THE IMPACT OF THE 2019 FLOOD ON THE ABIOTIC AND BIOTIC STRUCTURE OF BOTTOM SEDIMENTS AND SOILS IN THE AMUR RIVER FLOODPLAIN  

Garetova L.A.^*1, Kharitonova G.V.¹, Imranova E.L.¹, Fisher N.K.¹, Kirienko O.A.¹ 

¹ Institute of Water and Ecological Problems, Far East Branch, Russian Academy of Sciences, Khabarovsk, Russia

  Large and catastrophic floods occur every 20–30 years in the Amur River basin. The current decade is characterized by an extremely high frequency of recurrence of large floods and prolonged (up to three months or more) flooding in the floodplain (2013, 2019–2021). Unfortunately, insufficient attention is paid to the analysis of the consequences of flood impact on individual components of ecosystems. This is especially true for the study of changes in the abiotic and biotic structure of edaphic components of ecosystems (bottom sediments and soils) during prolonged flooding. In this work, we have analyzed and assessed changes in the content of organic matter, hydrocarbons and volatile organic compounds, particle-size distribution and pigment composition, structure of microbial communities in bottom sediments (BS) and floodplain soils after the 2019 flood. Samples of BS and soils were taken in 3 areas of the floodplain of the Amur River channel, including the mouth area of a small tributary of the Krasnaya Rechka River, before the flood and after the flood (August 1 and October 18, respectively). The coordinates of sampling points were verified using the JPS navigator. The study of the granulometric composition of samples by laser diffraction (Shimadzu-2300, Japan) revealed the features of particle-size distribution in the composition of BS and soils after prolonged flooding (about 3 months). It is shown that in the mixing zone of the waters of the Amurskaya branch and the Krasnaya Rechka, after the water drained, the content of the most representative fraction of suspended sediments of the Amurskaya branch (10–50 µm) in the surface layer of BS and soils (0–20 cm) increased significantly and amounted to more than 50% (59.8 and 56.1%, respectively), which is due to the deposition of substantial amount of silt. It was accompanied by an increase in the content of C_org; in addition, the amount of hydrocarbons in BS increased in comparison with the pre-flood period. Analysis of the structure and abundance of the biotic community (ecological and trophic groups of microorganisms, phytopigments) showed that soil flooding during the flood led to an increase in the total content of phytopigments (up to 5 times) and the proportion of carotenoids in their composition. The change in the structure of microbial communities in BS and soils is due to the synergistic effect of flooding and pollution by emergency sewage. Compared with the pre-flood period, the number of heterotrophic bacteria in soils increased up to 10 times, in bottom sediments up to 90 times. The study of the qualitative composition of volatile organic compounds by vapor-phase analysis in combination with gas chromatography showed that prolonged flooding contributed to the creation of reducing conditions in soils, resulting in an expansion of the spectrum of identified volatile organic compounds and an increase in their content up to 10 times compared with the pre-flood period.

Key words:  flood, soil, bottom sediments, particle-size distribution, organic matter, hydrocarbons, phytopiglents, microorganisms, volatile organic substances

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