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Field investigations of apparent optical properties of ice cover in Finnish and Estonian lakes in winter 2009
University of Helsinki, Finland.ORCID iD: 0000-0002-0861-3341
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2011 (English)In: Estonian journal of earth sciences, ISSN 1736-4728, E-ISSN 1736-7557, Vol. 60, no 1, p. 50-64Article in journal (Refereed) Published
Abstract [en]

A field programme on light conditions in ice-covered lakes and optical properties of lake ice was performed in seven lakes of Finland and Estonia in February–April 2009. On the basis of irradiance measurements above and below ice, spectral reflectance and transmittance were determined for the ice sheet; time evolution of photosynthetically active radiation (PAR) transmittance was examined from irradiance recordings at several levels inside the ice sheet. Snow cover was the dominant factor for transmission of PAR into the lake water body. Reflectance was 0.74–0.92 in winter, going down to 0.18–0.22 in the melting season. The bulk attenuation coefficient of dry snow was 14–25 m–1; the level decreased as the spring was coming. The reflectance and bulk attenuation coefficient of snow-free ice were 0.1–0.4 and 1–5 m–1. Both were considerably smaller than those of snow cover. Seasonal evolution of light transmission was mainly due to snow melting. Snow and ice cover not only depress the PAR level in a lake but also influence the spectral and directional distribution of light.

Place, publisher, year, edition, pages
2011. Vol. 60, no 1, p. 50-64
Keywords [en]
lake ice, snow, optics, reflectance, transmittance, attenuation coefficient
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
URN: urn:nbn:se:su:diva-157411DOI: 10.3176/earth.2011.1.05OAI: oai:DiVA.org:su-157411DiVA, id: diva2:1220043
Available from: 2018-06-18 Created: 2018-06-18 Last updated: 2018-06-21Bibliographically approved
In thesis
1. Light conditions in seasonally ice-covered waters: within the Baltic Sea region
Open this publication in new window or tab >>Light conditions in seasonally ice-covered waters: within the Baltic Sea region
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seasonal ice cover is a major driver of seasonality in aquatic ecosystems in the Baltic Sea region. Ice cover influences the underwater light conditions directly by limiting the light transfer and indirectly by modifying the mixing and circulation under the ice. Light conditions and stratification are key factors controlling the onset of the phytoplankton spring bloom. Therefore, the seasonal ice cover has an important role in setting the time frames for the primary production and in influencing the seasonality of the ecological processes. This thesis investigates the optical properties of the ice cover and the bio-optical substances in the water column.

Bio-optical substances, suspended particulate matter (SPM), Coloured dissolved organic matter (CDOM) and Chlorophyll-a (Chl-a), determine the availability and spectral distribution of light. Measuring turbidity is quick and easy compared to the gravimetrical determination of the SPM concentration. Paper I provides a new model to estimate the concentration of SPM from turbidity. The new SPM-turbidity model predicts SPM concentrations well, despite the high CDOM absorption and the optical differences in the coastal northwestern and southeastern Baltic proper. Therefore, the new SPM-turbidity model offers a cost-effective and reliable method to monitor SPM concentration.

The light transfer through the snow and ice cover was studied both in freshwater lake ice and in brackish sea ice (Papers II and III). Additionally, the seasonal evolution of light transmission through lake ice was investigated during spring. The crystal structure of the ice cover was analysed both in the coastal fast ice zone and in drift ice in the open Baltic Sea. The snow and ice cover was found not only to reduce the amount of light, but also to change its spectral and directional distribution. The light field under ice depended strongly on the snow cover. In addition, the bio-optical substances were analysed within sea ice and in the underlying water, as well as their effect on the light conditions.

The seasonal sea ice cover also limits the wind-driven mixing of the water column. The development of stratification was investigated in a coastal bay in the northwestern Baltic proper (Paper IV). The preconditions for an under-ice plume development were defined along with the spatial and temporal dimensions of the stratification pattern. Furthermore, an under-ice plume was found to cause a delay in the onset of the phytoplankton spring bloom, but the timing of the Chl-a maximum was not affected. The results also show that although diatoms dominate the phytoplankton community with and without under-ice plume, the dynamic conditions without under-ice plume seem to favour the motile photosynthetic ciliate Mesodinium rubrum. Overall, this thesis contributes to better understanding of the current role of seasonal ice cover on the light conditions and consequently on to the ecosystem.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, 2018
Keywords
Seasonal sea ice, Fast ice, Drift ice, Phytoplankton spring bloom, Under-ice plume, Turbidity, Suspended particulate matter, Chlorophyll, Coloured dissolved organic matter, Baltic Sea
National Category
Environmental Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-157483 (URN)978-91-7797-312-6 (ISBN)978-91-7797-313-3 (ISBN)
Public defence
2018-09-07, Vivi Täckholmsalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 09:30 (English)
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Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Accepted.

Available from: 2018-08-15 Created: 2018-06-20 Last updated: 2018-09-03Bibliographically approved

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