Comparison of Euphotic Layer Criteria in Lakes

The water layer where photosynthesis takes place (euphotic zone) was studied, and criteria for determining its thickness were compared. Published works give alternative definitions of the euphotic depth: 1) the depth at which radiation falls to 1 % of the subsurface irradiance in the photosynthetically active radiation (PAR) region of the spectrum; 2) the depth of some small constant value of downwelling irradiance; 3) the depth of the photocompensation point. We compared values of the euphotic depth ob-tained by these criteria with each other and with the depth where primary production approaches zero. The data describing the vertical distribution of irradiance in the PAR region (90 profiles) and primary production (41 profiles) in 13 Estonian and Finnish lakes collected in 1995–97 were used. Additionally, criteria 1 and 2 were investigated by model calculations. The regression formulae describing the rela-tionship between criteria for of the euphotic zone and the level of zero primary production were obtained. The respective correlation coefficients were different for each criteria and depended on the conditions of the data collection. The relationship between the 1%-level and constant irradiance level was strong when the incoming irradiance varies within narrow limits. The 1%-depth, widely used in practice, corre-sponded well to the level where primary production approached zero. By our results the 1%-depth and the depth of some constant irradiance describe the zone of positive gross primary production rather than the zone of positive net production. Key words: Optical properties of lakes, light field in lakes, photosynthesis in water 1.