Photosynthetic Active Solar Radiation in Lake Teletskoye During the Open Water Period

Abstract

Spatial and temporal variability of flux density of photosynthetic active solar radiation (PAR) in the waters of large deep-water oligotrophic Lake Teletskoye (Republic of Altai) is considered based on the analyzed experimental data for the years 2016-2021. Probing of the lake water column was implemented using Satlanic PAR sensor as a part of CTD of the SBE 25plus probe at the beginning and the end of the summer heating period. During various years of studying, PAR magnitude monotonically decreased with lake depth increase and practically became zero at horizons from 3 to 14,8 m according to the data received from two weather stations at the reservoir, i.e. in the pelagic zone of rivers Chulyshman and Kokshi. Flux density of photosynthetic active solar radiation in the upper layer made up to 27-1000 μmol/(m2·s) during the observation period. The photic depth zone (at one percent irradiation) in Lake Teletskoye at the Chulyshman station on sunny days reached 14.8 m, while at the Kokshi one — 11.7 m. According to PAR measurements, the attenuation coefficient (extinction coefficient) along the vertical was estimated as 0.1-2.6 m-1. The maximum weakening coefficient at the Chulyshman station was recorded in June 2016 in the surface 3-m layer of the lake (1.8-2.6 m-1), while below 3 m it had values of 0.1-1.9 m-1. The attenuation coefficient varied within 0.1-1.6 m-1 at the Kokshi station. Relative transparency of the Secchi white disk at the Chulyshman and Kokshi stations ranged within 1.3-5.3 and 2.4-6.5 m, respectively. The paper also presents vertical lake profiles of water temperature in June-July — characteristic of direct summer stratification of the reservoir.