DYNAMICS OF NUTRIENTS IN ESTUARINE AREA OF THE LENA RIVER: RESULTS FROM CRUISES IN SEPTEMBER 2015 AND 2017

In the paper performed results from observations of hydrochemical structure of estuarine are of the Lena River delta as well as in the central part of the shelf and above the continental slope of the Laptev Sea during 63^d and 69^th cruises of R/V “Academik Mstislav Keldysh” in September 2015 and 2017. Despite the large amount of freshwater input (estimated 540 km³/year only from the Lena River), the primary production of the Laptev Sea water is very poor. Except the estuarine area freshwater influence on the se water could be negative owing to worsening of vertical exchange, decrease of insolation due to raise of turbidity level. Moreover, nutrients coming with the riverine water are hardly consumed by marine organisms. In general, for productivity of seawater it is necessary not only abundance of nutrients but stability of the environment. Our studies included determination of dissolved oxygen, pH, alkalinity and nutrients. Concentration of phosphates and mineral forms of nitrogen was practically close to analytical zero despite significant influence of the continental runoff. Comparison of the structure in 2015 and 2017 showed strong resemblance. In addition, we have calculated dynamics of nutrients in the riverine-sea mixing zone. The most active emission of dissolved silicate and phosphorus was observed in the bottom layer and was accompanied by reduction of dissolved oxygen and oxygen saturation level. Based on the calculations, it is possible to distinguish three types of waters in the study area: surface waters that change their properties as the influence of riverine water decreases, seawater not directly affected by fresh water, and bottom waters in the estuarine area with elevated salinity and affected by exchange with upper layer of sediments.

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