Abteilung Oberflächengewässer

Lake Baikal, with a depth of 1637 m, is characterized by deep-water intrusions that bridge the near-surface layer to the hypolimnion. These episodic events transfer heat and oxygen over large vertical scales and maintain the permanent temperature stratified deep-water status of the lake. Here we evaluate a series of intrusion events that reached the bottom of the lake in terms of the stratification and the wind conditions under which they occurred and provide a new insight into the triggering mechanisms. We make use of long-term temperature and current meter data (2000–2013) recorded in the South Basin of the lake combined with wind data produced with a regional downscaling of the global NCEP-RA1 reanalysis product. A total of 13 events were observed during which near-surface cold water reached the bottom of the South Basin at 1350 m depth. We found that the triggering mechanism of the events is related to the time of the year that they take place. We categorized the events in three groups: (1) winter events, observed shortly before the complete ice cover of the lake that are triggered by Ekman coastal downwelling, (2) under-ice events, and (3) spring events, that show no correlation to the wind conditions and are possibly connected to the increased spring outflow of the Selenga River.

Deepwater renewal by intrusions and turbulent diffusion in Lake Baikal is very effective despite the enormous depth of up to 1642 m and the permanently stable stratification below ∼300 m depth. Temperature time series recorded at the bottom of a mooring installed since March 2000 in the South Basin of the lake indicate recurrent freshwater intrusions with volumes of 50 to 100 km³, about one order of magnitude larger than previously observed intrusions. Numerous mechanisms have been proposed to explain the advective deep water renewal. Here we present for the first time direct observations which prove that they are caused by coastal downwelling and subsequent thermobaric instability along the steep lake shores. Understanding these mechanisms is an important prerequisite for studying biogeochemical cycles, for predicting the effects of climate change on this unique ecosystem and for evaluating the local climate history from the extraordinary sedimentary record of Lake Baikal.

We studied the methane (CH₄) budget of Lake Baikal, the most voluminous lake in the world and the only freshwater body with known occurrences of methane hydrates in the sediments. CH₄ concentrations were measured in water samples taken during six expeditions between October 2002 and June 2004; these expeditions covered the entire lake volume. A one-dimensional model was applied to (1) estimate the large-scale vertical CH₄ fluxes within the South Basin of Lake Baikal, (2) determine the exchange with the atmosphere, and (3) constrain the CH₄ inputs from seeps and mud volcanoes to the deep water. Fluxes were generally several orders of magnitude below previous estimates. The annual internal source of CH₄ to the pelagic surface mixed layer was roughly estimated to be 40 Mg CH₄. A large part of this input diffuses downwards and is consumed in the water column, with a CH₄ residence time of about 4 yr. The input of CH₄ from deep gas seeps and mud volcanoes is less than a few 10 Mg CH₄ yr^-1, most of which is oxidized before reaching the surface. The net CH4 flux between the atmosphere and the main waterbody distant from shallow areas is negligible and not significantly different from zero. However, occasional high CH₄ concentrations, both in the surface water and in the atmosphere, indicate that the region near the Selenga delta is a local CH₄ source to the atmosphere. CH₄ fluxes in the Central Basin are very similar to those in the South Basin, whereas in the North Basin, the shallow CH₄ sources are weaker.

The internal cycles of carbon, silica, nitrogen, and phosphorus in the South and North Basins of Lake Baikal were quantified in the frame of a multidisciplinary collaboration. Fluxes of particulate organic matter from the epilimnion to the deep water were quantified with integrating sediment traps deployed at 200- to 250-m water depth and compared with fluxes measured in near-bottom traps to reveal mineralization in the water column. Sedimentation rates were determined with dated sediment cores to calculate mass accumulation rates of elements in the sediment. Advective and turbulent transport of dissolved nutrients in the water column was based on a set of monitoring data, which included temperature and current data, as well as hydrochemical data of the water column. Diffusive fluxes from the sediment to the overlying water column were determined by applying different porewater sampling techniques. The combination of these data resulted in consistent internal budgets for carbon, nitrogen, and phosphorus in Lake Baikal: the new production in the South Basin was 1730 mmol C m^-2 year^-1 and the mass accumulation rate in the sediment 220 mmol C m^-2 year^-1, whereas in the more secluded North Basin, new production was only 1220 mmol C m^-2 year^-1 and mass accumulation rate 125 mmol C m^-2 year ^-1. Fluxes of particle-bound nitrogen, phosphorus, and biogenic silica were by about 30% smaller in the North Basin than in the South Basin. Export fluxes of nitrogen from the surface zone to the deep water were 150 mmol N and 100 mmol N m^-2 year^-1. Denitrification rates in the sediment were estimated from mass-loss calculation to 38 and 53 mmol N m ^-2 year^-1 for the South and North Basin, respectively, corresponding to 25% and 52% of the total nitrogen input to the hypolimnion. Nitrogen (19 and 13 mmol m^-2 year^-1) was finally buried in the sediments of the South and North Basins; 10.1 and 3.5 mmol P m^-2 year^-1, and 1830 and 1400 mmol Si m^-2 year^-1 were transferred to the deep water in the South and North Basin where 28% and 70% P, and 64% and 54% Si were retained in the sediments. A diatom bloom occurred during our sampling period of 2 years, which usually occurs only every 3 to 5 years. Accordingly, Si flux data from sediment traps were increased by an estimated 50% compared with the long-term average illustrating the necessity of several years of field measurements to compensate for the natural dynamics of Lake Baikal.

We studied cold, deep-water intrusions in the South Basin of Lake Baikal on the basis of 2 yr of data (December 1995-November 1997) from near-bottom and near-surface thermistor strings, monthly conductivity-temperature-depth (CTD) profiles, and a near-bottom current meter, all collected near the South Basin maximum depth of 1,461 m. The data show intrusions into the greatest depths with temperatures of 0.08–0.20°C below ambient (~3.33 to ~3.38°C at maximum depth). The intrusions were observed three times per year between January and June, when surface water is always cooler than deep water, with durations of a few (at least 1–3) days. The estimated water input ranged from 1 to 10 km3 per event, and the annually accumulated volume was estimated to be 10–30 km³, which is significantly less than the steady-state indirect estimates of 30–70 km³ yr^-1 to the permanently stratified deep water (depth > 300 m). This indicates that not all of the cold intrusions reach the deepest area. Because the cooling of the near-bottom waters was not accompanied by a significant increase in ion or particle content and because deep sediment traps did not contain significant enrichments of minerogenic particles, we concluded that Selenga River inflow is not a possible source of the cold intrusions. Two CTD profiles in June 1996 and 1997 showed lower temperature throughout the deep water, as expected from thermobaric instabilities. The required downwelling is definitely not occurring in the pelagic interior but most probably by near-coast counterclockwise currents. The source of the regularly occurring deep intrusions is clearly cold surface water, but the actual mechanism is uncertain.

The water column of Lake Baikal is extremely weakly—but permanently—stratified below 250 m. Despite the thickness of this relatively stagnant water mass of more than 1000 m, the water age (time since last contact with the atmosphere) is only slightly more than a decade, indicating large-scale advective exchange. In the stratified deep water, the fate of water constituents is determined by the combined action of advective processes (deep-water intrusions) and small-scale turbulent vertical diffusion.
Here, vertical diffusivity is addressed through the analysis of 25 temperature microstructure profiles collected in the three major basins of Lake Baikal to a depth of 600 m. In addition, in the 1,432-m deep south basin, monthly CTD profiles and a two year record of near-bottom currents were analyzed. Balancing turbulent kinetic energy and small-scale temperature variance leads to the conclusions that (1) vertical diffusivity in the stratified deep water ranges from 10-90 X 10^-4 m² s^-1 (between 600 and 250 m), which is three orders of magnitude more than estimated by Killworth et al. (1996), (2) the mixing efficiency is ~0.16, comparable to that found in stronger stratification (e.g, the ocean interior), (3) turbulence under ice decays with a time scale of 40 ± 2 d and (4) the interior of the permanently stratified deep water below 250 m and the bottom boundary layer contribute roughly equally to the TKE production. The latter implies, that mixing in the deep water of Lake Baikal's three sub-basins is dominated by bottom boundary mixing as found in smaller lakes and ocean basins.