, then the CO2 gas obtained is analyzed the composition of the 13 C isotope ratio using EA-IRMS (elemental analyzer-isotope ratio mass spectrometry); and are expressed relative to the GDP standard (Craig, 1957).Analytical precision is ± 0.05 and measurements can be reproduced to ± 0.1 ‰.
Furthermore, DIC was measured using the titration method (Giggenbach and Goguel, 1989), with the principle of basing on pH changes after HCl and NaOH were added.DIC is obtained from the sum of HCO 3-and CO3 2-in units of µmol/kg.DIC concentrations were obtained simultaneously with isotope values through linear regression from the standard internal DIC used in this study.The concentration of each sample is proportional to the area of chromatography obtained during isotope measurements.

III. RESULTS AND DISCUSSION
The average isotopic values, other chemical and environmental variables at the western coastal waters of South Sulawesi.Overall, from the two seasons of data collection, presented in Table 1.Coastal waters in the study locations including estuary waters with hydrographic conditions (water temperature and pH) in different seasons did not show significant differences.The water temperature in coastal and marine waters in both seasons is relatively the same, with an average value (± standard deviation) of 30.62±1.77 in the dry season, and 32.00±0.88 in the rainy season.Coastal water temperatures sometimes have higher temperatures; due to the effect of hotter land, while the temperature of the water from the estuary to the sea, has a lower temperature, this is likely due to the encouragement of sea water masses from offshore to the bay.
The results of pH measurements in coastal and marine waters showed the average (± standard deviation) pH values in the dry and rainy seasons were 7.29±0.28and 7.32±0.36,respectively.The pH value is close to the normal pH; this is because it receives input from the land, namely household wastes, and acidic industries through the river.When approaching the lower pH values, the concentration of DIC in water rises, and the δ 13 C-DIC decreases (less negative).Increase in sea surface temperature can reduce the solubility of CO2 waters so that it can also cause a decrease in the pH value of the waters.
Furthermore, measurements of the isotope composition of dissolved inorganic carbon (δ 13 C-DIC) are carried out, which are the main indicators of changes in the aquatic surface system.In determining season and location factors, we compare the results obtained from each sampling.The global average value of carbon isotope composition in DIC (δ 13 C-DIC) at the west coast waters of South Sulawesi is using two data collection season's ranges from -4.34 ‰ to -7.74 ‰ while the DIC concentration ranged from 8.5 mg C/L to 11.7 mg C/L.Isotope ratio values of δ 13 C-DIC and DIC in coastal and marine waters vary between seasons and locations (Table 1).Isotope ratio value δ 13 C-DIC and concentration of DIC concentration in the rainy season (March 2018) is higher than the dry season (June 2017).The isotope ratio value δ 13 C-DIC with mean values (± standard deviation) ranging from -5.36 ‰ to -7.74 ‰ (-6.54 ‰ ±0.7) V-GDP in the wet season and around -4, 34 ‰ to -6.71 ‰ (-5.53 ‰ ±0.9) V-GDP in the dry season.While the DIC concentration with an average value (± standard deviation) ranged from 9.5 to 11.7 mg C/L (10.59 mg C/L±0.7) in the wet season, and in the dry season it was 8.5 to 9.3 mg C/L (8.91 mg C/L±0.2) (Figure 2).
The isotope ratio value of δ 13 C-DIC starting from the estuary to some of the outer islands has a smaller value and a less negative tendency due to the period of sampling and the occurrence of extreme climate events lately.In the dry season, there is high evaporation of water and vegetative stress resulting in a value of δ 13 C -DIC less negative as expected from respiration.Unlike the increase in photosynthesis in the aquatic system which will remove 12 C.As a result, the residual DIC will increase (enrich) the 13 C value (Table 1).Besides being caused by a dry season that produces less negative isotope values, this is also due to the dominance of biological processes and the gradual increase in the partial pressure of CO2 in water (Barth and Veizer, 1999) as shown in Figure 3.The mean value for the calculated partial pCO2 was 22.93 mgL -1 .There is no unreasonable statistical difference observed at the mean value of pCO2, with different sampling times (p-value = 0.0957).These results indicate that pCO2 is not affected by seasonal effects.However, the temporal pattern has a higher value in the rainy season (Figure 3).
Based on the pCO2 value of the sea and the atmosphere, the nature of waters can be determined as a sink or source of CO2.Water acts as a 'source' or releases CO2 into the air/atmosphere if the pCO2 value is higher than the atmospheric value (positive value) because there will be a flow of CO2 from the water into the atmosphere.Instead, it acts as a sink of CO2 from the atmosphere if the water pCO2 value is lower than pCO2 atm (negative value).The result shows that the changes to more positive values occur due to a variety of biological and physicochemical effects and generally have seasonal patterns that depend on climatic conditions and more specifically by temperature.Probably two substrates for photosynthesis have different isotope compositions, CO2(aq) being depleted 13 C by ~ 9 ‰ relative to bicarbonate at 25°C (Mook, 1974).Atmospheric input or DIC generated from carbonate dissolution will produce more enriched δ 13 C signs (> -5‰) because the DIC produced by dissolving carbonates is composed of soil CO2 (δ 13 C depending on C3/C4 abundance) and carbonate (δ 13 C~0 Asumsi assuming ancient sea carbonate).Furthermore, based on location, the isotope ratio δ 13 C-DIC and DIC concentration in Tallo waters are higher than Pangkep waters, but the value of δ 13 C-DIC to the sea is smaller until some of the outer islands are based on horizontal lines from Tallo and Pangkep waters.The isotope ratio value of δ 13 C-DIC in Tallo waters to the outer islands ranges between -5.23 ‰ to -7.74 ‰ (-6.68 ‰±0.7)V-GDP whereas Pangkep waters ranged between -4.34 ‰ to -6.56 ‰ (-5.38 ‰±0.8)V-GDP.Furthermore, the DIC concentration in Tallo waters ranged from 8.5 to 10.8 mg C/L (9.52 mg C/L ±0.8), while in Pangkep waters ranged from 8.7 to 11.7 mg C/L (9.98 mg C/L ±1.1) (Figure 4).DIC concentrations in rivers vary greatly depending on the geology of the catch and the degree of weathering (Brunet et al., 2009).DIC concentration values and water quality parameters are influenced by the situation and condition of the waters, both in the form of anthropogenic and weather factors.The influence of seasonal factors on the composition of δ 13 CDIC values and DIC concentrations is very significant.There are significant differences in the composition of CD13CDIC values and the concentration of DIC between seasons (dry and rainy) and between locations (Makassar and Pangkep).
This shows that the role of the season in the composition distribution of δ 13 C-DIC and DIC concentrations in the waters is quite large where the largest composition of δ 13 C-DIC and DIC concentrations occur in the wet season.The high composition of δ13C-DIC during the rainy season or the season with high precipitation rates is also shown in several estuaries and coastal water systems in the world although the land character is different, such as on the Rhône River (1-3 mMC and δ 13 C-DIC -5 to -10 ‰ V-GDP, and Northeast US Rivers (0.6 mMC and a value of δ 13 C-DIC -10 ‰ V-GDP (Raymond and Bauer, 2011).Overall, the process causing a decrease in the value of δ 13 C-DIC including respiration or oxidation photos of organic matter in a water column which has a lower dominance than processes that can increase δ 13 C-DIC such as photosynthesis and carbon equilibrium between DIC and atmospheric CO2.Released by the nearest source, the spatial-temporal character becomes information that is important enough to understand the hydrological and biogeochemical processes in a particular season.

IV. CONCLUSION
In the study area the main carbon sources are vegetative decomposition and to a lesser extent decomposition of carbonate and atmospheric CO2.Different mechanisms acting together as long as the DIC isotope composition has been identified as carbonate decomposition, metabolic processes (respiration and photosynthesis), and degasification of CO2 to the atmosphere.Dominance depends on the factor of the season when the measurement is done.The composition of δ 13 C-DIC and DIC concentrations in Makassar coastal waters and Pangkep waters during the rainy season have more negative and less negative values in the dry season.On the other hand, that determines the effect of seasonal effects on isotope composition δ 13 C-DIC and DIC concentration, due to high sample variability and extreme climatic factors that occur at baseline isotopes.

Figure 2 .
Figure 2. (a) δ 13 C-DIC values and (b) concentration of DIC during the dry and wet season.

Figure 3 .
Figure 3. Spatial variations of pCO2 (mg L -) during the dry and wet season.

Figure 4 .
Figure 4. Spatial variations in the δ 13 C-DIC values and concentration of DIC.

Table 1 .
The hydrographic condition of the sampling location.