Primary production and destruction of organic matter in Lake Arakhley (Eastern Transbaikalia, Russia)

. Global climate changes affect water ecosystems. Long-term observations of climatic variables are indicative of cyclic climate changes. The fluctuations result in changes in water levels in lakes, which, in turn, alter primary production processes. Planktonic algae are the main drivers of primary production. This research continues long-term studies on hydroecology of Lake Arakhley. The lake is used for fishery and recreation. This work aims to estimate production and destruction processes in Lake Arakhley during the current transitional period between dry and wet years. To this end, we performed field studies on Lake Arakhley in August from 2016 to 2021, during the season of maximum precipitation, in the central and littoral zones of the lake. Primary production of phytoplankton in water was estimated by the oxygen light and dark bottle method. Sampling was carried out to assess physical and chemical parameters of water using a multiparameter water quality instrument. Lake Arakhley features the low level of mineralization under alkali conditions and relatively stable level of mineralization registered from 2016 to 2021. Macrophytes zone expanded after the massive flooding of the shore in 2021 hence alterating the values of such environmental factors as oxygen levels, total and mineral phosphorus, with destruction prevailing over organic matter production. Depth and transparency however contribute to significant production of organic matter in the lake. The findings can be used as the groundwork for the sustainable management of the lake.


Introduction
Lake Arakhley is the deepest (14 m) water body located of the system of the Ivan-Arakhley Lakes that form the lynchpin of the Ivan-Arakhley Natural Reserve and belong to the Lake Baikal basin. Lake Arakhley lies in the south of the Vitim Plateau between the Yablonovy Ridge and Osinovy Ridge under ultracontinental climate conditions. The lake is situated in the forest and steppe zone at latitude 52˚48̍ -52˚10̍ N and longitude 112˚45̍ -113˚04̍ E at an altitude of 970 m above sea level (Baltic Height System, BHS-1977), with a surface area of 58.2 km 2 , a volume of 0.63 km 3 , the maximum depth of 17 m and the average depth of 10.4 m [1]. Lake Arakhley is used for fishery and recreation.
Global climate changes have been widely discussed over recent decades. Regionally, climatic shifts affect such environmental drivers as lake water levels, temperatures, etc. [2][3][4]. The fluctuations of the water level in Lake Arakhley observed since 1956 are cyclical in nature [2]. The minimum level (BHS-1977) was recorded within the latest cycle dated from 1980 to 2017 [5], when the depth of the lake decreased by 2 m.
Primary production in aquatic environments clearly reflects the impacts of climate change on the ecosystem [6,7]. Primary production and destruction of organic matter are considered not only as the basis for the functioning, but also for the sustainability of water ecosystems [8]. Hydrobiological studies which were conducted in Lake Arakhley since the 1960s and published in [1] cover spatio-temporal dynamics of the phytoplankton, biodiversity of species, seasonal and annual oscillations in primary production of the lake. According to the observations, Lake Arakhley is defined as a mesotrophic water body. The investigations on the lake are currently continued by [5,[9][10][11][12][13].
The study aims to estimate production and destruction processes in Lake Arakhley during the current transitional period between dry and wet years.

Materials and methods
Field studies on Lake Arakhley were carried out in August from 2016 to 2021, during the season of maximum precipitation, in the central and littoral zones of the lake (Figure). The period under study features a transitional phase within a long-term climate cycle. Samples were collected at each station (based on water depth and transparency) from the following water column horizons: for the central regionsurface layer -½ of transparency zonetransparency zonebottom layer; for the littoral zonesurface and bottom layers. Water transparency (CD) and depth (h) of the sampling areas were measured using a Secchi disc (diameter of 30 cm). The material samples from each layer were taken with a Patalas bathometer. For the primary production of phytoplankton was estimated by measurements of oxygen in dark and light bottles with 24 hours incubation period. Primary production and destruction of organic matter in the lake were calculated according to [14][15][16]. The total value of primary production (1 m 2 ) (ΣА) was calculated as Аmax (production maximum value) multiplied by the value of water transparency at the station; the total value of destruction (ΣR) was measured as the values of destruction multiplied by the average depth of the lake [7]. The content of dissolved organic carbon (Corg) was assessed using the ratio of carbon to oxygen: 1 mg O2 = 0.375 mg of Corg [17]. The concentration of phosphates was measured using reagent mixed with ascorbic acid; the content of total phosphorus was determined by oxidation of samples with potassium persulfate [18].

Environmental factors
Mountainous conditions of Lake Arakhley and its continental subarctic climate affect the physical and chemical properties of the ecosystem. The mean values of the physical and chemical properties of Lake Arakhley ecosystem are shown in Table 1.
From 2016 to 2021, the depth of the lake in the regular central zone increased by 0.6 m with transparency alterating from 4 to 6 m and the shoreline extending landwards for 1 m. Lake Arakhley features the low level of mineralization and alkali conditions. Oxidationreduction potential (ORP) of the ecosystem of Lake Arakhley ranges from 148 to 190 mV, which is conducive to oxidative conditions and by attendance a number of elements in the highest form of valence. Within the littoral zone, the content of dissolved oxygen is higher than in the central zone due to macrophytes vegetation. The concentration of total phosphorus is relatively low (0.023-0.057 mg L -1 ) with organic phosphorus only prevailing in 2021 compared to the dominance of orthophosphates detected during 2017-2020. The level of total phosphorus and orthophosphates is within the long-term fluctuations which is typical for lakes located in the forest and steppe zone [19]. no data 0.014±0.005 0.013±0.007 0.014±0.003 0.003±0.00 0.038±0.011 Note: numerator -indicators of the central zone; denominator -indicators of the littoral zone; h -water depth; T -water temperature; pH -active hydrogen index; TDS -total mineralization; EC -electrical conductivity; ORP -redox potential; DOdissolved oxygen; DO %degree of oxygen saturation of water; TP -total phosphorus; Pmin -orthophosphate.
The relatively stable level of mineralization in Lake Arakhley during 2016-2021 was observed by [20,21]. Changes in water level of the lake do not affect its mineral content by contrast with soda lakes of the steppe zone in the Zabaikalsky Region exhibiting a manifold increase in mineralization within the same period [22]. During the transitional phase between low water periods to high water periods, the depth and transparency of the lake gradually increased with macrophytes zone expanding after the massive flooding of the shore in 2021, and hence alterating the values of most environmental factors (Table 1).

Primary production and destruction of organic matter
Primary production is mainly driven by planktonic algae communities. In the 1960s, the predominant primary producers were diatoms. In this period of time destruction processes in Lake Arakhley are parallel to changes in production and are 2-3 times more intensive [23][24][25][26]. In the 1980s, 144 species and subspecies were registered in Lake Arakhley with diatoms making up more than 80% of annual average phytoplankton biomasses [27]. In 2017, 97 taxons were detected in the phytoplanktonic communities with predominant diatoms, Chlorophyta, Chrysophyta, and Cyanobacteriae. The total phytoplankton abundance ranged from 37.51 to 3733.4 thd cells L -1 and biomass varied from 36.9 to 5193.9 mg/m 3 . Changes within a year correlated with natural seasonal successions without extreme outbursts of particular groups or species [11].
Primary production and destruction of organic matter is indicative of the ecosystem stability under changes in environmental conditions. One of the main characteristics of the organic matter balance in a water body is the ratio of phytoplankton production to organic matter destruction. If the ratio is <1, the ecosystem is capable of self-purification [28]. The increase in depth of the lake by 2021 resulted in the dominance of destruction over production. In the littoral zone of water bodies, as the area of interference between water and land, destruction mainly exceeds production. The littoral zone of Lake Arakhley retains its barrier function at the boundary line between water and land. The mean values of primary production during the period of research are given in Table 2. The photosynthesis intensity in Lake Arakhley during 2016-2021 is relatively low ranging from 0.53 to 1.45 mg O2 L -1 in the central region and from 0.22 to 3.10 mg O2 L -1 in the littoral zone. Depth and transparency however contribute to significant production of organic matter under surface reaching to 1,540 mg C (m 2 ) -1 per year (Table 3). 2.9±0.33 3.4±0. 19 1.09 1.090 Note: ∑А and ∑R, mgО2/m 2 ·dayprimary production and destruction of OM for water column with a section of 1 m 2 per day; ∑А, mgC/m 2 ·daytotal primary production under 1 m2 expressed as carbon content; ∑∑А mgC/m 2 ·year -primary production per year According to the long-term observations on Lake Arakhley by [2], the period of the minimum water level in Lake Arakhley was recorded in 2017. Production values of the lake during this period are shown in Table 4. In the late May, after the ice melting in the central region, production is registered only in warm surface water layers with the temperature of 7.5 °С in the surface layer and 6.8 °С in the bottom layers. Production is hardly detected in the water column below the photic level up to the bottom horizons. In the warm shallow littoral zone, the photosynthetic production is distributed uniformly within the water column from the surface to the bottom with the temperature of 9.1 °С in the surface layer and 8.2 °С in the bottom layers. In August, higher solar exposure of the surface horizons (21.5 °С) leads to greater production contents in the layers with transparency of 1-1.5, whereas the bottom layers with lower transparency and temperature of 9.5 °С demonstrate lower concentrations of organic matter production. The volume of production in the littoral zone is relatively similar to the production in the surface layers of the central region.
In December, the highest contents of production are observed from the area of transparency to the bottom horizons due to longer range of insolation and higher temperatures at the bottom (surface 0.5 °С, bottom 3.5 °С). The findings correlate with regional characteristics of the lakes in the Eastern Transbaikalia with photosynthesis of water vegetation throughout the whole ice-cover period that is conditioned by winds blowing snow cover off the ice and by crystal ice structure with higher transparency [29]. Approximately 30 % of annual phytoplankton volume is produced during the ice-cover period that lasts longer than 7 months [27].
In the central region of the lake, higher destruction of organic matter is detected in the bottom layers in May, in the photic layerin August, whereas in December, destruction only occurs in lower horizons with the temperature of 4 °С. The littoral zone mainly exhibits destruction in the surface layers in May and August with the increase observed in the bottom layers in December.

Conclusion
The observations on long-term fluctuations in water chemistry under global climate changes contribute to the current state forecasting of aquatic ecosystems. The transitional period between dry years and wet years is characterized by changes in the structure of the ecosystem including those in production and destruction process. The level of mineralization in Lake Arakhley is relatively stable with higher content of dissolved oxygen and lower levels of oxygen saturation. Higher content of total phosphorus is due to the dominance of organic phosphorus produced within the lake.
The littoral zone during wet years is significant for primary production after the flooding of the shore with an expanded area of aquatic vegetation. By 2021, primary production and destruction of organic matter increase with greater share of destruction processes. In contrast to the littoral zone, the central region is less prone to changes that are however observed at deeper levels with higher transparency and greater organic matter production. In the dry period (2017), the maximum production was recorded within upper horizons during spring and within the photic layer in August with lower production at the bottom. The findings confirm the previous observations on photosynthesis during the icecover period. The destruction of organic matter increases in spring within bottom layers, whereas in August destruction processes occur simultaneously with changes in production, in Decemberdestruction is detected within the bottom horizons. The observations provide the groundwork for the sustainable management of the lake.