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3. Doktoritööd

Selle kollektsiooni püsiv URIhttp://hdl.handle.net/10492/2490

Sirvi

Sirvi 3. Doktoritööd Autor "Arst, Helgi (advisor)" järgi

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  • Pisipilt
    Kirje
    Dissolved organic matter and its ecological role in large and shallow water bodies
    (Eesti Maaülikool, 2013) Toming, Kaire; Nõges, Tiina (advisor); Arst, Helgi (advisor)
    About 90% of the organic matter in most aquatic systems is dissolved organic matter (DOM) and the rest forms the pool of particulate organic matter. DOM plays a significant role in the carbon and energy cycle of water bodies and as the main source of energy for microbial metabolism it can have a broad effect on food chains and on the proportions of auto- and heterotrophic processes. The role of DOM in ecological and biogeochemical processes depends on its source and composition. Autochthonous DOM (Au-DOM) is produced in the pelagic and littoral zones of the lake by phytoplankton and other photosynthetic organisms and it consists mainly of non-humic substances. Allochthonous DOM (Al-DOM) is transported from the catchment area, primarily originates from vascular plants and soil organic matter, and consists mainly of humic substances. The composition and sources of DOM in an aquatic ecosystem may vary spatially and temporally. In many lakes, especially within the cool boreal region, net ecosystem production is negative (lakes release carbon), suggesting that appreciable Al-DOM runoff from the catchments increases the respiration within the lake ecosystem to a level that exceeds photosynthetic carbon sequestration. Many studies have considered the origin and the variability of DOM in humic lakes or in rather deep water bodies with relatively clear water while the information from large, shallow and eutrophic water bodies is scarce and the present study aims to fill this gap. The thesis focuses on DOM in eutrophic large and shallow water bodies (coastal areas, large lakes) and the main aims of the study are to estimate: the range of temporal and spatial variability of DOM; the origin and the proportion of allochthonous and autochthonous DOM; the controlling factors of DOM variability and origin; the impact of CDOM on underwater light attenuation; the factors driving long term changes of underwater light field. The conclusions of the study were: the discharges from the main rivers were mainly responsible for CDOM spatial variability and its high values in the coastal/onshore areas in all studied large and shallow water bodies; CDOM values were highest in spring and lowest in autumn with most distinct seasonal pattern in onshore areas close to the river mouths. The impact of the riverine CDOM and share of Al-DOM in lakes and in the bay increased together with the ratio of the catchment area to the lake/bay volume; contrary to our prediction, in Lake Võrtsjärv Au-DOM had lower δ13C values than Al-DOM. This is a consequence of low δ13C values of DIC, phytoplankton and other seston in this lake; with increasing water level the proportion of Al-DOM and the heterotrophy of the ecosystem of Võrtsjärv increased; with increasing water temperature the proportion of Au-DOM increased and the autotrophy of the ecosystem enhanced; according to the DOM δ13C values, the average proportion of Al-DOM in Võrtsjärv was 68% during the active growing season (May-September) and 81% in October- April. The high share of Al-DOM indicates the strong influence of catchment processes and rapid degradation of Au-DOM in the lake ecosystem; the main factor controlling the light attenuation in the Pärnu Bay was CDOM, which overshadowed the impact of organic/inorganic particles. The latter played the main role in Peipsi and Võrtsjärv; the reconstructed time series of diffuse attenuation coefficient (Kd,PAR), based on regressions with optically active substances, enabled the reconstruction of long-term time series of primary production, euphotic depth (z1%) and the mean light intensity in the mixed layer (Emix), which corresponded to the eutrophication history and changes in water levels of studied water bodies; Emix values below the critical threshold showed that in Peipsi phytoplankton was likely light limited in autumn and in Võrtsjärv throughout the year.