Nutrient and Chl. a distributions in surface waters of Lake Baikal before and after the thermal stratification development

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Soluble reactive phosphorus (SRP), ammonia, nitrite+nitrate and Chl. a were measured in 24 surface waters from southern Lake Baikal in June 2002, while those in 17 surface water samples from southern and partly central Lake Baikal were determined in August of the same year. Water temperatures indicated that these two months were before and after the thermal stratification development, respectively. SRP and nitrite+nitrate decreased significantly and Chl. a, dominated by the <10 μ m fraction, increased 3-fold from June to August, while a similar low level of ammonia was recorded in these two months. Averages and standard deviations in June were 0.18±0.04 μ mol l-1 for SRP, 0.2±0.1 μ mol l-1 for ammonia, 3.3±0.7 μ mol l-1 for nitrite+nitrate and 0.6±0.1 μ g l-1 for Chl. a. Those in August were trace for SRP, 0.3±0.1 μ mol l-1 for ammonia, 0.4±0.1 μ mol l-1 for nitrite+nitrate and 1.7±0.5 μ g l-1 for Chl. a. These results suggest that SRP and nitrite+nitrate were supplied to the surface from deeper waters by seasonal mixing and taken up by phytoplankton during the development of thermal stratification. On the other hand, the distribution of ammonia in the surface water was independent from water movements, suggesting that the ammonia uptake and regeneration was in equilibrium at such a low concentrations throughout the year.
2011-02-15