Seasonal change in heat flux at the water-bottom sediment boundary in a small lake

Heat exchange with bottom sediments is the main component of the thermal regime of ice-covered shallow lakes of the temperate zone, which explains the importance of its study and parameterization for inclusion in numerical models. Circulations arising in ice-covered lakes due to heat exchange with bottom sediments, and existing for several months, can make a significant contribution to the transport of dissolved and suspended particles along the water column. The aim of this work was to study the seasonal variability of the heat flux at the waterbottom boundary in a shallow lake during the under-ice period, including the period of spring under-ice convection. Based on the analysis of data from high-frequency (minute) long-term measurements of water temperature in the bottom area of a small lake in the temperate zone, a wide range of variability of the heat flux across the water-bottom boundary during the winter from minute to daily fluctuations was established. The role of the spring under-ice heating in the change in the heat flow at the water-bottom boundary is shown. It is shown that shallow areas of the lake bottom, falling into the zone of influence of spring subglacial convection, can accumulate heat already at the end of the ice period. The comparison of temperature fluctuations in the deep-water part of the lake and tin he area with depths close to the average is carried out. It is shown that the spectrum of temperature fluctuations has similar periods, however, in time, sharp temperature jumps in different areas of the lake do not coincide.

Current methods of subsea production systems survey in the conditions of the Arctic region

The development of deposits in the Arctic region and the Far Eastern shelf of Russia is complicated by the presence of ice conditions. The duration of the ice period can vary from 5 up to 9 months, during which the underwater equipment of the field is inaccessible for inspection and repair. This work discusses the methods of inspection and maintenance of subsea production systems that can be used in the development of Arctic deposits.

Adaptive strategies of the microbial community in meromictic soda Lake Doroninskoye (Transbaikalia, Russia)

The adaptive strategies of biogeochemical systems are considered as a criterion for natural fluctuations of the territory using the example of the microbial community in meromictic soda Lake Doroninskoye (Transbaikalia, Russia). The features of the phylogenetic and functional diversity of the microbial community of the lake have been investigated. In the off-season, a change in the dominant component of the community in the chemocline of the lake was established. During the ice period, the lake is dominated by metabolically flexible, anoxygenic, photoheterotrophic non-sulfuric purple bacteria Rhodospirillaceae and Rhodobacteraceae (class Alphaproteobacteria), which can switch from anoxic photosynthesis to aerobic chemotrophic metabolism. During the open water period, facultative aerobic bacteria of the families Enterobacteriaceae (class Gammaproteobacteria) and Alcaligenaceae (class Betaproteobacteria) predominate, which switch to denitrification in the absence of oxygen.

Dissolved oxygen variability in a small ice-covered lake during the spring under-ice convection

<p>A decrease in the ice-period on lakes against the background of climate warming improves its oxygen regime in the cold half of the year by reducing the winter anoxia. A decrease in the thickness of the snow-ice cover can contribute to an increase in under-ice irradiation, which can provoke an earlier onset of spring under-ice convection and activation of algal blooms. Do these processes affect the oxygen content in ice-covered lakes? This study examines the variability of dissolved oxygen, water temperature, currents, chlorophyll "a" and under-ice irradiation according to field measurements carried out in 2007-2020 during spring under-ice convection in a small Lake Vendyurskoe (northwestern Russia). Field data were obtained at autonomous stations with an interval of one minute. Measurements of temperature and dissolved oxygen (RBR TR- and DO-sensors) were carried out from October to May, covering the entire ice-period, while measurements of currents (ADCP), solar radiation fluxes («Star-shaped pyranometer»  «Theodor Friderich & Co, Meteorologishe Gerate und Systeme»), and chlorophyll "a" (BBE Moldaenke) were carried out for 3-12 days from late March to the third decade of April in different years. The thickness of the snow-ice cover was also measured. Analysis of the data showed that in 2007-2020 the thickness of the snow-ice cover of Lake Vendyurskoe in spring (late March – mid-April) varied significantly from 35 to 70 cm, depending on weather conditions. The under-ice solar radiation fluxes varied from close to zero to more than 150 W/m2. The duration of spring under-ice convection ranged from two to seven weeks. Chlorophyll "a" was fairly uniformly distributed within the convective layer, even below the photic zone. We assume the dual role of convective currents in the development of subglacial plankton: ascending currents facilitate the entry of algal cells and nutrients into the photic zone, activating photosynthesis, while descending currents carry them out of it, suppressing photosynthesis. With well-developed convection, oscillations of dissolved oxygen were recorded with a daily frequency, reaching 1 mgO2/L in the upper part of the convective layer. Presumably, an increase in the content of dissolved oxygen is associated with a daytime increase in photosynthesis against the background of an increase in under-ice radiation, and a decrease is associated with the destruction of organic matter. Convective currents also affect the vertical distribution of dissolved oxygen, involving the oxygen-depleted bottom waters in mixing, which leads to a certain decrease in oxygen concentrations in the convective layer. The total amount of oxygen in the water column during the period of spring under-ice convection can increase by 10% due to the photosynthesis of phytoplankton. Oxygen fluctuations from minutes to hours were identified, which can be caused by seiche activity, the convective cells, advective transport, and the dynamics of internal waves. The results obtained in this work will contribute to a better understanding of the variability of oxygen in ice-covered lakes, caused by the total impact of biological and hydrophysical processes. The study was supported by an RFBR grant 18-05-60921.</p>

CHANGE OF HEAT FLUX AT THE WATER-BOTTOM SEDIMENTS BOUNDARY IN A SMALL LAKE DURING THE YEAR

The features of the variability of the heat flux at the boundary of the water column with bottom sediments in the central part of the small polymictic Lake Vendyurskoe (southern Karelia) were analyzed according to the data of year-round temperature measurements from November 2018 to October 2019. It was shown that the heat flux directed from the bottom sediments into the water reached 6–8 W/m2 in the pre-ice period and at the moments of ice formation and breaking, and did not exceed 1 W/m2 during most of the winter. A week after the ice-off (May 3–4, 2019) the water column was completely mixed, the heat flux changed its sign, and heat accumulation in the upper layer of bottom sediments began. The heat flux directed to bottom sediments was 4–6 W/m2, periodically increasing to 8-12 W/m2 from mid-May to late July and from the second decade of August to the first decade of September. Against the background of a prolonged cooling during the first ten days of August, the heat flux changed sign with daily frequency, the amplitude of daily fluctuations reached ± 2.5 W/m2. Autumn cooling of the Lake Vendyurskoe began in early September 2019, the heat flux sign changed on September 14–16, 2019.

ICE CONDITIONS IN THE SOUTHEASTERN BALTIC SEA FROM SATELLITE DATA (2004–2019)

The Russian part of the Southeastern Baltic Sea has a specific ice regime, which is determined by geographical location and climate conditions. Satellite data are widely used to study the propagation of sea ice. The advantages of radar images (RLI) from satellites equipped with synthetic aperture radar (SAR) are a large area coverage and independence from daylight and cloud-cover conditions. There were 840 SAR images of the Southeastern Baltic Sea analyzed for the period from December 1 to March 31, 2004–2019. It is shown that in modern conditions ice phenomena in the southeast Baltic do not occur every year. It is shown that in recent conditions ice phenomena do not occur every year in the Southeastern Baltic Sea. The revealed spatial and temporal variations in the ice regime are determined by the consequences of climate changes in the Southeastern part of the Baltic Sea. The intensity of ice formation depend on temperature conditions. Ice formation begins mainly in the second half of January-early February. The average duration of sea ice period is 22 days per season. The probability of observing sea ice from a satellite is maximum in February. Stable ice fields occur when the daily average air temperatures drop below –5 °C during 5 or more days. The maximum duration of ice period and maximum ice extent are observed in February.