1Introduction
Chromophoric dissolved organic matter (CDOM) is the optically active fraction of the dissolved organic matter (DOM) pool which is present in natural waters. CDOM strongly absorbs light in the ultra violet and visible ranges and as a consequence the presence of CDOM in natural waters can have important optical and ecological effects.
Optically, the presence of CDOM can affect estimates of ocean color and primary productivity. In coastal region, CDOM comprises up to 70 % of total of DOM (Nieke et al., 1997). As a consequence light absorption properties, CDOM can act as a sunshade to phytoplankton thus reducing PAR (Photosynthetically Active Radiation) penetration of the water column. Conversely, CDOM can act as a sunshade protecting organisms from harmful UV B radiation (Ha et al., 2010). The history of active research on CDOM is relatively short and the origin of CDOM research has three different backgrounds. The first is related with the harmful effect of UV B radiation to the aquatic organisms. Since the CDOM can absorb the harmful UV B radiation and control the depth of penetration of the radiation, CDOM in the surface water can protect the aquatic biota from the harmful UV radiation. The second is photoreactivity of CDOM. The absorption of light by CDOM can produce the photochemical intermediates including various reactive oxygen speices such as super oxide, CO and CO2, which is important trace gas in the atmosphere. The third is related with satellite ocean color measurements to examine biogeochemical processes over the spatial and temporal scales. Now it is well recognized that high level of CDOM absorption im many coastal and estuarine waters can seriously compromise the determination of phytoplankton biomass from satellite measurements. Thus a great deal of efforts has been expended on improving algorithms for the passive remote sensing of phytoplankton biomass and CDOM absorption. However, there is a new research interest related with CDOM very recently. That is the heat absorbing effect of CDOM in the water column (Hill, 2008; Granskog et al., 2015; Pavlov et al., 2016). High concentrations of CDOM can also affect heat exchange and, through photolysis, can be a source of inorganic nutrients and small carbon moieties (Hill, 2008; Granskog et al., 2015; Pavlov et al., 2016). An impact on upper ocean heating and stratification (Hill, 2008; Granskog et al., 2015; Pavlov et al., 2016) was reported, which was due to its light absorbing characteristics of terrigenous DOM entering surface waters of the Arctic Ocean. Granskog et al. (2015) reported that high concentration of CDOM in the polar water resulted in 50-60 % more heat deposition in the upper meters relative to clearest natural waters. Recent evidence pointed towards a microbial source of CDOM in the aquatic environment and led to the proposal that phytoplankton are not a direct source of CDOM, but that bacteria, through reprocessing of DOM of algal origin are an important source of CDOM (McCallister et al., 2006; McKnight and Aiken, 1998; Rochell-Newall and Fisher, 2002).
Recently the researches on correlation between DOC (Dissolved Organic Carbon) and CDOM (Stedmon et al., 2011) were reported at the polar environments. The main research interest is related with heat absorbing properties of CDOM, which would be provided from the rivers surrounding the Arctic ocean. The accelerated melting of permafrost can flush out the DOM, especially CDOM can increase the melting speed of Arctic sea ice. Stedmon et al. (2011), collected the data of DOC concentration from 10 major rivers around the Artic ocean and they matched DOC data with CDOM from the satellite data. From this monitoring, they reported the close correlation between DOC and CDOM. They suggested that CDOM from the satellite data can be used as proxy for monitoring of DOC input to Arctic ocean.
At present, the main focus of CDOM is related with climate change and UV-protection of aquatic biota from harmful UV B radiation (Ha et al., 2010; Hill, 2008; Granskog et al., 2015; Pavlov et al., 2016). And the composition of CDOM is still not well known at the molecular level, so slope values are used as a proxy for chemical characteristics of CDOM. S varies with the source of the CDOM, but also can be altered through the biological and chemical processing of a source material (Hansell and Carlson, 2002). Values of S for humic substances and for CDOM from a wide variety of sources range from as low ~ 0.01 nm-1 for terrestrial humic acids to as 0.02 ~ 0.03 nm-1 for oligotrophic seawaters (Table 3). For humic substances, the relationship between S and the molecular properties of CDOM can be summarized as follows (Blough and Green, 1995), (1) S is larger for fulvic acids than for humic acids: (2) S increase with decreasing molecular weight: (3) S increase with decreasing aromatic content. A number of studies (Carder et al., 1989; Green and Blough, 1994; Nelson and Guarda, 1995) reported that S values are larger for offshore seawaters (≥ 0.02 nm-1) than coastal waters influenced by river input (0.013-0.018 nm-1) or for most fresh waters. Although there is few reported exceptions (Stedmon et al., 2000), S is usually observed to increase with decreasing absorption and increasing salinity during transit of the terrestrial CDOM to offshore waters (Blough et al., 1993), suggesting that this material is being altered or replaced with a marine form. However there is few study reported on CDOM at the estuary and coastal environments in Korea. The understanding of mixing process, source of CDOM to the offshore, spatial and temporal variation of CDOM in the marginal marine environment is necessary to understand the production and removal process of CDOM in the ocean.
Our study area, the Seomjin river has an open estuary, which is one of the rare cases in Korea. Most estuaries in Korea are blocked by man-made dams. So Seomjin river estuary is the best location to study the mixing process and the end points of CDOM, to find the source of CDOM and the change of characteristics of CDOM along the salinity gradients. Seomjin river has the flow length of 223.86 km and takes up the total area of 4911.89 km2, among which 74 % is composed of forest land (Fig. 1). Seomjin river has is relatively less contaminated than other rivers and has the open estuary. As the river-mouth weir has not been installed yet, the fresh water is mixed with sea water in the river-mouth area, thus showing the feature of natural river mouth area.
We investigated the distribution and characteristics of CDOM of Seomjin river estuary in order to find the spatial and temporal variability of CDOM, FDOM, and the characteristics of CDOM by slope (S300-500). And we also examined environmental conditions such as temperature, salinity, nutrients, Chlorophyll a to find the controlling factors for source and characteristics of CDOM and FDOM in Seomjin river estuary.
2Material and Methods
2.1Sampling
The water samples were collected for analyses of CDOM, FDOM, Chlorophyll a, and nutrients from surface and bottom using Niskin water sampler along the salinity gradient from the river mouth toward the upstream of Seomjin river (35 km). Locations of sampling stations are shown in Fig. 1. Sampling was made during dry (March, June, 2012) and wet seasons (July 2012) to find the variation of CDOM. The sampling stations were 16 stations (March) and 19 stations (June and July). In March, water sample was not collected at station 1 and 2 because of absence of water and there was no sampling at station 19 in March. Water samples were transferred to the acid-wash bottle and kept in dark and cold condition until the lab analysis.
2.2Temperature and Salinity
The water temperature and salinity were measured by vertically lowering the CTD SBE-19 plus (Conductivity, Temperature and Depth Sea-Bird Electronics, USA) from the surface to the bottom.
2.3CDOM and Slope values
The samples were collected from surface and bottom using Niskin water sampler. The samples were filtered through the membrane filter in the lab, and stored at the dark place. SHIMADZU 2550 UV-VIS Spectrophotometer was used for the measurement of absorption by CDOM and the analysis was conducted by setting every 1nm for the range of 190 ~ 750 nm using 10 mm quartz cell with the scan speed set to 1 nm/0.2 sec and slit width set to 5 nm. The baseline was set by arranging the Milli-Q water to the zero of absorbance and the samples were measured three times in a row. The measured data was deducted by the average absorbance in the wave length of 700 ~ 750 nm for background calibration. The absorption spectra of CDOM are broad and unstructured and typically decrease with increasing wavelength in an exponential fashion. And these spectra can be expressed as eq. (1):
where a(λ) and a(λ0) are the absorption coefficients at wavelength λ and reference wavelength λ0 respectively, and S defines how rapidly the absorption decrease with in creasing wavelength (Green and Blough, 1994). The absorbance coefficients are calculated as in eq. (2), where A(λ) indicates the absorbance of CDOM in the wavelength(λ) while ι means the value of the passage length of cell in the unit of m.
2.4FDOM
The fluorescence of DOM (FDOM) was measured by Spectrofluorometer (SINCO FS-2) with the setting of PMT Voltage(V): 900, Integration time: 0.1 sec, Scan speed: 932 nm/min, Silt width ex/em: 5 nm, and Scan mode: emission option1. In Excitation Emission Matrixes (EEMs), 250 ~ 500 nm of the excitation spectrum was analyzed for every 5 nm while 280 ~ 600 nm of the emission spectra was analyzed for every 5 nm. During the measurement of EEMs, the fluorescence intensity varies depending on the conditions of the Raman scattering. So the blank EEMs are measured using the distilled water and that value was then deducted from the EEMs of the samples to remove the Raman peak and normalize the EEMs. The count per second (CPS) was used as the unit of fluorescence. The analyses were conducted two times and the blank EEMs were measured using the distilled water prior to the analysis. The fluorescence of FDOM from different sources are shown in Table 1 (Coble, 1996).
2.5Chlorophyll a
The samples were filtered through the membrane filter using the filtering set and extracted by 90 % acetone. Then, the absorbances of the extracts acquired from each wavelength through absorption spectrometer put into the following formula to get the concentration of chlorophyll-a.
Ca :
11.6E665 - 1.31E645 - 0.14E630
ν :Volume of extracted acetone (mL)
V :Volume of filtered sample water (L)
2.6Nutrients
The water samples were filtered by GF/F and kept in freezer before the analysis of nitrate and phosphate carried by auto nutrients analyser (QUAARTRO, BRAN UEBBE).
3Results and Discussion
3.1Precipitation and stream flow of Seomjin river
We collected the precipitation and stream flow data at Hwagae observatory, in order to find the effect of fresh water input on concentration and properties of CDOM during the dry season and wet season of Seomjin river estuary in 2012. The monthly average precipitation are shown in Fig 2. The highest precipitation, 656 mm was recorded on August and the lowest precipitation, 6 mm was on January, 2012. The stream flow was following the precipitation and maximum flux was 12,705 m3/s on September and minimum flux was 426 m3/s on February 2012. About 60 % of precipitation was concentrated on summer season. We selected the sampling period March and June for dry season and July as wet season. The average depth of the Seomjin river was increased in July (6.0 m) compared to March and June (5.2 m).
3.2Water Temperature
The distribution of water temperature measured along the Seomjin river estuary during dry and wet season are shown in Fig. 3. The range and average water temperatures on March were 9.0 ~ 10.0°C and 9.7°C, respectively. The water temperature increased slightly towards seaward and relatively homogeneous. The water temperature significantly increased on June and July. The range and average temperature on June and July were 21.9 ~ 25.7°C (23.6°C) and 22.8 ~ 29.1°C (26.6°C), respectively (Table 2). And the water temperature decreased towards the river mouth, which is in contrast with March.
3.3Salinity
In March (dry season), when the stream flow is low and the tidal flow is predominant, the distribution pattern of salinity was well-mixed type and salinity was vertically homogeneous (Fig. 4). The salinity increased towards downstream up to 33.0 psu. In June, when the stream flow was minimum level (400 m3/s), the average salinity was higher than the average salinity of March and July. Because of low precipitation and flux of fresh water in June (dry season), tidal input from the river mouth resulted in adding more saline water towards upstream of Seomjin river. As shown in Table 2. the average salinity of June (dry season) was almost 10 psu higher than those of July (wet season) in 2012. The dense salinity gradients was located at the upstream (St. 2 ~ St. 5) in June. Compared to June, distribution pattern of salinity in March showed gradual increase of salinity to downstream and vertically well mixed column. So the freshwater flux was in balanced with tidal input of seawater from the downstream in March. In July, however, the increased stream flow induced by high precipitation deter the tidal flow from the sea and resulted in the very low salinity (ave. 14.1 psu) the dense salinity gradients was located at downstream (St. 14 ~ St. 18).
3.4Nutrients
Distributions of concentration of nitrate and phosphate are shown in Fig. 5 and Fig. 6. Average concentrations of nitrate in July was highest (70.9 μM) and the lowest was in June (19.6 μM). The relatively high concentration of nitrate at upstream (> 50 μM) decrease along the salinity gradients less than 10 μM at downstream. The average phosphate concentrations were also highest in July, but lowest in March (0.15 μM), due to extensive utilization for photosynthesis of spring bloom. The highest abundance of phytoplankton was confirmed by the highest Chl a in March. As Kwon et al. (2001) reported that phosphate is the limiting factor for photosynthesis in the Seomjin river, the N/P ratio was 224 in average, which is far above the Redfield ratio.
3.5CDOM
3.5.1CDOM and slope values
The concentrations of CDOM in the water samples were measured indirectly from the optical properties of organic matter and expressed by a355 (CDOM). The average values and range of CDOM measured during dry and wet season are shown in Table 3. The average CDOM measured was highest in July (1.4 ± 0.3 m-1) and the lowest was 1.0 ± 0.3 m-1 in March, 2012. The CDOM distribution along the salinity gradient was shown in Fig. 7. The high CDOM at upstream tends to decrease when the salinity increase along the downstream at all season. At shallow upstream, CDOM was high and laterally changing the concentration of CDOM along the salinity gradient. The high temperature favored the degradation by microbial activity and relatively higher CDOM at the riverine fresh water compared to the marine water seems to affect on the highest CDOM in July (wet season) with the highest fraction of fresh water in the Seomjin river estuary.
In general, CDOM concentration was higher at the upstream and decreased along the increase of salinity. The high CDOM of fresh water at the upstream during dry season was above 1.5 m-1 and the low CDOM at downstream (saline seawater) was less than 0.5 m-1.
The CDOM is known to be produced from the biogenic matter and the microbial degradation product of POM (particulate Organic Matter) and large DOM from phytoplankton is the main source in aquatic environment (Hansell and Carlson, 2002). The maximum flux of fresh water with high content of CDOM from the river head is thought to be the reason for the highest average CDOM in July (wet season) and the highest S300-500 because of activated microbial activity in increased temperature of water column compared to March and June. The CDOM and S300-50 values showed that not only the concentration of CDOM but also the chemical properties of DOM changed from upstream to downstream at Seomjin river estuary.
3.5.2CDOM, slope values vs Salinity
Along the stream, not only CDOM, but also slope values are varying (Fig. 8) and CDOM tends to decrease when the salinity increases and slope values were almost constant at upstream till the salinity reach up to 20 psu (CDOM concentration is about 1.0 m-1). When salinity increases higher than 20 psu, the slope values showed a wide variation (Fig. 9). The most high saline water has the highest slope values (> 0.025) but interestingly in June very low values of slope values. Important things is that CDOM is changing its characteristics along the salinity gradient, although what is main reason for change of properties of CDOM is not clear. And the locations for this kind of transition is at about 20 psu. The typical S300-500 values of sea water are 0.025 ~ 0.028 nm-1 and river water is 0.010 ~ 0.015 nm-1. Our data (Table 4) measured at Seomjin estuary was in the range of 0.006 ~ 0.029.
3.6FDOM and Chl a
In general, FDOM concentration varied according to the change of water temperature. This similarity between FDOM and water temperature implies the correlation with biological activity with increased temperature, especially with microbial activity rather than the primary production. Since there is no clear correlation between Chl a and FDOM (R2 = 0.01 ~ 0.17). Among the FDOM, C-peak showed the close correlation with salinity (R2 = 0.79 ~ 0.91). C-peak, visible-terrestrial humic component among the FDOM is known to originated from the terrestrial source, which has the fluorescence maximum at 420-460 nm (Table 1). During the dry season (June), the correlation between FDOM (C-peak) and salinity, but the source of FDOM (C-peak) was clearly from the upstream to the downstream. Over the 2 × 104 cps at the upstream of the Seomjin river decreased to less than 0.4 × 104 cps in June. On the contrary, FDOM (T-peak), protein-like peak of FDOM can be used as indicator of the biological activity, since the Tryptophan-like peak (T-peak) produced when microbial degradation of particulate OM and exudation of OM from the membrane of the cells as extra celluar release of remnant OM.
In this study the T-peak was high in July compared to June and March. And the T-peak was higher at the upstream compared to the downstream. In March there was no clear difference between upstream and downstream and surface and bottom. However relatively the FDOM (T-peak) tends to be high at the upstream compared to the downstream. FDOM and CDOM showed a negative correlation with salinity and CDOM was positively correlated with FDOM (Fig. 10).
3.7Mixing of CDOM along salinity gradients
As plot of salinity and CDOM from all season data (Fig. 11), CDOM showed close correlation with salinity (R2 = 0.8797). Although at the surface of upstream showed an additional removal of CDOM, in general the mixing pattern between fresh water with high CDOM and saline water with low CDOM was conservative in Seomjin river estuary. At downstream, additional sources of CDOM at the surface layer exist. There is small branch stream near st. 7, St 10, st. 13 (Fig. 1) and these might be the source of the additional sources of CDOM.
4Conclusion
CDOM concentrations at Seomjin river estuary were in the range of aCDOM (355) : 0.3-2.4 m-1 and the distribution of CDOM during dry season (March, June) and wet season (July) showed different patterns. The concentration of CDOM of the fresh water and seawater were higher than 1.5 m-1 and less than 0.5 m-1 respectively. Depending on the variation in precipitation, the input of saline tidal flow into the Seomjin river estuary has changed. The maximum intrusion of saline water from downstream was observed in June (dry season), when the lowest precipitation recorded in the study area and the lowest was observed in July, which means that there is the least dilution by low CDOM seawater. The average salinity in June was the highest as 23.8 psu and this resulted in the maximum dilution with low CDOM-seawater. However between two dry season, March and June, the average CDOM measured in June was not the lowest. In fact, the average salinity was highest in June with maximum tidal input of saline water, the average CDOM in June was slightly higher than CDOM in March. This result suggests that the dilution (mixing) is not the only main controlling factor for concentration and distribution of CDOM in this study area. The increase in average water temperature from March to June was ~ 14℃ and the higher CDOM in June might resulted from the enhanced biological activity and increased insolation, which is the major source of CDOM by the degradation and photodegradation of DOM. The average CDOM concentrations increased in the following order: March < June < July. The high CDOM at upstream showed the decreasing tendency towards the downstream when salinity increased. At shallow upstream region showed homogeneously high CDOM with low salinity and gradual decrease of CDOM by dilution of CDOM with saline water from downstream.
The mixing between fresh water with high CDOM and the saline water with low CDOM at Seomjin river estuary showed a conservative mixing, with small removal at upstream. The change in CDOM in molecular composition was clear along the salinity gradients in Seomjin river estuary based on the S300-50 values. S300-50 of the fresh water was 0.008 ~ 0.011 nm-1 and S300-50 of the seawater was above 0.020 nm-1 and upto 0.029 nm-1. It is known that humic properties of DOM from land source is gradually changing after the fresh water mixed with coastal saline water (Hansell and Carlson, 2002). The acidic and high molecular weight fraction of humic composition is known to decrease and less acidic and lower molecular weight moiety increase the fraction in total DOM. The change in slope values, which is the sign of change in molecular characteristics was clear when the salinity is increased higher than 20 psu and CDOM is diluted up to aCDOM (355) = 1.0. When the high concentration of CDOM from fresh water at upstream is diluted with saline water from tidal flow, not only the removal of CDOM by flocculation and adsorption on the surface of particle, but also change in molecular size and characteristics by biodegradation and photo-dissociation in the water column occurs. The similarity between FDOM and water temperature implies the correlation with biological activity with increased temperature, especially with microbial activity rather than the primary production. Since there is no clear correlation between Chl a and FDOM (R2 = 0.01 ~ 0.17). FDOM and CDOM showed a negative correlation with salinity and CDOM was positively correlated with FDOM. This correlation implys that the main source of CDOM in this study area is not the phytoplankton and the important role of microbial degradation and photodegradation for the production of CDOM. This process is closely related with temperature. Thus the concentration and spatial distribution of CDOM are controlled by salinity (dilution by mixing) and variation precipitations, but temporal variation of CDOM governed by the water temperature which is related biological and photo-chemical processes in Seomjin river estuary.
