Comprehensive studies of river mouths flowing into the Onezhskiy Bay of the White Sea

. Comprehensive oceanological studies depending on the tidal phase are important for an objective assessment of the patterns of biogeochemical parameters in the White Sea. Rivers are the main contributors of pollutants from the watershed to the White Sea. The studies of the mouths flowing into the Onezhskiy Bay of the White Sea were carried out from June 20 to June 28, 2023. The distributions of hydrophysical, hydrochemical and biological parameters were obtained depending on the phase of the tidal cycle. The maximum of total phosphorus content was observed in the Nizhny Vyg River Mouth – 42 μg/l, the highest concentration of total nitrogen was in the Onega River Mouth – 0.37 mg N/l. In the mouth of the Kem’ River, concentration of ammonium nitrogen prevailed among inorganic forms, like directly in the river, which indicates anthropogenic impact. The highest values of chlorophyll-a were observed at the most desalinated station, in the Onega River the highest were detected – 2.20 mg/m 3 . The influence of the phases of the tidal cycle in the study area on the spatial and temporal variability of environmental characteristics was manifested in changes in temperature and salinity.


Introduction
The study of river mouths from the point of view of oceanology has been a pressing task for many researchers over the past few years [1][2][3][4].This applies to both private research related to environmental pollution and complex work that combines the efforts of specialists from different fields of science [5,6].
For more than five years the hydrological and biogeochemical regimes of Onezhskiy Bay of the White Sea under the influence of tidal processes have been studied comprehensively by our group.The condition of observing a regular grid of stations and a set of studied parameters is fulfilled.It is necessary to obtain objective and regular information on the functioning of the marine environment, the interruption of which is extremely undesirable.It is important to carry out integrated studies for a more complete description of the objects under study.In this context the emphasis is on studying the mouths of the rivers flowing into the White Sea, as these objects are indicators of hydrological and biogeochemical processes occurring in the catchment area of each river.In modern conditions of development of the northern territories of the Russian Federation, an important place is given to the issues of rational use of water resources and protection of water bodies.The main source of pollutants entering directly into the White Sea is river runoff.
The purpose of the expedition is to carry out comprehensive studies of the hydrological and biogeochemical regime of the rivers mouths of Onezhskiy Bay of the White Sea under the influence of tidal processes, based on methods of analyzing contact observation data.
Expedition works in the mouths of the Nizhny Vyg, Onega and Kem' rivers (Fig. 1) include water sampling at 12 stations from different horizons; measurement of hydrological characteristics (temperature, salinity); determination of the biogeochemical parameters (nutrient content); determination of the concentration of chlorophyll-a; estimation species composition, abundance and biomass of phytoplankton; along-track measurements, vertical profiling with CTD-probes.The expedition was carried out on June 20-28, 2023.The Kem' River is the largest tributary of the White Sea from the territory of the Republic of Karelia.The length of the Kem' River is 191 km, catchment area -27,700 km 2 , annual flow volume 8.18 km 3 .In its catchment area it is the largest water system in Republic of Karelia: the length of Chirko-Kem' system is 221 km and, thus, the total (hydrographic) length of the Kem' lake-river system is about 410 km.
The Vyg River.Complex lake-river system.It consists of two practically independent watercourses -the Verhniy (Upper) and Nizhniy (Lower) Vyg and they are connected by the Vygozero (Vygozersko-Ondskoye reservoir).The catchment area is 27.2 thousand km 2 , the total length of the system is 313.7 km.The length of the Verhniy Vyg is 135 km, Nizhny (from the source from the Vygozersky reservoir) -102 km.Verhniy Vyg remains almost in a natural state, Nizhny Vyg -since the mid-30s of the last century has been the northern slope of the White Sea-Baltic Canal.In addition, Kem' and Nizhny Vyg are fully regulated in terms of energy.There are cascades of hydroelectric power stations on both rivers.
The Onega River.Length 416 km, basin area -56.9 thousand km 2 .The average water flow at the source is 74.1 m 3 /sec, at the mouth 505 m 3 /sec.

Materials and Methods
The following hydrophysical equipment was used in the expedition: multi-parameter probe CTD-90M Sea&SunTechnology (Germany) (pressure measurement range from 0 to 200 dbar, accuracy 0.1%; temperature measurement range from -2 to +35ºC, accuracy +/-0.005ºC;conductivity measurement range from 0 to 60 mSm/cm, accuracy +/-0.020mSm/cm); CTD-probe CastAway (USA); CTD-48 Sea&SunTechnology (Germany) for performing multi-hour stations.There were 31 hydrophysical stations where profiling by CTD-probe was carried out.The stations were numbered from the river towards the sea.These are the stations at the mouth of the Nizhny Vyg River -total number 4 (V-1, V-2, V-3, V-4), of which 3 stations (V-2, V-3, V-4) carried out measurements in high and low water of the tidal cycle.At the mouth of the Onega River -a total of 4 (O-1, O-2, O-3, O-4), of which 3 stations (O-2, O-3, O-4) were also probed at both low and high water.At the mouth of the Kem' River -8 stations (K-1...K-8).In the river stations, where the influence of tides on salinity distribution is not traceable, measurements were made only once.In addition to vertical sounding during different phases of the tidal cycle, multi-hour measurements were made at four stations (O-4; V-4; K-8; K-7) by two multiparameter CTD probes placed at different horizons.Thus, the total number of hydrophysical stations amounted to 36.Water sampling for hydrochemical and hydrobiological studies was conducted by using a plastic 5-L Niskin bottle.Sampling was performed at 9 stations located in the mouth of the Vyg River (stations V-2, V-3 and V-4), mouth of the Kem' River (stations K-3, K-7 and K-8) and mouth of the Onega River (stations O-2, O-3 and O-4).Also water samples for chemical analysis were collected at 3 river stations (V-1, O-1, K-1).
Biological studies include determination of chlorophyll-a concentration (Chl).It was analyzed according to the manual: Radchenko, I.G., Kapkov, V.I. and Fedorov, V.D. Practical Manual on Collection and Analysis of Samples of Marine Phytoplankton.Moscow: Mordvintsev Press, 60 p. (2010).Water samples for determination of Chl were collected from 3-4 horizons (surface, above and below pycnocline, bottom layer).For determination of Chl content, 1 L water samples were deposited on Whatman GF/F filters under vacuum at a discharge of 0.3 atm [7].After filtration, the filters were dried at room temperature for 1-2 hours, frozen and stored at -20ºС for further determination in laboratory conditions.Extraction was carried out with 90% aqueous acetone solution during the day.Chl was determined fluorimetrically in acetone extract before and after acidification with aqueous 1 N HCl solution using a Trilogy Turner Designs fluorimeter (USA).To estimate phytoplankton abundance 1L water samples were concentrated by reverse filtration [7] in a chamber equipped with a nuclear membrane filter (Joint Institute for Nuclear Research, Dubna) with a pore diameter of 2 µm.Concentrated samples were fixed with Lugol's solution.The selected samples were stored in a dry dark place for further processing under a Micromed-3 light microscope in a 0.05 ml Nojott chamber at ×40×10×0.65 magnification.Samples will be analyzed at the Department of General Ecology and Hydrobiology, Faculty of Biology, Lomonosov Moscow State University.Water samples for chemical analysis were collected in clean plastic vials from the surface (0.5 m) and near-bottom (1 m from the bottom) layers at both low and high water of tide cycle.River water was sampled from surface layer only.To determine the total phosphorus content, water was collected in 250 ml polyethylene vials and then preserved with 4N H2SO4 solution.Chemical analyses (determination of inorganic and total phosphorus (IP and TP), NH4-N, NO2-N, NO3-N, total nitrogen (TN) and Si content were performed by spectrophotometric method in the Laboratory of Hydrochemistry and Hydrogeology, Northern Water Problems Institute of Karelian Research Centre of the Russian Academy of Sciences according to the manual: Analytical, kinetic and computational methods in hydrochemical practice / Ed.P.A. Lozovik, N.A. Efremenko.St. Petersburg: Nestor-Istoriya, 272 p., (2017).The concentration of TON was calculated as the difference between TN and the sum of inorganic forms.

Hydrophysics
Water in the mouth of the Nizhny Vyg River, where the influence of sea water is not traced by salinity, was 17ºC, in the mouth of the Onega River -17ºC, in the mouth of the Kem' River -18ºC (Table 1).The water temperature varied naturally from a mouth towards the sea and was influenced by tidal currents.The horizontal gradient in temperature and salinity at the mouths of all three studied mouths is comparable.

Nutrient content
Among the rivers, the maximum TP content was observed in the Nizhny Vyg River (station V-1); the highest concentration of TN was in the Onega River (station O-1) (Table 2).In the studied rivers, water organic compounds dominated among the forms of phosphorus and nitrogen with the exception of the Kem' River, where ammonium nitrogen predominated.The silicon content in the water of the Nizhny Vyg and Kem' rivers was close (Table 2) and corresponded to the regional background concentration (1.9 mg/l).
In the mouths of the rivers, the content of TP was similar (Table 2).Its concentration ranged from 14 to 29 µg/l, and it was represented by organic forms (62-95%, on average 81% of TP).IP content varied from 1 to 9 µg/l, its share was 5-40%, on average 19% of TP.Among the forms of nitrogen, organic nitrogen predominated (on average 98% of TN).In the mouths of the Nizhny Vyg and Onega rivers, the content of inorganic forms of nitrogen was below the detection limit.the mouth of the Kem' River, concentration of ammonium nitrogen prevailed among inorganic forms, like directly in the river, which indicates anthropogenic impact.The silicon content in the mouths of the rivers varied widely (0.1-1.9 mg/l) (Table 2).

Chlorophyll-a concentration
In the Nizhny Vyg River Mouth, the maximum of Chl was detected in the surface horizon of the most desalinated station V-2 (Table 3).At stations V-3 and V-4, Chl values varied insignificantly and were comparable.In the Kem' Mouth the Chl was the lowest of all investigated areas and varied in the surface horizon from 0.14 to 0.47 mg/m 3 .The highest values, as well as in the Vyg River Mouth, were observed at the most desalinated station K-3.In the Onega River Mouth at station O-2 the highest Chl were detected -2.20 mg/m 3 .At these stations during the sampling period a bloom of brown algae of the genus Fucus was detected.Filters used for chlorophyll analysis had a characteristic brown color after filtration of water aliquots.

Discussion
Water temperature at all mouths was practically the same in the most desalinated section of the rivers.During the development of the tidal wave, the temperature of the water mass decreased and salinity increased at the more seaward stations.On the contrary, when the tidal phase changed to low tide in the offshore part of the mouths, temperature naturally increased and salinity decreased.Undoubtedly, these processes affected the distribution of nutrients and chlorophyll-a concentration throughout the day.
In June 2023, the distribution of nutrients in the mouths of the three studied rivers in different phases had similar patterns.From river stations to sea stations, a decrease in silicon and TP content was observed, since the river runoff is characterized by increased concentrations of these components, and there is also an increase of TN concentration.The organic components predominated among forms of nitrogen and phosphorus due to occurrence of production processes, which determines the minimum concentrations of inorganic forms of nitrogen and phosphorus in the spring-summer period.The increased content of ammonium nitrogen in the water of the Kem' River and its mouth area was associated with the impact of wastewater from the city of Kem', which is discharged into the river without treatment.
The spatial distribution of nutrients in the White Sea depends mainly on hydrodynamics.River runoff waters are richer than surface sea waters in nutrients (especially silicon and phosphorus compounds), and accordingly, areas exposed to them are characterized by increased concentrations of these elements.
Data on the water chemical composition in the mouths of the studied rivers are consistent with the results of chlorophyll-a determination.Chl was higher at the more desalinated stations and lower at the more marine stations of a mouth.At low water of the tidal cycle, concentrations increased.This is natural, as algal development is more intense in the shallower and warmer parts of a mouth.

Conclusions
The influence of phases of the tidal cycle in the study area on the spatial and temporal variability of environmental characteristics was manifested in changes in temperature and salinity.The distribution of nutrients and phytoplankton species depended on tidal dynamics.The maximum of total phosphorus content was observed in the Nizhny Vyg River; the highest concentration of total nitrogen was in the Onega River.In the mouth of the Kem' River, concentration of ammonium nitrogen prevailed among inorganic forms, like directly in the river, which indicates anthropogenic impact.In the Nizhny Vyg River Mouth, the maximum of Chl it was detected in the surface horizon (0.89 mg/m 3 , high tide).In the Kem' Mouth the Chl was the lowest of all investigated areas and varied in the surface horizon from 0.14 to 0.47 mg/m 3 .The highest values were observed at the most desalinated station.In the Onega River Mouth the highest Chl were detected -2.20 mg/m 3 .

Fig. 1 .
Fig. 1.Layout of stations in Onezhskiy Bay of the White Sea (1 -mouth of the Kem' River, 2mouth of the Nizhny Vyg River, 3 -mouth of the Onega River) Information about the morphometric characteristics of rivers is contained in the database of the Northern Water Problems Institute of Karelian Research Centre of the Russian Academy of Sciences (River flow in the White Sea basin, Svid.about the state reg.database No. 2018621833.November 19, (2018)).The Kem' River is the largest tributary of the White Sea from the territory of the Republic of Karelia.The length of the Kem' River is 191 km, catchment area -27,700 km 2 , annual flow volume 8.18 km3 .In its catchment area it is the largest water system in Republic of Karelia: the length of Chirko-Kem' system is 221 km and, thus, the total (hydrographic) length of the Kem' lake-river system is about 410 km.The Vyg River.Complex lake-river system.It consists of two practically independent watercourses -the Verhniy (Upper) and Nizhniy (Lower) Vyg and they are connected by the Vygozero (Vygozersko-Ondskoye reservoir).The catchment area is 27.2 thousand km 2 , the total length of the system is 313.7 km.The length of the Verhniy Vyg is 135 km,

Table 1 .
Hydrophysical characteristics of stations where chemical and biological studies were carried out.

Table 2 .
Hydrophysical characteristics of stations where chemical and biological studies were carried out.

Table 3 .
Stations, sampling horizons and concentration of chlorophyll a (Chl, mg/m 3 ) in the studied areas of the White Sea.