An isotopic model for the origin of autochthonous organic matter contained in the bottom sediments of a reservoir

International Journal of Sediment Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Original Research

An isotopic model for the origin of autochthonous organic matter contained in the bottom sediments of a reservoir Piotr Koszelnik n, Renata Gruca-Rokosz, Lilianna Bartoszek Department of Environmental Engineering and Chemistry, Faculty of Civil and Environmental Engineering and Architecture, Rzeszów University of Technology, al. Powstańców Warszawy 6, PL35-959 Rzeszów, Poland

art ic l e i nf o

a b s t r a c t

Article history: Received 17 January 2017 Received in revised form 30 September 2017 Accepted 19 October 2017

Geochemical analysis of surface sediment samples collected in 2005 and 2006 was used to evaluate the potential sources of the organic matter present in sediments of southeast Poland's Solina Reservoir. Statistical analysis of sediment variables (carbon to nitrogen ratio, and the carbon 13 and nitrogen 15 isotope ratios) determined for the organic fraction indicated significant spatial variability with respect to sources of organic matter. A binary mixing model was developed from literature sources to predict the relative contributions of allochthonous and autochthonous production to sediment organic matter. Autochthonous production was shown to account for 60–75% of bulk sedimentation in the lacustrine parts of the reservoir, near the dam. In contrast, autochthonous production accounted for only 25% of sedimentation in the riverine zone receiving stream inputs. Statistical analysis identified the δ15N of organic matter as the best predictor of the source of organic matter. Multiple regression analysis indicated that two water-quality variables (nitrate and dissolved silica) were significantly related to the δ15N signature of organic matter. This led to a conclusion that limnetic nitrate and dissolved silica concentrations were regulating organic matter production in the Solina Reservoir. & 2017 International Research and Training Centre on Erosion and Sedimentation/the World Association for Sedimentation and Erosion Research. Published by Elsevier B.V. All rights reserved.

Keywords: Reservoir Bottom sediments Organic matter Stable isotope

1. Introduction Identification of the sources, directions of transport, and fate of organic matter (OM) in aquatic ecosystems currently represents one of the main courses of research development of limnology and oceanography (Alin & Johnson, 2007; Kruger et al., 2016; Smal et al., 2015). The formation of bottom sediments, as inseparably linked with a reservoir's accumulation of matter from its drainage basin, is related to the sedimentation and simultaneous early diagenesis (Patience et al., 1995) of OM containing biogenic elements. Quantitative estimation of the permanent accumulation of carbon or nitrogen in bottom sediments is vital in determining the global balance of these elements (Chen et al., 2015; Lehmann et al., 2002), especially where immobilization of nitrogen or phosphorus in bottom sediments represents a key mechanism curbing eutrophication (Gołdyn et al., 2003; Grochowska et al., 2015). Sources of OM in aquatic ecosystems are typically classed as either allochthonous (terrigenous) or autochthonous (planktonic). Given the abundance of biogenic compounds, OM deriving from ecosystem production can exert a vital influence on the amount of n

Corresponding author. E-mail address: [email protected] (P. Koszelnik).

bottom sediment present in stagnant water in reservoirs. Various studies show that up to 35% of the OM produced in the euphotic layer of a reservoir goes on to enrich the surface layer of bottom sediment (de Junet et al., 2005; Lehmann et al., 2002; O'Beirne et al., 2015; Wiatkowski, 2010). On a global scale, the mineralizationrelated formation of sediments leads to the permanent accumulation of only 0.1% of the net primary production (Lehmann et al., 2002). However, paleolimnological studies show that the process of aggradation of bottom sediments is directly proportional to an increase in primary production (Tadonléké et al., 2000; Tyson, 2001). Thus far, the most common method to identify the origin of organic matter in bottom sediments has been analysis of changes in the content of organic carbon (TOC) relative to nitrogen (TN) (the C/N ratio). A high C/N ratio is characteristic of organic matter originating from the sedimentation of terrigenous substances; this is the same as for cellulose, as a component of land plants. Autochthonous (planktonic) matter is characterized by lower values for this ratio (approximately 6/1), as algal debris contains more nitrogen than matter originating on land (Calvert, 2004, de Junet et al., 2005; Gu et al., 1999; Koszelnik, 2009). Analysis of the stable-isotope ratios of carbon and nitrogen has become increasingly popular as a method by which to evaluate the origins of bottom sediments (e.g., Hou et al., 2013; Koszelnik et al., 2008;

http://dx.doi.org/10.1016/j.ijsrc.2017.10.002 1001-6279/& 2017 International Research and Training Centre on Erosion and Sedimentation/the World Association for Sedimentation and Erosion Research. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Koszelnik, P., et al. An isotopic model for the origin of autochthonous organic matter contained in the bottom sediments of a reservoir. International Journal of Sediment Research (2017), http://dx.doi.org/10.1016/j.ijsrc.2017.10.002i

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Murase & Sakamoto, 2000; Zigah et al., 2012). Such methodologies are based on the rationale that the deposition of organic matter in bottom sediments represents the final stage in a series of transformations of carbon and nitrogen compounds within the land and water column that affect the relative abundance of isotopes. This deposition results in a difference in content with respect to the 15 N/14N and 13C/12C isotopic ratios, in bottom sediments of different origins. While the isotopic ratios of carbon or nitrogen, expressed as δ13C or δ15N, often serve as tools in the identification of OM origins in bottom sediments, they have been used only rarely in the quantitative analysis of sediment deposition (Hou et al., 2013; Koszelnik et al., 2008; Zhang et al., 2017). Considering the possibilities for isotopic analysis to serve this kind of purpose was, thus, the main aim of the work described in this paper, which considered a large piedmont dam reservoir of mesotrophic character. The highlights of the study described concern ways in which stable-isotope datasets from reservoir surface sediments can complement site-specific studies of isotope processes, helping identify key processes controlling the accumulation and degradation or reservoir sediment. Solina Reservoir's close similarity to many others ensured its status as a perfect testing ground for interpretation of the foregoing kind of analysis.

2. Materials and methods 2.1. Study site The research detailed here was done in the dimictic, mesotrophic Solina Reservoir (Fig. 1), a part of Poland's Vistula-San river system, situated in the upper part of the catchment area (at c. 450 m above sea level). The reservoir accounts for approximately 15% of the overall water storage in Poland, having a volume of c. 470 M m3, and a surface area of 22 km2. The hydraulic retention time (HRT) is on the order of 150–350 days, while mean depth is 22 m (maximum 60 m). The fluctuation in water levels is up to 10 m, leading to a reduction of the littoral zone. The reservoir came into existence in 1968. Its (1175 km2) catchment area comprises the Bieszczady Mountains, which are dominated by forests and mountain pastures. Farmland and settlements are mainly located at the mouths of the tributary valleys. The reservoir supplies a hydroelectric power plant, but also serves for flood control, recreation and the water supply. 2.2. Sampling strategy Bottom sediment cores were collected using a gravity sampler in the March-November periods of 2005 and 2006 (on 18 occasions), at four sampling sites (S1-S4) located along the studied

reservoir's longitudinal axis (Fig. 1). The shallower (~15 m-deep) Sites S1 and S2 were located in upstream areas of the reservoir (Fig. 1), and were characterized by an OM sedimentation rate of ~3000 t yr–1 (Bartoszek & Koszelnik, 2016). The downstream Sites S3 and S4 were in turn at greater (respectively 45, and 55 m) depths, and their OM sedimentation rates were lower (at ~1400 t yr–1; Bartoszek & Koszelnik, 2016). Reservoir water was sampled at the aforementioned stations on the same occasions as bottom sediment was collected. 2.3. Laboratory analysis Each analysis related to a thin (2 cm) upper layer of sediment, which was dried at 60 °C, milled, and subjected to the removal of the carbonate fraction via 72-h contact with the vapour of 30% hydrochloric acid in a desiccator. The content of OM was analyzed using the loss ignition method, at 550 °C for 4 h. The contents of TOC and TN were measured using an elemental analyzer (Flask 1112, ThermoQuest), with a standard deviation less than 0.05 and 0.01%, respectively. Biogenic silica (BSi) was analyzed after Teodoru et al. (2006), to a precision of 10%. All parameters were expressed as percentage of dry mass (%). The N and C isotopic compositions were detected using a DELTAPlus isotopic ratio mass spectrometer (Finnigan Mat), as coupled with an elemental analyzer. δ15N and δ13C values were expressed in permil (‰), as set against the respective Air and Pee Dee Belemnite (PDB) standards as follows:
δ15 N ord 13 C ¼ Rsample =Rstandard –1  1000 ð1Þ where R denotes 15N:14N and 13C:12C. The methods were calibrated using International Atomic Energy Agency-N (IAEA-N) standards for δ15N and the National Bureau of Standards 22 (NBS22) standard for δ13C. The standard deviations of the isotopic analysis standards were o 0.4‰ and o 0.1‰, respectively (n ¼ 10). Waters collected were analyzed for nitrogen (total nitrogen TNwater, and inorganic nitrogen – IN as the sum of nitrate, nitrite (NO3-) and ammonia nitrogen) and phosphate (PO43-), dissolved silicon (DSi), chlorophyll a (Chl a) and dissolved oxygen content. Well–known, standard colorimetric methods were used in these determinations (Rand et al., 1976). 2.4. Calculations and statistical analysis The fractional contributions of autochthonous OM to the thin (0–2 cm) upper layer of bottom sediment were estimated by reference to the binary mixing model (Murase & Sakamoto, 2000; Thornton & McManus, 1994). Relevant analysis concerned suspended matter collected at a depth of ca. 0.7 m from Sites S3 and S4 in July and August, and from Sites S1 and S2 in April. Determinations involved C/N, δ15N, and δ13C, with values obtained serving as end-member values of autochthonous and terrigenous OM. The binary mixing model is: Y s ¼ Y a ⋅ua þY t ⋅ut ua ¼

Y t −Y s Y a −Y t

ð2Þ ð3Þ

where:

Fig. 1. Bathymetric chart of the Solina Reservoir showing the locations of the sampling sites (S1-S4).

Yp is the value of C/N, δ15N, or δ13C in the bottom sediment, Ya is the value of the C/N, δ15N, or δ13C autochthonous endmember, Yt is the value of the C/N, δ15N, or δ13C terrigenous end-member, ua is the share accounted for by autochthonous OM sources, ut is the share accounted for by terrigenous OM sources.

Please cite this article as: Koszelnik, P., et al. An isotopic model for the origin of autochthonous organic matter contained in the bottom sediments of a reservoir. International Journal of Sediment Research (2017), http://dx.doi.org/10.1016/j.ijsrc.2017.10.002i

P. Koszelnik et al. / International Journal of Sediment Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 1 Variability nitrogen, phosphorus, silicon derivatives (mg L−1), and Chl a (μg L−1) concentrations and oxygen saturation (XO2, %) as well biogenic silica (% of sediment dry weight) in the Solina Reservoir. Abbreviations as in the text. Studied site

XO2 TNwater N-NO3IN P-PO43DSi BSi Chl a

S1

S2

S3

3

Table 2 ANOVA results of the average values for the bottom sediments’ parameters. Abbreviations as in the text. Parameter

F

p

OM (%) TN (%) TOC (%) C/N δ15N (‰) δ13C (‰)

9.52234 35.27909 18.97885 16.38004 10.46649 4.95980

0.000000 0.000000 0.000000 0.000000 0.000000 0.000472

S4

Mean

SD

Mean

SD

Mean

SD

Mean

SD

89.32 2.31 1.42 1.52 0.03 1.05 0.041 2.90

9.91 0.99 0.61 0.64 0.02 0.85 0.042 3.12

89.19 2.94 1.51 1.63 0.03 1.01 0.050 3.74

9.71 0.66 0.42 0.46 0.02 0.78 0.033 3.18

88.33 2.62 1.60 1.73 0.03 0.98 0.071 2.99

9.25 0.825 0.50 0.56 0.02 0.77 0.051 3.35

86.79 2.41 1.53 1.63 0.02 1.00 0.064 2.78

13.08 0.66 0.40 0.47 0.01 0.81 0.054 2.27

Results obtained were then processed for basic descriptive statistics such as the minimum, maximum, mean, and standard deviation values, while analysis of variance (ANOVA) was also carried out. The MS Excel 2007 program was used in these calculations. For linear relations, Pearson's correlation coefficient (R) with the corresponding p level of significance was calculated. The Student t-test was used to compare means for the two groups, while a non-parametric Kruskal–Wallis test assessed differences between a few groups. Multiple regression analysis was in turn used to determine the simultaneous influence of a few (at least two) independent variables on a dependent variable. This was done using the Statistica 10 PL Statistical Package. The significance level utilized was p o 0.05.

3. Results 3.1. Water quality The concentrations of the analyzed forms of nitrogen and phosphorus, as well as the level of chlorophyll a, indicated that Solina Reservoir is mesotrophic (Table 1). This was associated with a stable and relatively high level of oxygenation (85%, Table 1). The average concentration of TNwater varied from one study site to another across the range 2.31–2.94 mg L−1, with approximately 60% consisting of IN. The average concentration of P-PO43- was 0.03 mg L−1 at three sites, and 0.02 mg L−1 at Site S4. No seasonal variability was recorded for these indicators, unlike chlorophyll a and dissolved silica. Average concentrations of chlorophyll a at the studied sites were in the 2.78 to 3.74 μg L−1 range, while the maximum concentration observed during the summer was 13 μg L−1, mostly in the lower part of the reservoir. Maximum values for chlorophyll a were accompanied by minimum values for DSi content in the water (on the verge of complete depletion), while the average concentration for this indicator fluctuated in the 0.98–1.05 mg L−1 range (Table 1). This issue was discussed thoroughly in earlier publications (Koszelnik, 2013; Koszelnik & Tomaszek, 2008), while spatial and temporal variations in both water and bottom-sediment parameters were also considered (Bartoszek & Koszelnik, 2016; Tomaszek et al., 2009). 3.2. Elemental and isotopic composition of the surface layer of bottom sediments ANOVA for average concentrations of OM, TN, and TOC, as well as the C/N ratio and e δ13C and δ15N in the surface layer of bottom sediments of Solina Reservoir points to significant spatial differentiation with respect to these analyzed indicators. Averages for all characteristics examined differed significantly among the four sites studied. In each case, the test probability, i.e. the significance

level p for the F statistic (Fisher – Snedecor) was below the accepted significance level α ¼ 0.05 (Table 2). Average concentrations of OM in the reservoir sediments ranged between 8.72% at Site S2 and 10.11% at Site S3 (Fig. 2). Analysis of variance showed statistically significant differences in the average contents of OM between certain sites (Table 3). The point closest to the dam (S4) has the widest range of values for OM content (from 5.6 to 12.0%). At the sites located in the river zone (S1 and S2), OM varied across a much narrower range of concentrations. When it came to concentrations of organic carbon in the surface layers of the reservoir's bottom sediment, two groups of sites with significantly different average concentrations of TOC were observed (Table 3). Significant similarity was only indicated between Sites S1 and S2 and between S3 and S4. Sediment collected at Sites S3 and S4 had the lowest concentrations of TOC (2.09 and 1.95%, respectively), while those from S1 and S2 showed slightly higher concentrations (2.92 and 2.77%) (Fig. 2). The progressively lower TOC values reported for bottom sediment along the axis of Solina Reservoir is distinctive, manifesting a significant correlation (n ¼ 4; R ¼ −0.9559; p o 0.05) between TOC and the average depth of the reservoir (D) at each studied site; in line with the function TOC ¼ −0.0232D þ 3.1091. The average concentration of TN in the surface layer of Solina Reservoir bottom sediment was rather balanced, oscillating in the 0.19–0.23% range, though with slightly lower values recorded in parts of the reservoir influenced by side streams (Fig. 2). There were no significant differences between sites for average values of TN (Table 3). The observed diversity of the chemical composition of the surface layer of bottom sediment collected from all the sites translated into a similar diversity of TOC and TN (C/N) ratios. Significantly the lowest C/N values (below 10) were recorded in the lacustrine zone (Sites S3 and S4) of the Solina Reservoir, while highest values characterized its riverine zone (Sites S1 and S2), with ratios at approximately 14 (Fig. 2). The bottom sediment at sites located in the Solina Reservoir's upper part (S1 and S2) had significantly similar (Table 3) and the lowest (~ 2.5‰) average values for δ15N. Proportions of 15N (δ15N 4 4.5‰) recorded in sediments collected at Sites S3 and S4 were in turn significantly higher (Fig. 2). Unlike the characteristics of bottom sediment analyzed previously, δ15N assumed a relatively wide range of values in the Solina Reservoir (Fig. 2). Average δ13C values in sediment varied across the narrow range of -27.39‰ at Site S2 to -27.66‰ at S3 (Fig. 2). Average values for δ13C within the reservoir were not found to differ significantly (Table 3). Analysis of variance for the four studied sites and six analyzed characteristics of sediment identified two distinct groups of sites. Except in the case of OM concentration, average values for indicators did not differ significantly between Sites S3 and S4, or between Sites S1 and S2.

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Fig. 2. Boxplots of parameters of the bottom sediment studied. Diamonds sequentially from top: maximum, mean and minimum values, dashes means 7 one standard deviations. Abbreviations as in text. Table 3 The matrix of statistical significance (p-value) of the exact F-test calculated of the average values for the bottom sediments’ parameters, α ¼ 0.05. The significant differences marked in bold. Abbreviations as in the text.

Table 4 The values of isotopic (δ) and elemental (C/N) ratios of suspension sampled from sites S1 and S2 and also S3 and S4. Source of OM

Studied site

S1

S2

S3

S4

OM (%) TOC (%) TN (%) C/N δ15N (‰) δ13C (‰)

1 1 1 1 1 1

1 1 1 1 1 1

1 1 1 1 1 1

1 1 1 1 1 1

S2

OM (%) TOC (%) TN (%) C/N δ15N (‰) δ13C (‰)

0.0402 0.6224 0.6992 0.7818 0.8382 0.8830

1 1 1 1 1 1

1 1 1 1 1 1

1 1 1 1 1 1

S3

OM (%) TOC (%) TN (%) C/N δ15N (‰) δ13C (‰)

0.6284 0.0057 0.1101 0.0000 0.0000 0.2875

0.0119 0.0214 0.1004 0.0000 0.0000 0.2266

1 1 1 1 1 1

1 1 1 1 1 1

S4

OM (%) TOC (%) TN (%) C/N δ15N (‰) δ13C (‰)

0.6405 0.0014 0.4520 0.0000 0.0000 0.5529

0.1177 0.0060 0.2587 0.0000 0.0000 0.4608

0.3465 0.6092 0.3818 0.8700 0.4312 0.6488

1 1 1 1 1 1

S1

3.3. End member study and origin of the bottom sediments OM Table 4 lists reference values determined for the indicators used in the model (C/N, δ15N, and δ13C), as conforming to the

Allochthonous

Site, no. of measurements

S1 and S2, n ¼ 4 Autochthonous S3 and S4, n ¼ 8

C/N

δ15N (‰)

δ13C (‰)

17.1 7 1.65 1.06 7 1.24 − 28.63 7 0.13 6.8 7 0.37

6.67 7 1.48 − 26.77 7 0.42

composition of matter of either planktonic or terrigenous origin. Fig. 3 in turn shows the distribution of reference values and actual values for C/N, δ13C, and δ15N, at the time of sample collection. The average values of C/N and δ15N in the reference samples (Table 4) differed significantly from the average values at specific sites (ANOVA, p o 0.05). In contrast, there were no significant differences in the case of δ13C. Moreover, by observation of the distribution of observed values of the aforementioned indicators in comparison with reference values, observed ranges of δ13C were found to overlap with respect to both C/N and δ15N. For this reason, and because the reference δ13C did not manifest statistically significant differences (p o 0.05), this indicator could not be used in the reference study using Eq. 3. This interpretation is in line with Lehmann et al. (2004), who concluded that the use of the C-isotopic composition of particulate organic matter (POM) as an indicator of primary productivity in surface waters may be limited in less-productive lakes in which terrestrial inputs are relatively large. The results of the model calculations, estimated on the basis of the total rate of sedimentary accumulation (Bartoszek & Koszelnik, 2016) of organic matter, organic carbon, and autochthonous

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Table 6 Relations between the δ15N, and δ13C values and chosen parameters of the studied waters and sediment expressed as the Pearson’s correlation coefficient with its statistical significance, dash – means insignificant. Abbreviations as in the text. S1 and S2, n ¼ 32

S3 and S4, n ¼ 31

δ15N

δ13C

δ15N

δ13C

– 1 – −0.3960c – 0.3720c

1 0.3526c – – – –

– 1 −0.4368c – −0.3674c –

– – – – – – – – –

0.4461c – 0.3675c – 0.4273c 0.4745b −0.4423c – 0.3720c

– – – – – – – 0.4259c –

Bottom sediment 15

δ N δ13C OM TN TOC C/N

1 0.6249a – −0.4292c – – Superficial water

TNwater N-NH4 þ N-NO3DSi Chl a IN/DSi O2 XO2 Δ NO3a b c

– – – – – – – – 0.4725b

p o 0.001; p o 0.01; p o 0.05

autochthonous sedimentation of OM, nitrogen, and carbon occurred at Sites S1 and S2 than in the lower parts of the reservoir. 3.4. The influence of external factors on ratios of tracer isotopes in the sediments

Fig. 3. The distribution of end members and actual values of elementary and isotopic ratios at the time of sample collection (note: terrygen. ¼ terrigenous, autocht. ¼ autochthonous).

Table 5 Share and amount of autochthonous origin OM, TN, and TOC accumulated in the Solina Reservoir sediment calculated on the basis of the binary mixing model using end-member values of C/N (uaC/N) and δ15N (ua δ15N).

S1 S2 S3 S4

ua C/N

ua δ15N (%)

uamean

OM

TN (t yr-1)

TOC

24 27 73 75

26 28 67 61

25.0 27.5 70.0 68.0

717 789 1001 972

18.2 20.1 25.9 25.2

217 239 301 292

nitrogen, are as listed in Table 5. It was concluded that, at sites located in bays under the influence of side streams, organic matter of autochthonous origin represented 24–28% of all OM in the surface layer of the bottom sediments. The reverse proportion was observed at sites located in the center of the lake and near the dam, where OM originating from production within the reservoir represented 61–75% of all accumulated material. Multiplication of the obtained values of material accumulated in the sediment by the sedimentation rate for each element allowed for determination of rates of sedimentation of OM, organic carbon, and autochthonous nitrogen in the Solina Reservoir's upper part (Sites S1 and S2) and its lower part (S3 and S4) (Table 5). Only slightly less

Statistical methods determined the influence exerted by OM (expressed as δ15N and δ13C values) on concentrations of the analyzed forms of nitrogen, phosphorus, and silica in the Solina Reservoir. Significant R values for Pearson correlations involving the two groups of studied sites are listed in Table 6. With one exception, no significant correlations are found when the isotopic composition of organic carbon in the bottom sediment is set against parameters describing the reservoir water. The exception relates to a positive influence of oxygen saturation of surface water (XO2) (Table 6). All studied sites featured a significant positive influence of consumption of nitrates (ΔNO3-, i.e. the change in concentration of nitrates between the two dates of collection of water samples) on the increase in δ15N. The remaining physicochemical parameters of water did not exert a significant influence in changing the isotopic composition of nitrogen in the bottom sediment of the Solina Reservoir Sites S1 and S2. However, at Sites S3 and S4, where a considerable proportion of matter of autochthonous origin was identified in bottom sediments, the content of nitrogen in the reservoir's euphotic zone did influence changes in δ15N significantly. Significant positive correlations were recorded for concentrations of total N, nitrates, and ΔNO3-. Moreover, an increase in 15N in sediment at these sites may be stimulated by an increase in the molar IN/DSi ratio (Table 6), as well as by an oxygen deficit in the reservoir bottom layer. A significant correlation also was obtained between isotopic and elementary indicators characterizing the bottom sediment (Table 6). Moreover, a significant interdependence of the two isotopic indicators was found, meaning that 15N enrichment of bottom sediment is everywhere associated with enrichment in 13C. This phenomenon is more noticeable at Sites S1 and S2 in the upper part of the reservoir. This correlation also is important near

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the dam, though the R coefficient there has a lower value. Additionally, there was a negative influence of TN content on values of both isotopic indicators at Sites S1 and S2. 3.5. An isotopic model for the origin of the Solina Reservoir bottom sediments Analysis of variance shows that δ15N is the parameter most accurately describing variations in the proportion of matter of autochthonous origin in the Solina Reservoir bottom sediment. Unlike carbon, nitrogen is characterized by average isotopic ratio values that differ significantly from one study site to another (see Table 2). Moreover, a more marked influence of external factors was recorded for changes in δ15N than for those involving δ13C (Table 6). Regression analysis of the influence of variables on values of δ15N (Table 5) shows that the required independent variable as a function of the remaining dependent variables is best described by:

δ15 N ¼ 1:92ð 7 0:54Þ þ 1:13ð 7 0:20ÞΔNO−3 þ0:08ð 70:03Þ

IN þ 0:45ð 7 0:19ÞTN water DSi

Fig. 4. Relations between carbon and nitrogen isotopic ratios for sites S1 and S2 (n ¼ 32; R ¼ 0.6249; p ¼ 0.0000), S3 and S4 (n ¼ 31; R ¼ 0.4339; p ¼ 0.0147).

autochthonous origin makes in the reservoir's bottom sediments. IN 1; 92 þ 1; 13ΔNO−3 þ0; 08DSi þ0; 45TN−δ Nt 15

ua ¼

δ15 N a −δ15 N t

ð5Þ

4. Discussion ð4Þ

where:

δ15N is the ratio of the stable isotopes of nitrogen in the bottom sediment of the Solina Reservoir (‰); Δ NO3- is the decrease in nitrate concentrations in the water of the Solina Reservoir (mg L-1); IN/DSi is the molar concentration ratio of assimilable forms of nitrogen (nitrates (V) þ ammonium-N) and of dissolved silica in water (-); TNwater is the concentration of total nitrogen in the epilimnion (mg L-1). Based on the summary of the regression characterizing the dependent variable δ15N (R ¼ 0.6437; R2 ¼ 0.4144; F(3.58) ¼ 13.683; p ¼ 0.0000; Standard Error of Estimate ¼ 1.0838), it can be concluded that the aforementioned model describes 41% of the variability in δ15N. Incorporation of additional dependent variables does not improve the parameters of the model significantly, and, as can be seen, it is changes in the nitrate content of the reservoir epilimnion that influence the value of δ15N most. Consumption of 1.13 g m-3 of NO3- causes a 1‰ increase in the content of the heavier isotope of nitrogen in bottom sediments. Likewise, an increase in the concentration of total nitrogen influences this value, with an additional 0.45 g m-3 also resulting in a 1‰ enrichment of 15N in sediment. The increase in the value of δ15N is dependent on the ratio of the molar concentrations of mineral nitrogen and dissolved silica. In the case considered, this influence is considerable. Enrichment of the water in nitrates (V) and ammonia, or a deficit of dissolved silica in the water causing a 0.08 change in the IN/DSi ratio, lead to a 1‰ increase in the content of 15 N. This will result in a significant increase in the amount of matter of autochthonous origin accumulated in the sediment. The validity of the model was further demonstrated through analysis of the correlations between the three independent variables included in Eq. (4). As these correlations did not achieve significance, and as a normal distribution of the data was present, the validity of the model is deemed to be confirmed. When the generated model for the isotopic composition of nitrogen in the bottom sediment of the Solina Reservoir is combined with the binary model (Eq. (3)), the result is the equation given below: a model of the contribution organic matter of

While the bottom sediment examined is relatively poor in organic matter, the amount of OM accumulated in such sediment is known to be significantly smaller in dam reservoirs than in natural lakes, where OM may exceed 25% and even reach 70% (Mielnik et al., 2009; Tarnawski et al., 2015). This is related to the much longer residence time, and hence to sedimentation of suspended matter. Comparison of results presented here with data from the same site in previous years (Tomaszek & Czerwieniec, 1996) implies a c. 10% increase in both OM and TOC in the sediment of the Solina Reservoir over the last 20 years. This could have been caused by internal factors, i.e. production. It was concluded that TOC accounts for approximately 30% of the OM at Sites S1 and S2, and for 20% at S3 and S4. Sediments of the surface layer of this body of water were poor in TN. Recorded values of 0.20–0.25% are average for bodies of water with drainage areas consisting mainly of deciduous forest (Mielnik et al., 2009). Sediment in an area affected by side streams had higher concentrations of TOC than those in a lacustrine area. This is in contrast with the behavior of TN, which is more abundant in the lower part of the reservoir than its upper part (see Fig. 2). The foregoing values translated into values for the C/N ratio above 14/1 in the cases of Sites S1 and S2 and below 10/1 at S3 and S4. The C/N o10/1 is assumed to indicate an autochthonous origin of OM, which is composed mostly of proteins, while higher values indicate supply of terrigenous matter, in which the main component is cellulose (Calvert, 2004; de Junet et al., 2005). However, this cannot be a definitive conclusion because a significant part of the OM may be degraded in the upper water column at sites of all depths, with preferential release of carbon relative to nitrogen. The quantity of this release depends on the type of organic compound, leaving the C/N ratio in the final deposited sediments as a function of both the source of OM and the mechanism by which it decomposes in the water column (Chen et al., 2008 and references therein). Variation in sources of origin of OM were also shown to influence values of the isotopic ratios involving δ13C and δ15N, in the sediments of the analyzed reservoir. Suspended and deposited OM is a mixture deriving from various sources with theoretically different isotopic compositions. Typically, carbon of autochthonous origin has lower δ13C values than allochthonous organic carbon (de Kluijver et al., 2014; Karlsson et al., 2012). For δ15N, the converse is true (Gu et al., 1999; Torres et al., 2012). Gu (2009) found higher

Please cite this article as: Koszelnik, P., et al. An isotopic model for the origin of autochthonous organic matter contained in the bottom sediments of a reservoir. International Journal of Sediment Research (2017), http://dx.doi.org/10.1016/j.ijsrc.2017.10.002i

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δ15N in the case of particulate OM in eutrophic lakes, as opposed to oligotrophic lakes, around the world. In the case of δ13C, an overall relation between isotope composition and trophic state has not been reported, presumably on account of the complex nature of carbon metabolism and cycling in lake food webs. Moreover, an important role may be played by watershed area and development (C3 or C4 sources) (de Kluijver et al., 2014; Karlsson et al., 2012; Torres et al., 2012). Lehmann et al. (2004) report enrichment of organic carbon in 13 C during the summer, accompanied by higher values of δ15N. This led the authors to propose that the assimilation of atmospheric N2 during summer fixation may have contributed to the low 15N/14N ratio. If dinitrogenation serves as an N source, the δ15N value of the bulk OM may be lowered significantly as isotopic fractionation associated with N2 fixation is limited, compared with photosynthesis, and the δ15N-N2 is close to 0‰. The δ13C seasonal variation noted results from stable isotope composition of the substrate, dissolved organic carbon, and low primary production without carbon dioxide (CO2) limitation. As the bottom sediment of the Solina Reservoir display no seasonal changes in isotopic compositions of carbon and nitrogen (Koszelnik et al., 2008; Tomaszek et al., 2009), it can be concluded that the temporal effect of OM enrichment in 15N and impoverishment in 13C reflecting primary production was overshadowed by the flow of terrigenous matter. However, this phenomenon is clearly visible once the spatial distribution of the isotopes of N and C is taken into consideration. In the lacustrine area, the values of δ13C were higher than in the river area, and the values of δ15N were lower. However, analysis of variance demonstrated that, in the case of carbon, the differences in isotopic composition between sites studied within the reservoir were not statistically significant, unlike in the case of nitrogen. As a result, previous studies on the proportion of autochthonous OM accumulated in bottom sediment instead made reference to differences in values of δ15N. In conclusion, reference to both C/N and isotopic ratios is seen to assist in the identification of OM sources in the Solina Reservoir. In the analyzed case, an explicit influence of the production of organic matter in the central and near dam areas of the reservoir can be seen, while the area augmented by side streams is mostly burdened by material from the drainage basin. A binary mixing model has increasingly been applied in estimating the influence on both coastal and inland ecosystems of organic matter originating from the two different sources. Thornton and McManus (1994) determined that the proportionality of matter of terrigenous origin to matter of marine origin was decreasing in the bottom sediments of the Tay Estuary. In their paper, the authors gave examples of ecosystems in which a significant correlation between values of δ13C and δ15N justifies the belief that one or other source prevails. A lack of this type of correlation indicates a high proportion of both sources. Confirmation can be found in the paper by Gu et al. (1999), in which such a correlation indicated a strong influence of primary production in bioslurry in the lakes of Florida. In the case of the Solina Reservoir, regression analysis points to a significant (Fig. 4; R2 ¼ 0.3890; p ¼ 0.0001) correlation between the isotopic indicators C and N at Sites S1 and S2, in which a predominance of allochthonous matter is indicated. In contrast, in the area where primary production dominates (at Sites S3 and S4), this correlation proves weaker (Fig. 4; R2 ¼ 0.1883, p ¼ 0.0147), in contrast with the finding reported in Gu et al. (1999). Similar conclusions to those of Thornton and McManus were arrived at by Graham et al. (2001), as well as Perdue and Koprivnjak (2007), in work on estuaries, as well as by Murase and Sakamoto (2000), as they identified areas of deposition in Lake Biwa.

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Applying this model to the conditions of the Solina Reservoir (Table 5), it is concluded that approximately 18–25 t of TN and 217–301 t of TOC accumulate in bottom sediment each year. Despite a stronger (approx. 70%) influence of primary production on the balance of organic matter in the lower part of the reservoir, as opposed to the upper part (where the corresponding figure is 25%), different rates of sedimentation at these sites result in similar weight ratios for the elements. Prus et al. (2006) reported that primary production of phytoplankton in the Solina Reservoir amounts to some 33,000 t of sediment yr-1, equating to approximately 25,000 net t yr-1 of organic carbon and 4000 t yr-1 of nitrogen of autochthonous origin. Thus, 0.6–1% of primary production undergoes accumulation in the bottom sediments of the reservoir. This value is clearly higher than the average given for aquatic ecosystems, which is approximately 0.1% of net primary production (Lehmann et al., 2002). Multiple regression shows that assimilable forms of nitrogen and silica influence the value of δ15N reported in bottom sediments, with this being identified as the most unequivocal indicator of the deposition of organic matter of autochthonous origin in bottom sediments (Eqs. (4) and (5)). Statistical analysis shows that, as a characteristic of autochthonous OM, a higher content of δ15N was directly proportional to the content of total and inorganic nitrogen, as well as to reduced nitrate-nitrogen in the water of Solina Reservoir's euphotic layer. Favoring positive values of δ15N in particular is a deficit of silica, and consequently increased IN/DSi ratio. This confirms the principle of the silica pump (whereby this compound takes over as a factor limiting production among diatomaceous algae, while an abundance of nitrogen and anthropogenic phosphorus is used in the intense production of other algae). Such scenarios are observed commonly in various ecosystems. For example, in Brest Bay (Ragueneau et al., 2002), a seasonal deficit of silica under 1 µmol causes an increase in primary production of an intensity similar to that observed in the Solina Reservoir (with the level of Chl a increasing from below 1 to above 4 mg m-3). An increase in the production of organic matter as a result of a deficit in silica and a constant supply of the remaining biogenic compounds has also been found in the waters of the Baltic Sea (Humborg et al., 2006) and the Black Sea (Humborg et al., 1997). A similar phenomenon occurs in the inland waters of Lake Biwa (Harashima et al., 2006) and the Great Lakes (Schelske, 1999). Such conclusions do not seem to be supported by low concentrations of BSi residues recorded in bottom sediments in this research (Table 1). The average concentrations found in the thin (2 cm) upper sediment layer vary from 0.041% at Site S1 to 0.071% at Site S3, and are statistically indistinguishable. However, slightly higher values were observed for Sites S3 and S4, where DSi depletion was greater than at Sites S1 and S2. There may be several reasons for the low BSi content. In reservoirs with strong flow, some BSi could be discharged from the body of water. Displacement of BSi beyond the production site may be characteristic of flow ecosystems in which resuspension is occurring. Teodoru et al. (2006) report that the concentrations of BSi in bottom sediment are considerably lower in the riverine zone of the Iron Gate Reservoir than its lacustrine part (0.2% vs 0.8%). The same paper reports sediment–core results whereby lower concentrations of BSi were identified in the thin upper layer as opposed to deeper ones. Discharge of BSi outside an ecosystem can be facilitated where this is soluble. Ran et al. (2013) reported that approximately 50% of the sedimented BSi could be dissolved to the DSi in the deep reservoir. This resource can leave even more readily, as the outflow from the Solina Reservoir takes place through a penstock located approximately 40 m under the water surface, with higher BSi concentrations expected near the dam. This conclusion may be supported by relatively limited (2–5%) DSi retention identified for

Please cite this article as: Koszelnik, P., et al. An isotopic model for the origin of autochthonous organic matter contained in the bottom sediments of a reservoir. International Journal of Sediment Research (2017), http://dx.doi.org/10.1016/j.ijsrc.2017.10.002i

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8

the reservoir during this period (Bartoszek & Koszelnik, 2016). However, the problem of silicon fluxes in the Solina Reservoir requires further in-depth analysis.

5. Conclusions Stable isotopes can prove useful, not only in the qualitative interpretation of fluxes of biogenic compounds, but also in quantitative research on the accrual of bottom sediment. When the results of the research detailed here are set against those in the subject literature, a need is seen for input data in quantitative modeling to be supplemented by microbiological elements that can complement the chemical data. In the case of reservoirs, comprehensive data on both water and sediment chemistry, stable isotope content, and biological indicators are all lacking. Nevertheless, the analyses carried out and described here sustain following conclusions: 1. Analysis of fresh organic matter accumulated in the surface layer of the reservoir bottom sediment points to significant spatial differentiation in chemical composition. 2. In the Solina Reservoir, accumulated OM was mostly of autochthonous origin in the lacustrine zone, and mostly terrigenous in the riverine zone. 3. There was no identified influence of seasonality on autochthonous sedimentation, probably because of overlap with the matter supplied from the direct drainage area. 4. Results show that the isotopic ratio of nitrogen (δ15N) is the indicator of greatest significance in describing the proportion of organic matter of autochthonous origin present in the Solina Reservoir bottom sediment. 5. Mathematical modeling sustained the conclusion that a significant influence on the generation of autochthonous sediment in the reservoir is being exerted by the concentration and consumption of nitrates, as well as the deficit of silica in the euphotic layer.

Acknowledgements The research gained financial support from Poland's Ministry of Science, via grants no. 2 PO4G 0842 and no. 2011/03/B/ST10/ 04998 from the Polish National Science Centre.

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Please cite this article as: Koszelnik, P., et al. An isotopic model for the origin of autochthonous organic matter contained in the bottom sediments of a reservoir. International Journal of Sediment Research (2017), http://dx.doi.org/10.1016/j.ijsrc.2017.10.002i