ISSN 00014370, Oceanology, 2015, Vol. 55, No. 3, pp. 339–346. © Pleiades Publishing, Inc., 2015. Original Russian Text © N.V. Lobus, V.I. Peresypkin, N.A. Shulga, A.N. Drozdova, E.S. Gusev, 2015, published in Okeanologiya, 2015, Vol. 55, No. 3, pp. 379–386.
MARINE CHEMISTRY
Dissolved, Particulate, and Sedimentary Organic Matter in the Cai River Basin (Nha Trang Bay of the South China Sea) N. V. Lobusa, V. I. Peresypkina, N. A. Shulgaa, A. N. Drozdovaa, and E. S. Gusevb a
b
Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia Papanin Institute of Biology of Inland Waters, Russian Academy of Sciences, Borok, Russia email:
[email protected] Received February 14, 2014; in final form, May 12, 2014
Abstract—The data were obtained on the content of organic carbon in the water, particulate matter, and bot tom sediments of the Cai River and its tributaries (the basin of Nha Trang Bay). The surface waters of the con sidered basin contained OM mainly in a dissolved form (DOC/POC = 3). The fraction of Corg in particulate matter amounted to 4.9% on average. Based on the analysis of nalkanes in bottom sediments, three OM types—autochthonous, mixed, and mainly terrigenous genesis—were distinguished. These types are alter nated in series in the lengthwise profile of the river. All the OM types are in close relation to the peculiarities of sedimentation and hydrodynamics of the waters in the treated aquatic area. The geochemical indices (Pr/Ph, OEP17–19, and CPI values) represent the influence of oxidative conditions on OM formation and the intense microbiological transformation of its autochthonous component. DOI: 10.1134/S0001437015030121
INTRODUCTION The study of the content, distribution, and compo sition of organic matter (OM) in the water, particulate matter, and bottom sediments allows one to character ize the fluxes in the ecosystem and reveal the genesis of OM. The OM in natural waters occurs in various dis persed and phase states; however, two principal forms—dissolved and particulate—are the most fre quently studied [1]. The ratio and total sum of the sup ply of these forms into final basins of runoff, being the key biogeochemical characteristics of the environ ment, depend on the climate conditions of a region, the diversity of land vegetation, and the soil cover of the drainage area [10]. The mass of primary organic carbon (Corg) produced in a basin (the autochthonous frac tion) is replenished with land OM (the allochthonous fraction). The OM is partially mineralized, resulting from transformations by heterotrophs and bacteria in trophic chains, whereas the components being more resistant to biodegradation are supplied to the bottom and buried within the sediment mass [2, 4, 9]. At the current time, the anthropogenous factor has a consid erable influence on the OM distribution and composi tion at several parts of both marine and freshwater aquatic areas [10, 13]. The basins of Southeast Asia are characterized by great volumes of the supply of sedimentary matter to the seas and ocean, which is under the study of geochemists [2, 4, 10]. The South China Sea in the area of Nha Trang Bay is characterized by compli cated hydrodynamic conditions resulting from quite powerful tidal currents and the presence of seasonal
upwelling, which affect the sedimentation process. The preceding study has shown that the formation of a composition of organic matter in the waters and bottom sediments of the bay was under the great influence of Cai River runoff [8]. This study aimed to research the content and gen esis of organic matter in the water, particulate matter, and the surface layer of bottom sediments of the Cai River and its tributaries, having Nha Trang Bay of the South China Sea as the final runoff basin. STUDY AREA The Cai River is the main watercourse of the region under consideration. The area of the drainage basin is 1904 km2. The basin of the upper and middle river is constituted by red and yellow podzols; alluvial soils prevail downriver. The river receives mountain streams and about 15 tributaries, among which the Khe, Cau, and Dau rivers are the largest. The former two tributar ies are characterized by a rapid current and are located in a mountainous, sparsely populated area; the latter is a lowland river with a slow current. The hydrological conditions of the Cai River show a pronounced sea sonal dependence: water discharge varies from 15–30 to 700–800 m3/s in dry and rainy seasons, respectively. The average annual runoff is 2 km3 [6]. The chemical composition of the treated water bodies is under the profound influence of anthropo genic activity, i.e., of domestic and municipal wastes, as well as the washouts from urban territories during heavy rains. Several private shipyards are situated in
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104°
106°
108°
110° N Khanh Hoa Province
China 22°
20° Laos
Gulf of Tonkin
18° SOUTH CHINA SEA
16°
Van Phong Bay
Thailand
14° Cambodia 12° Gulf of Thailand
Cai River Nha Phu Bay st. 1 Khe Cau
st. 2
st. 3 st. st. 6 Dau 4, 5
Suoi Dau reservoir
Tre Island
Phu Quoc Island
10° E 0 100
400 km Cam Ranh Bay
Fig. 1. Schematic map of the explored area.
the estuary, which deal with manufacturing and repairing of boats and small vessels of wood and com posite materials using polyester resins and hardeners. Manufacturing procedures for covering the vessel hulls use preparations containing anticorrodant and anti fouling substances [5, 12]. MATERIALS AND METHODS The water, particulate matter, and bottom sedi ments were sampled during the dry season (July 2010) at the following stations: stations 1, 2, and 3 at the upper, middle, and lower Cai River, respectively; sta tions 4 and 5 in the river estuary; station 6 in the adja cent aquatic area of Nha Trang Bay; and in the Khe, Cau, and Dau tributaries (Fig. 1). To separate the dissolved and particulate OM in lab oratory conditions, the water was filtered with a Milli pore device through calcined glassfiber GF/F filters (Whatman Co., 47 mm diameter). The filtrate was acidified with hydrochloric acid to pH 2 and kept in a refrigerator. The GF/F filters were dried in a desiccator at 60°C. To collect the particulate matter and to deter
mine its mass concentration mg/L, the membrane nuclear filters of 47 mm diameter and 0.45 μm pore size were used [3]. Bottom sediments were collected with a sampler; after the lithological description, the samples were dried in a desiccator at 60°C [7, 8]. Organic carbon (Corg) in the water, particulate matter, and bottom sediments was determined with a ShimadzuEurope TOC 5000VCPH analyzer. To determine the dissolved organic carbon (CD, DOC), the technique of hightemperature (680°C) ther mocatalytic oxidation with dispersionfree IRdetec tion was used. The quantitative analysis of organic carbon in the particulate matter (CP, POC) and in bottom sediments was carried out by hightempera ture (900°C) combustion in airflow using an SSM 5000 A device. The content of Corg was calculated by determining the difference between the total and car bonate carbon. The ranges of measured concentra tions amounted to 0.05–25000 mg C/L for CD, 5– 10000 μg C/L for CP, and 0.02–30 mass % for bot tom sediments (the dry mass). Device error accounted for 1%. The reproducibility of the results was within ±5%. OCEANOLOGY
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DISSOLVED, PARTICULATE, AND SEDIMENTARY ORGANIC MATTER D
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Table 1. The content of particulate matter, C org , C org and chl a in the Cai River and its tributaries Aquatic site
Particulate matter, mg/L
P
D
C org , mg/L
C org , mg/L
P
D
C org / C org
Corg fraction in particulate matter, %
chl a*, mg/m3
chl a**, mg/m2
5.0 4.1 2.8 2.6 5.5 4.9
0.6 1.1 1.9 1.7 1.4 0.8
0.4 21.3 24.2 n/d n/d n/d
6.3 6.4 6.2
0.8 1.1 11.6
6.2 3.9 43.7
Lengthwise profile of the Cai River Station 1 Station 2 Station 3 Station 4 Station 5 Station 6
10.9 42.7 68.0 48.3 15.2 4.7
2.2 2.6 2.3 2.2 2.1 1.4
0.5 1.8 1.9 1.3 0.8 0.2
4.0 1.5 1.2 1.8 2.6 6.0 Tributaries
Khe Cau Dau
3.0 22.9 32.2
1.0 3.1 5.5
0.2 1.5 2.0
5.3 2.1 2.8
* chl a concentration in phytoplankton. ** chl a concentration in phytobenthos. n/d⎯not detected.
Chlorophyll a (chl a) was determined by means of spectrophotometry using a HACH DR2010. The sam ples of phytoplankton were concentrated in Whatman glassfiber filters. Immediately after the filtration, the samples were placed into test tubes containing 90% ethanol and exposed in the dark at 4–7°C for 16–20 h [19]. To determine chl a in phytobenthos, the bottom sediments were sampled with a plastic pipe of a known capturing area. The photosynthetic pigments were extracted as described above. The chl a concentration was converted by means of the common procedure and expressed in mg/m3 for water and in mg/m2 for sediments [16, 19]. Aliphatic hydrocarbons (HC) from probes of bot tom sediments were extracted with a Branson1210 ultrasound bath. As a solvent, methylene chloride was used, ensuring the most complete HC extraction. The fraction of nalkanes was isolated by column chroma tography (column filler, silica gel; eluent, hexane). Hydrocarbons were analyzed with a GS 2010 Shi madzu gas chromatograph under the following condi tions: heating from 60 to 300°C at a rate of 4°/min and isothermal conditions at 300°C for 30 min, with helium as a carrier gas, at 1.5 mL/min gas flow in the column. The temperatures of the injector and detector amounted to 300 and 320°C, respectively. The obtained chromatograms were treated by means of GC solution 2.30 SU4 Snimadzu software. The peaks were identified by the retention times; the identification was verified comparing to the spectra of nalkane stan dards (Solution C8C20 and С21С40). To calculate the concentrations, an inner standard (squalane) was used. OCEANOLOGY
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RESULTS AND DISCUSSION The dissolved and particulate organic matter sup plied to the seas by riverine runoff considerably affects the distribution and composition of OM in marine bot tom sediments [7, 8]. As a rule, the bulk of riverine CD is presented by humic compounds supplied from the soils of the drainage area. This value is increased by vital metabolites of hydrobionts and the decomposed prod ucts of dead tissues of plants and animals [1, 9, 10]. The DOC concentration in the waters of the explored area varied from 1 to 5.5 mg/L. The minimum and maxi mum values were associated with the mountain and lowland Khe and Dau tributaries. The DOC content in the Cai River as such was quite unvaried and amounted to 2.1 mg/L on average all along the river (Table 1). The distribution of particulate organic carbon in the basins is less uniform and often related to the pro cesses of autotrophic biosynthesis [4]. The POC sources are phyto and zooplankton, periphyton, bac teria, mosses, grasses, macrophytes, and residues of wood plants [1, 9]. POC concentrations varied from 0.2 to 2 mg/L; however, as in the case of CD, the max imum and minimum values were associated with the Khe and Dau tributaries. The average CD content over the river basin amounted to 1.3 mg/L (Table 1). The POC fraction in the particulate matter of the Cai River varied from 2.8 to 5.0% (4% on average). The particulate matter in the tributaries contained a large amount of organic carbon, with values as high as 6.4% (Table 1). The freshwater part of the Cai River showed an increase of CP concentration by the decrease of its percentage in particulate matter from 5 to 2.8%, which was probably caused by the “dilution” of biogenic organic detritus with mineral matter sup
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plied intensively to the basin from the drainage area (Table 1) [11]. The distribution of particulate matter and CP in the mixing zone of the riverine and marine waters is characterized by a drastic decrease in the river → sea direction. POC content decreased by a factor of 2–8 under the water mixing and the POC fraction in particulate matter increased (to 5.5% at stations 5 and 6), which represents qualitative changes in PM composition at the geochemical barrier [3]. The CP supply to the waters of the Cai River estuary resulting from the processes of primary production is minor, as shown by the pronounced decrease of chl a concentration (to 0.8 mg/m3 at station 6, Table 1). Further from the river mouth, the POC fraction in the particulate matter of the surface water layer of Nha Trang Bay increased to 10–20% and reached its maximum value (25%) in the seaward parts of the aquatic area [8]. The content of organic matter in the PM of the waters of the Cai River Basin (~4.9%) is higher com pared to the world average value for rivers, taken to be 2.5% [9, 10]. However, the published values for Corg fraction in the riverine particulate matter of Southeast Asia are lower, e.g. 2.8, 1.4, and 1.2% for the Mekong, Ganges, and Yangtze rivers, respectively [11]. One must note that the explored basins are considered oli gotrophic in view of the amount of photosynthetic pigments of plankton and benthos, as well as the value of primary production (PP) [12]. The preceding study has shown that the total PP was very small and amounted to about 18–20 g C/m2 per year at the mid dle river (station 3) and to 40–45 g C/m2 in the estuary (station 5) [12]. The Cai River waters are sufficiently provided with nutrients, therefore the potential factors limiting productive characteristics might be the hydro logical parameters (the current velocity and water dis charge), as well as a deterioration of light conditions because of high turbidity [19]. One must also point to the specific ionic composition of the waters of low mineralization [16]. In view of the poor development of the primary producers in the Cai River and its trib utaries, the enrichment of particulate matter in organic carbon in the considered basin might be caused by the supply from land. A similar situation is seen in the waters of the Amazon River where CP con tent in particulate matter in OM is of terrigenous ori gin and amounts to 4.7% on average [1, 11]. However, this assumption requires additional study. At the same time, a considerable contribution into the OM supply (CD + CP) at the down Cai River, its lowland Dau trib utary, and, especially, in the estuary might be made by human economic activity [5, 8]. As shown by our studies, the organic matter in the surface waters of the Cai River Basin occurred mainly in a dissolved form, which was attested by the CD/CP ratio. This value varied from 1.2 to 6, amounting to 3 on aver age. In view of the data on the average CD and CP con tent in the river and on the volume of its runoff (by the hydrological service of Khanh Hoa Province), it was
found that the Cai River supplied about 1.5 × 109 g of CD and 0.8 × 109 g of CP to Nha Trang Bay during the dry season (from April to September). Bottom sediments in the lengthwise profile of the river are constituted by coarse quartz sand with pebble and by silted sand and aleuritic ooze at the upper and down river, respectively. The zone of riverine and marine water mixing is characterized by the settling of fine–clayey oozes of dark grey color with a small admixture of sandy matter. The content of Corg in the sediments of the Cai River varies from 0.03 to 1.0%. The minimum concentrations are associated with the upper and middle river parts (stations 1 and 2), and the maximum values are characteristic of the river mouth area (station 6). The sediments in the Khe and Cau tributaries are presented by quartz sand of various grain sizes with the admixture of pebble and minimum Corg concentrations (0.03%). The sediments in the lowland Dau tributary are constituted by silty–clayey oozes with a high content of organic carbon (1.83%). The distribution as such of Corg by grainsize types of sediments is caused by the disperse composition of the OM bulk and by sorption processes on clayey minerals and oxides [4, 9]. The notion of genetic variety of the composition of organic matter is given by the typization of molecular markers for alkane hydrocarbons [18]. The content of nalkanes varies from 0.65 to 4.6 μg/g of dry sediment in the surface layers of the bottom sediments in the treated basins and watercourses. The spread in values mainly results from the difference in lithological compositions. The fine clayey oozes are characterized by more intense HC sorption compared to sands in which a part of accu mulated HC is rapidly washed out [2, 7]. The content, molecular and group composition of nalkanes, as well as the calculated geochemical parameters representing the genesis and diagenetic transformations of OM in the sediments, are given in Table 2. The autochthonous (hydrobiont) type of initial organic matter is characterized by the maxima of the distribution of nalkanes within С15С19. Organic mat ter genetically related to higher plants shows the max ima at С25С35; some of the middlechain С20С24 homologues are bacterial biomarkers [18]. All exam ined samples showed the pronounced bimodal charac ter of the distribution of nalkanes with prevailing ranges of С15С18 and С29С33 (Fig. 2). The fraction of hydrocarbons designated commonly as those of microbial origin ΣС20С24 is low (
