In recent decades, the amount of phytoplankton in the lakes of the planet has increased significantly, which is often associated with global warming. Scientists from the Kola Science Center of the Russian Academy of Sciences studied diatoms from the Norwegian lake Rabbvatnet and identified other possible reasons for the change in their numbers.
Diatoms are unicellular organisms surrounded by a silica (silicon oxide) shell. The special properties of this shell, designed for capturing the luminous flux, are used, in particular, for the production of solar cells. The ability of diatoms to synthesize inorganic substances allows them to be used in the creation of semiconductors and ultra-strong biocomposites.
It is difficult to overestimate the importance of diatoms in the food chain of aquatic ecosystems and the formation of the composition of the earth's atmosphere: they inhabit almost all water bodies of the Earth. In the process of photosynthesis, diatoms produce almost a quarter of all atmospheric oxygen and bind up to forty percent of organic carbon.
The surprisingly widespread occurrence of diatoms is coupled with their hypersensitivity to environmental changes. Any changes in temperature and content of impurities in water, solar radiation and other parameters lead to a change in the species composition.
Silica shells of dead algae settle to the bottom and are perfectly preserved for many years. By linking the preferred habitat of a particular algal species and its amount in bottom sediments, diatom analysis can be used to reconstruct climate change and environmental quality.
Employees of the Institute of Industrial Ecology of the North of the Kola Science Center conducted a diatom analysis of a small subarctic lake Rabbvatnet (northern Norway) for the period from 1895 to 2012. The data obtained from the analysis of the species composition of bottom sediments were compared with the tree-ring chronology of the Murmansk region and information from the Vardo meteorological station.
Since the 1940s, the lake located in the border zone with Russia has been affected by atmospheric emissions of sulfur dioxide and the heavy Pechenganikel plant. The highest concentrations of heavy metals in lake sediments since the 1970s-1980s are associated with the beginning of processing of the Norilsk ore with a high sulfur content. Compared to this period, SO2 emissions have now decreased by four times.
Analysis of bottom sediments showed that significant changes in the composition and abundance of diatom complexes of the lake were characteristic of the second half of the 20th century. Meteorological data show a tendency for air temperature to rise from 1895 to 2012 at approximately the same rate up to 0, 012 degrees per year. In the last decade, the rate of temperature increase has varied depending on the season: in autumn the warming is the most intense, in the spring it is the least. In summer, the average temperature has even dropped over the past ten years.
The increase in the surface temperature of lakes in recent decades significantly exceeds the increase in air temperature. The generally accepted hypothesis is that anthropogenic carbon dioxide emissions are to blame for the warming of the air. However, the increase in its concentration in the atmosphere due to exchange processes between the atmosphere and the ocean can significantly exceed everything that man produces, and diatoms play a decisive role in the processes of carbon exchange and the transfer of carbon to the deep layers of the ocean.
Changes in solar radiation are comparable to the effect of carbon dioxide on air temperature. They can explain up to half of the warming in the twentieth century and up to 35 percent of the rise in temperature since the 1980s. If we take into account the indirect effect (cosmic rays on the cloud cover, and ultraviolet radiation on the chemical composition of the atmosphere and the ozone layer), then the contribution of solar activity to climate change increases.
Temperature peaks in the study area were noted much earlier than the total number of diatoms began to increase. Usually, an increase in water temperature causes thermal stratification of water bodies and leads to the appearance of small diatoms, for example, the genus Cyclotella. However, in recent decades, the number of representatives of this genus has decreased, while the number of larger diatoms, for example, Aulacoseira alpigena, has increased.
The characteristics of the diatom community can change not only due to warming or cooling, but also due to changes in pH and nutrient content, as well as pollution. However, studies have shown that these parameters did not change during the study period or did not change significantly.
Since the 1920s, the concentration of phosphorus, which limits the growth of diatoms, in the lake water gradually decreased, but in general remained typical of Arctic lakes. The pH values throughout the studied period varied from 7 to 7.05, which indicates the neutral status of the water. Even significant emissions of sulfur compounds by Pechenganikel enterprises in the second half of the 20th century did not provoke an excess of its natural background.
The spring bloom under the ice cover of Arctic lakes indicates that the low temperature does not hinder the growth of phytoplankton. Solar radiation plays a huge role in the bioproductivity of diatoms. This influence is especially evident at high latitudes. For photosynthesis, the conditions of illumination are extremely important - solar radiation of the visible spectrum, the length of the day, the transparency of the water, and cloudiness. Clouds over ice-covered lakes can significantly increase the dose of solar radiation through multiple scattering.
Most diatoms have solar absorption peaks in the wavelength range of about 400 nanometers (blue light) and 680-750 nanometers (near infrared and infrared radiation). Phytoplankton is negatively affected by UV-B radiation (280–315 nanometers), which is well absorbed by atmospheric ozone. In the last decade of the last century, a significant decrease in the atmospheric ozone content was observed, accompanied by a decrease in the total number of the diatom community of Lake Rabbvatnet. Scientists at the Kola Science Center believe that this decrease is caused by the damaging effects of UV-B radiation.
The maximums of the total number of diatoms in 1965 and 1980 coincided with the maximum growth of tree rings on the Kola Peninsula. When studying the width of tree rings, it was found that spectral solar radiation is one of the main solar agents that affect the growth of trees in the Arctic Circle.
Direct measurements aboard the Solar Radiation and Climate Experiment satellite showed: from 2004 to 2007, against the background of a decline in the eleven-year solar cycle, changes in solar radiation in the visible and infrared ranges of the spectrum were opposite to variations in the UV range and integral solar radiation, that is, against the background of a general decrease solar activity, the observed fluxes of solar radiation in the visible range increased. The observed change in FID fluxes in this spectral range can be significant for the composition and temperature of the atmosphere.
The authors conclude that the increase in the number of diatoms in Lake Rabbvatnet and other Arctic lakes is mainly associated not with air temperature, but with an increase in photosynthetically active solar fluxes recorded by SORCE measurements. However, it is impossible to completely exclude the influence of other factors (including the change in water temperature). It is necessary to conduct further studies of diatom communities in Arctic lakes, as this will open up new aspects of the use of diatoms in paleoclimatology.