Reconstruction of long-term changes of the underwater light field in large shallow lakes Peipsi and Võrtsjärv, North-East Europe

Abstract

The main objective of our study was to reconstruct the multi-decadal changes of the underwater light field in two large, shallow and polymictic Estonian lakes Võrtsjärv and Peipsi in order (i) to assess the potential role that light limitation may have had on phytoplankton growth in the past and (ii) to get an insight into the factors driving underwater light climate in shallow turbid lakes in the long term. We reconstructed the long-term variations of the diffuse attenuation coefficient of water (K d,PAR ) in the photosynthetically active region (PAR, 400-700 nm) partly on the basis of measured beam attenuation spectra and partly using regression analysis. From K d,PAR we calculated the depth of the euphotic zone (z 1% ) and the mean light availability in the mixed layer (E mix ). The reconstructed time series of these bio-optical parameters gave a plausible picture of the long-term development of light conditions in the two lakes studied, which was in accordance with their eutrophication history and changes in their water levels. Better light availability in both lakes generally coincided with years of a low water level, and the coincidence was more distinct in the shallower Võrtsjärv. Values of E mix revealed a probable light limitation in Peipsi in autumn and in Võrtsjärv throughout the year.

Radiocarbon Dates and Stable Isotope Data from the Early Bronze Age Burials in Riigiküla I and Kivisaare Settlement Sites, Estonia

“…The two lakes are located in the same hydrogeological province (UNESCO 1981) and have similar geological settings (Saarse 1990;Eberhards 2006). Like Lake Burtnieks, Lake Võrtsjärv is shallow and moderately alkaline; δ 13 C values in dissolved organic matter (DOM) at the outflow of the lake are consistently depleted and the dissolved inorganic carbon (DIC) δ 13 C values are similar to that in a modern water sample from the River Salaca at the outflow of Lake Burtnieks (Toming et al 2013;Meadows et al 2014). Thus, it would be surprising if the isotopic baselines in prehistoric freshwater resources from the two lakes were very different.…”