СПЕКТРАЛЬНЫЙ ВКЛАД В ПОКАЗАТЕЛЬ ОСЛАБЛЕНИЯ СВЕТА ОПТИЧЕСКИ АКТИВНЫХ КОМПОНЕНТОВ В ВОДАХ ТЕЛЕЦКОГО ОЗЕРА | Опубликовать статью ВАК, elibrary (НЭБ)

СПЕКТРАЛЬНЫЙ ВКЛАД В ПОКАЗАТЕЛЬ ОСЛАБЛЕНИЯ СВЕТА
ОПТИЧЕСКИ АКТИВНЫХ КОМПОНЕНТОВ В ВОДАХ ТЕЛЕЦКОГО ОЗЕРА

Научная статья

Акулова О.Б.^1, *, Букатый В. И.², Кириллов В.В.³ 

¹ ORCID: 0000-0002-3677-090X;

^{1, 2, 3} Институт водных и экологических проблем Сибирского отделения Российской академии наук,
Барнаул, Россия

* Корреспондирующий автор (akulova8282[at]mail.ru)

Аннотация

В работе представлены результаты измерений спектрального показателя ослабления света ɛ(λ) и показателя поглощения света жёлтым веществом κ_жв(λ) озёрной водой в диапазоне 400–800 нм по акватории Телецкого озера (Горный Алтай), полученные летом (19–23 июня) 2018 г. в ходе экспедиций лаборатории гидрологии и геоинформатики ИВЭП СО РАН. В исследуемый период значения показателя ослабления (рассчитан при натуральном основании логарифма) в различных точках отбора проб варьировали в пределах 0,2–4,4 м^–1. Дополнительно, для определения трофического статуса озера, рассчитывали концентрации хлорофилла а Chlа, которые за исследуемый период находились в диапазоне 0,4–1,8 мг/м³, средняя величина составила 0,8 мг/м³. Максимальные значения Chlа выявлены для мелководных, защищенных от волноприбойных процессов хорошо прогреваемых участков зарастающей макрофитами литорали. Минимальные значения концентрации хлорофилла а в поверхностном слое отмечены на участках открытой пелагиали. В результате экспериментов получено, что трофический статус оз. Телецкое в различных точках отбора проб можно охарактеризовать как олиготрофный водоём с элементами мезотрофии на литорали, на участках впадения крупных притоков и расположения населённых пунктов. Также рассчитан относительный спектральный вклад основных оптически активных компонентов озёрной воды (чистой воды, жёлтого вещества, взвеси и фитопланктона) в показатель ослабления света по акватории исследуемого водоёма. Выявлено, что во всех точках (всего 22 точки отбора проб) озера максимальный вклад в ɛ(λ) вносит жёлтое вещество. Анализ пространственного распределения показателей ослабления и поглощения света жёлтым веществом озёрной воды показал, что озеро Телецкое существенно отличается не только своими гидрооптическими характеристиками, но также и гидробиологическими, характеристиками, следовательно, общее ослабление света озёрной водой может служить объективным маркером гидрофизической структуры водоёма и его экологического состояния.

Ключевые слова: спектральный показатель ослабления света, жёлтое вещество, взвесь, фитопланктон, хлорофилл а, чистая вода, Телецкое озеро.

SPECTRAL CONTRIBUTION OF OPTICALLY ACTIVE WATER COMPONENTS
OF LAKE TELETSKOYE TO LIGHT ATTENUATION

Research article

Akulova O.B.^1, *, Bukaty V.I.², Kirillov V.V.³ 

¹ ORCID: 0000-0002-3677-090X;

^{1, 2, 3} Institute for Water and Environmental Problems, Siberian Branch of the Russian Academy of Sciences,
Barnaul, Russia

* Corresponding author (akulova8282[at]mail.ru)

Abstract

The paper presents the measurement data on spectral light attenuation ɛ(λ) and light absorption by yellow substance κ_ys(λ) in waters of lake Teletskoye (Altai) in the range from 400 to 800 nm, which were obtained in June 19–23, 2018 during expeditions arranged by the Laboratory of Hydrology and Geoinformatics, IWEP SB RAS. In different sampling sites, attenuation (calculated at the natural logarithmic base) varied as 0.2–4.4 m^–1. To identify the lake’s trophic status, we calculated chlorophyll a C_hla concentrations (0.4–1.8 mg/m³, at the average of 0.8 mg/m³). Maximal C_hla was recorded in the shallow, well-warmed areas of the littoral overgrown with macrophytes and protected from wave-breaking processes. Minimal concentrations of chlorophyll a were observed in the surface layer of the open pelagial areas. The experiments suggest that lake Teletskoye in various sampling sites can be characterized as oligotrophic (by its trophic status) with mesotrophic elements in the littoral, at the confluence of large tributaries and the settlements’ location. The relative spectral contribution of main optically active components of lake water (pure water, yellow substance, suspension and phytoplankton) to light attenuation in the study water body was also calculated. It was found that yellow substance was responsible for the greatest contribution to ɛ(λ) in all 22 sampling sites of the lake. The analysis of spatial distribution of light attenuation and absorption by yellow substance is evidence of a big difference in hydro-optical and hydrobiological characteristics of lake Teletskoye. Therefore, total light attenuation in water can serve as an objective marker of a hydrophysical structure and ecological state of the lake.

Keywords: spectral light attenuation, yellow substance, suspension, phytoplankton, chlorophyll а, pure water, Lake Teletskoye.

Introduction

Optically active components of any natural (ocean, sea, lake, large pond) or artificial (reservoir, channel, catchment/basin, dam) water body are yellow substance, suspension, phytoplankton and pure water affecting strongly on total light attenuation [5], [14], [27].

The main optical component of water is its base, i.e. pure water that is a chemically pure substance consisting of atoms of various isotopic varieties/combinations of hydrogen H and oxygen O without admixture of any other substances. As a reference sample for measuring spectral water transparency by means of a spectrophotometric method, we use distilled water of high purity.

Suspension is among the most active optical components of natural water affecting absorption and, especially, light scattering. In the literature, water suspension is often understood as a combination of large (with a diameter of 0.5 µm–1 mm), small (0.45÷1 µm) and colloidal (0.001÷0.1 µm) particles. Depending on particle composition, it is divided into terrigenous (mineral) and biological (organic) components [16]. The absorbing properties of suspension particles (mineral and organic) differ considerably. For mineral particles, absorption rates are low and often neglected, especially in the surface waters [5]. The same applies to dead organic particles, i.e. detritus that is a single/uniform complex consisting of particles of dead organic at different stages of transformation and associated microorganisms. It can be as organic as organo-mineral detritus [10]. Light absorption in a living organic suspension (phytoplankton) occurs differently. Two maximal absorption peaks, i.e. blue and red ones appear at 430–440 nm and 670–680 nm, respectively. Besides of two major maxima in phytoplankton absorption spectra, some less pronounced peaks are also noted. In large reservoirs, phytoplankton-related absorption by natural water is observed only in the surface layers, where the amount of solar radiation is sufficient for photosynthesis. In deep waters below the euphotic zone, where suspension is represented by detritus and mineral particles, absorption bands do not exist at all [7]. In reservoirs, light scattering by suspended particles is one of the most important optical phenomena. It is a common knowledge that light scattering parameters depend on optical constants of suspended particles, particle size and wavelength of light. The relative refractive index n for mineral particles varies within 1.15−1.20 or even wider (1.13–1.25) depending on their mineral composition [5]. Bearing in mind that in natural waters mineral suspension exists mainly in the form of organo-mineral complexes, the real average refractive index demonstrates a shift towards lower values. The nature of light scattering also changes. As for the relative refractive index of organic particles, it is in the range 1.02–1.05 [5], [13]. Thus, suspension is understood as a set of organic and mineral particles with a diameter of 0.001 µm –1 mm.

Yellow substance is a major component affecting the processes of biological self-purification and water quality formation, the ecological state of water bodies to be exact. In line with the generally accepted approach, yellow substance (YS) is a constituent of dissolved organic matter (DOM), i.e. the water that passes through a filter with a pore size of 0.45−1 µm. Dissolved organic substances, which intensively absorb UV and blue rays (thus making water yellow-brown) are referred to YS as well. According to [17], yellow substance consists of two main groups of compounds, i.e. phenol-humic and hydrocarbon-humic acids. It should be noted that spectral light absorption by yellow substance is exponential. For instance, with an increase in wavelength, absorption decreases exponentially.

Our research is aimed at assessing optically active water component effects on total light attenuation in waters of lake Teletskoye in the summer of 2018.

Materials and methods

The study of spatial distribution of spectral light attenuation ɛ(λ) in waters of lake Teletskoye is based on field data obtained by the Institute for Water and Environmental Problems (IWEP SB RAS) in June 2018. A detailed description of sampling sites, their toponymy and coordinates are given in [1]. We got the data on morphometry and bathymetry from [11].

In June 19−23, a total of 22 samples were taken from the lake’s surface layer (the distance from the water – atmosphere boundary to a 10−15 cm depth). In addition, 528 measurements of spectral water transparency were made using a PE-5400UF spectrophotometer (spectral range: 190−1000 nm; wavelength setting error does not exceed 1 nm; when measuring spectral transmittance at T (315−1000 nm), the allowable absolute error is 0.5%). For all samples, two measurements were performed (before and after filtration through «Vladipor» type MFAS-OS-1 membranes with a pore diameter of 0.22 µm) in the range of 400−800 nm in increments of 30 nm. Disk-shaped filters had a diameter of 35 mm and a total porosity of 80−85%. To determine water spectral transparency (transmittance), we used the spectrophotometric method based on measuring the ratio of two intensities of light fluxes passing through the tested and reference mediums. The latter was a distilled water of high purity (a control sample) obtained due to double distillation at the electric type aquadistillator VO AE-10 MO in accordance with GOST 6709-72. The calculation of ɛ(λ) (at the natural logarithmic base) was made by the formula

    (1)

derived from the Booger’s law, where L is the cuvette length,  – the transparency (transmittance) in relative units, I(λ), I₀(λ) – the intensity of transmitted and incident light, respectively, λ – the wavelength of light. The absolute error of ε(λ) is induced by the spectrophotometer instrument error at transmittance measurement (ΔТ=0.5%) and the error measurement of a cuvette length. In the experiment, we used cuvettes of L=50 mm long. The maximal absolute error in defining the light attenuation coefficient made up 0.1 m^–1.

The concentration of chlorophyll a in acetone extracts of phytoplankton algae was detected by means of the standard spectrophotometric method according to GOST 17.1.4.02-90.

The relative spectral contribution of optically active components of water (suspension, yellow substance, phytoplankton and pure water) to ɛ(λ) was calculated using a modified semi-empirical spectral model of light attenuation [2], which was first proposed by O.V. Kopelevich in [5]. It looks as

     (2)

where κ_ph(λ) and κ_ys(λ) – spectral parameters of absorption by phytoplankton and yellow substance, respectively, σ_mol(λ) – spectral molecular scattering by pure water, σ_s(λ) – spectral scattering by entire suspension, κ_pw(λ) –spectral absorption of pure water. When analyzing expression (2), it is necessary to keep in mind that depending on functional features of the device, the experimentally defined ɛ(λ) may produce an incorrect result. In our device, when measuring Т(λ), a single-beam photometric scheme is used to compare light fluxes passing through the reference sample (pure water) and the test mediums. Therefore, attenuation calculated by formula (1) does not contain the data on pure water attenuation ɛ_pw(λ)=κ_pw(λ)+σ_mol(λ). Thus, in formula (2), we sum up values of ɛ(λ) obtained from spectrophotometer measurements and ɛ_pw(λ) taken from reference data [21], [25].

Light absorption by yellow substance κ_ys(λ) is defined after measuring spectral transparency of lake water, treated from water suspension and chlorophyll a by filtration through membrane filters. The calculation of κ_ys(λ) is performed by the formula according to expression (1), where instead of ɛ(λ) we take κ_ys(λ) without κ_pw(λ). The effective value of κ_ys(λ) in formula (2) is calculated similar to the previous scheme through summing up κ_pw(λ). A filtration unit represents a filled with natural water funnel with a “nest” for the membrane filter at the bottom. The funnel is connected to the Bunsen flask to provide a vacuum of 10^–1–10^–2 mm Hg with the help of a forevacuum pump.

The index of phytoplankton absorption is calculated using the formula

     (3)

Here, C_chl is chlorophyll a concentration, in mg/m³, κ_sp.ph(λ) – specific index of phytoplankton absorption, in m²/mg; its values are given in the paper [5]. Tabular data are used for calculations of ĸ_pw(λ) [21], [25], and σ_mol(λ) – [25]. Spectral scattering by suspension σ_s(λ) is identified below by the formula from expression (2)

     (4)

Results and discussion

In June 19–23, 2018, spectral light attenuation ɛ(λ) in the surface layer in various sites (points) of lake Teletskoye was 0.2–4.4 m^–1 at 400–800 nm. For instance, at λ=430 nm, ɛ(λ) varied within 1.2–3.8 m^–1, and at λ=550 nm as 0.4–2.3 m^–1 (see figure 1). Wavelengths 430 and 550 nm are preferred because the first is maximal light absorption by chlorophyll a, while the second – the greatest solar radiation. Interestingly, these wavelengths are taken into account in developing optical vision devices and used as a marker in hydro-optical studies.

Fig. 1 – Schematic map of distribution of spectral light attenuation

 

In the littoral zone, at Chulyshman (site111) and Kyga (site 103) river mouths, light attenuation shows its maximum (2.2 and 2.3 m^–1, respectively) at a wavelength of 550 nm. This may be induced by intensive removal of suspensions (mainly of terrigenous origin) by river waters and coastal erosion. In this coastal zone with shallow waters, wind (drift) currents raise bottom sediments and mix them throughout the water column. As the pelagic site of lake Teletskoye stretches to the north up to the thermobar boundary, the values of ɛ(λ) decrease to 0.9–1.7 m^–1. Thermobar affects the lake’s ecosystem because it separates two zones with different aquatic characteristics. To the south of site 115, water temperature in the surface layer (a warm layer) is +10–12°C, whereas in the north (a cold layer) – +3.6–4.0°C) that provides spatial difference both in hydro-optical and hydrobiological properties as well. Concentrations of chlorophyll a C_hla varies from 0.4 to 1.8 mg/m³ with its average of 0.8 mg/m³. Maximal values of C_hla are registered in the shallow, well-warmed areas of the littoral overgrown with macrophytes and protected from wave-breaking processes. Minimal C_hla are marked in the surface layer in the open pelagial.

As an example, two characteristic spectra of light attenuation ɛ(λ) and light absorption by yellow substance κ_ys(λ) in the range of 400–800 nm with the highest and lowest values for the pelagic and littoral zones of the Kamga river are given in figure 2.

Fig. 2 – Spectral dependence of total light attenuation and absorption by yellow substance on λ
in the surface layer of lake Teletskoye:

1– ɛ(λ) in site 037 (Kamga river, pelagial); 2 – ɛ(λ) in site 038 (Kamga river, littoral);
3 – κ_ys(λ) in site 037 (Kamga river, pelagial); 4 – κ_ys(λ) in site 038 (Kamga river, littoral)

 

From figure 2 it follows that spectral light absorption by yellow substance is exponential; with an increase in wavelength, κ_ys(λ) decreases exponentially that is true only for the short-wave optical spectrum. However, at wavelengths >650 nm the exponential dependence disappears because of considerable absorption by pure water present in yellow substance according to the method used.

The paper [4] presents the measurement data on decimal light attenuation (in ten spectral bands obtained in 2000 during a couple of summer expeditions. The authors analyzed spatial distribution of ɛ(λ) for ten fixed wavelengths and did not establish any stable dependence of light attenuation on the coordinates of sampling site (9 sites). They defined average geometric values of ɛ(λ) based on data from both field trips to all study sites (0.67–2.46 m^–1). In July 22–27, 2013, other researchers [3] got new quantitative data on light attenuation dynamics in the surface layer of lake Teletskoye. Measurements were made in the spectral range of 400–800 nm in 14 sampling sites. At the natural logarithmic base, values of ɛ(λ) were within 0.1–5.3 m^–1. In the paper [12], the trophic status of lake Teletskoye was determined based on the measurements (2016) of light attenuation in a visible range. The values of ɛ(λ) were within 1.5–5.2 m^–1. Unlike [3], [4], [12], we managed to assess the effect of optically active water components on total light attenuation in the surface layer of lake Teletskoye due to study results of ɛ(λ) (2018) present in this paper.

Of course, the hydro-optical structure of lake Teletskoye is closely related to its hydrological regime, regional meteorological conditions, including optically active water components. Concentrations of the latter change both in time and in space under the influence of intra-water processes occurring in close relation with the catchment – a land area from which water with sedimentary and dissolved material enters the water body. Therefore, the quantitative and qualitative composition of optically active water components and their ratio will differ across the lake’s water area.

The calculated spectral contribution of optically active components of lake water to spectral light attenuation ε(λ) in the surface layer of lake Teletskoye in various sampling sites are evidence of essential optical influence of yellow substance and suspension on total attenuation (see table 1). Here, values of ε(λ) and κ_ys(λ) are given at the natural logarithmic base.

 

Table 1 – Spectral contribution of water components (%) to light attenuation in the surface layer of Lake Teletskoye
(June 19–23, 2018)

Wavelength of light λ, nm	Light absorption			Light scattering	Light attenuation ε(λ), m–1
site 002 (Chlа=1,4 mg/m3)
430 550	0,1 3,5	90,9 62,5	3,4 0,5	5,6 33,5	3,3 1,6
site 008 (Chlа=0,8 mg/m3)
430 550	0,2 5,6	82,6 60,0	2,8 0,5	14,4 33,9	2,3 1,0
site 014 (Chlа=1,0 mg/m3)
430 550	0,1 3,5	81,2 50,0	2,5 0,4	16,2 46,1	3,2 1,6
site 019 (Chlа=1,2 mg/m3)
430 550	0,2 4,7	62,9 50,0	3,5 0,6	33,4 44,7	2,7 1,2
site 021 (Chlа=0,5 mg/m3)
430 550	0,2 5,6	68,2 50,0	1,8 0,3	29,8 44,1	2,2 1,0
site 023 (Chlа=0,6 mg/m3)
430 550	0,3 8,0	87,5 71,4	3,0 0,5	9,2 20,1	1,6 0,7
site 025 (Chlа=0,7 mg/m3)
430 550	0,4 11,3	66,6 40,0	4,6 0,9	28,4 47,8	1,2 0,5
site 028 (Chlа=0,5 mg/m3)
430 550	0,3 9,4	85,7 50,0	2,8 0,5	11,2 40,1	1,4 0,6
site 031 (Chlа=0,4 mg/m3)
430 550	0,4 11,3	75,0 60,0	2,6 0,5	22,0 28,2	1,2 0,5
site 033 (Chlа=0,5 mg/m3)
430 550	0,4 11,3	75,0 60,0	3,3 0,6	21,3 28,1	1,2 0,5
site 036 (Chlа=0,7 mg/m3)
430 550	0,3 8,0	94,1 71,4	3,3 0,6	2,3 20,0	1,7 0,7
site 037 (Chlа=0,7 mg/m3)
430 550	0,3 8,1	82,3 71,4	3,3 0,6	14,1 19,9	1,7 0,7
site 038 (Chlа=0,4 mg/m3)
430 550	0,3 9,4	85,7 50,0	2,3 0,4	11,7 40,2	1,4 0,6
site 040 (Chlа=0,9 mg/m3)
430 550	0,3 11,3	80,0 80,0	4,8 1,2	14,9 7,5	1,5 0,5
site 045 (Chlа=0,6 mg/m3)
430 550	0,4 14,1	75,0 50,0	4,0 0,9	20,6 34,7	1,2 0,4
site 101 (Chlа=1,3 mg/m3)
430 550	0,1 3,3	84,3 76,4	3,2 0,5	12,4 19,8	3,2 1,7
site 103 (Chlа=1,8 mg/m3)
430 550	0,1 2,4	86,8 91,3	3,7 0,4	9,4 5,9	3,8 2,3
site 106 (Chlа=0,5 mg/m3)
430 550	0,3 14,1	85,7 50,0	2,8 0,7	11,2 35,2	1,4 0,4

 

End of table 1 – Spectral contribution of water components (%) to light attenuation in the surface layer of Lake Teletskoye
(June 19–23, 2018)

site 111 (Chlа=1.5 mg/m3)
430 550	0,1 3,1	78,3 66,6	3,2 0,5	18,4 29,8	3,7 1,8
site 112 (Chlа=0.5 mg/m3)
430 550	0,3 9,4	82,3 66,6	2,3 0,5	15,1 23,5	1,7 0,6
site 113 (Chlа=0.6 mg/m3)
430 550	0,4 14,1	83,3 50,0	4,0 0,9	12,3 35,0	1,2 0,4
site 115 (Chlа=1.0 mg/m3)
430 550	0,3 9,4	87,5 83,3	5,0 1,0	7,2 6,3	1,6 0,6

 

The largest contribution of YS at λ=430 nm falls on the pelagial of rivers Kamga (site 036) and Chulyshman (site 002), i.e. 94.1 and 90.9%, respectively. At a wavelength of 550 nm, the YS contribution varies from 40.0% (site 025 – the Adamysh river, pelagial) to 91.3% (site 103 – the Kyga river, littoral). Maximal suspension contribution to attenuation at λ=430 nm in site 119 (cape Syraktu, pelagial) reaches 33.4%, and at λ=550 nm – 47.8% (site 025, the Adamysh river, pelagial). Clean water makes an insignificant contribution (0.4%) to light attenuation at λ=430 nm in all sites. However, in the long–wave region at λ=550 nm it increases sharply – up to 14.1%. The contribution of phytoplankton at λ=430 nm is in the range from 1.8% (site 021 – the Kokshi river, pelagial) to 5.0% (site115 – the thermobar boundary), whereas at λ=550 nm – from 0.3% (site 021) to 1.2% (site 040 – Yaylu village, pelagial). The contribution of molecular light scattering by pure water in the study spectral range is inessential (about 0.1%).

Figure 3 shows comparative pie charts of relative spectral contributions to total light attenuation over three years.

 

Fig. 3 – Diagrams of distribution of spectral contribution of main optically active water components to ε(λ) in the surface layer of sampling site 021 (the Kokshi river, pelagial) of lake Teletskoye

 

Thus, yellow substance and suspension are major optically active components affecting total light attenuation in waters of lake Teletskoye.

Conclusion

The paper presents the limnological experiment findings on assessing the influence of main optically active components of water (pure water, yellow substance, suspension and phytoplankton) on light attenuation in the surface layer of lake Teletskoye in various sampling sites. The data were obtained due to a series of hydro-optical studies made in June 2018. It was found that yellow substance was responsible for maximal contribution to total light attenuation.

Specific hydrological and complex hydrothermal regimes of lake Teletskoye form the zones with different hydro-optical characteristics of ε(λ) and κ_ys(λ). These values are higher in the littoral (at the confluence of rivers Chulyshman and Kyga) as compared to the deep-water (pelagic) part. During the study period, ε(λ) in various sampling sites varies in the range of 0.2–4.4 m^–1, κ_ys(λ) – 0.1–4.0 m^–1 at 400–800 nm. We have calculated the concentrations of chlorophyll a and yellow substance and revealed a heterogeneity in their distribution. The concentration of yellow substance calculated from its measured transmittance coefficient varies as 2.6–14.1 g/m³, whereas the chlorophyll a content – within 0.4–1.8 mg/m³.

These results confirm the possibility of using hydro-optical characteristics in studies of large lakes for monitoring water pollution, qualitative assessment of suspended and dissolved substance concentrations as well as for monitoring the ecological state of lake waters.

Финансирование Работа выполнена в рамках Научно-исследовательской программы ИВЭП СО РАН (государственный регистрационный номер проекта ААА 17-117041210241-4).	Funding The work was carried out within the framework of the Research Program of IWEP SB RAS (state registration number of the project AAAAA 17-117041210241-4).
Благодарности Мы хотели бы поблагодарить Марусина К. В., научного сотрудника Лаборатории гидрологии и геоинформатики ИВЭП СО РАН за помощь в отборе проб воды.	Acknowledgement We would like to thank Marusin K.V., the researcher of the Laboratory of Hydrology and Geoinformatics, IWEP SB RAS for assistance in water sampling.
Конфликт интересов Не указан.	Conflict of Interest None declared.