Dynamics of carbon pools and fluxes in the Don River Delta, Southern (European) Russia, and the estuary under the conditions of increasing marine factors

The Don River Delta, bordering the Taganrog Bay in the Sea of Azov, is one of the major deltas of Europe, providing important ecological and economic services. The Sea of Azov is an enclosed sea, which is also the shallowest sea on the globe (the mean depth is 7 meters) with rich biological productivity.

It was indicated recently that both the Sea of Azov (Berdnikov et al, 2023) and the Don Delta (Venevsky et al, 2023), as well as the estuary area have undergone significant environmental transformations in the last four decades. The water temperature and salinity in the sea and the estuary increased to the never observed values mostly due to climate change (Berdnikov et al, 2023) and the prevailing wind directions changed to the westerlies bringing strong upward surges to the delta. Meanwhile, the Don River runoff significantly dropped started from 2007, while fluvial sediments delivery to the Don Delta were steadily diminishing already during 70 years due to the constructions of dams, human land use and runoff regulation (Venevsky  et al, 2023). Significant amount of suspended sediments from the Taganrog Bay enters the delta and salty waters intrusions to the delta are frequent during surges driven by the westerlies. Thus, the role of marine factors in the delta and estuary area of the Don increased in the last few decades in comparison with fluvial factors. Carbon sequestration in coastal areas considered to be the so-named natural solution for climate change mitigation. Thus, it is important to estimate the past, present and future carbon balance of the Don Delta and the estuary, especially accounting that the delta undergoes changes from being fluvial dominated to marine (wave and surge) dominated one.

We are currently involved in the study focusing on the quantification of carbon pools and fluxes in the Don Delta and the estuary. The study combines modelling approach with field observations and remote sensing data. Our field data included seasonal observations for 2006-2020 of total suspended solids, salinity, concentration of dissolved and suspended organic matter, and chlorophyll-a concentration in the river-delta-estuary continuum (the Lower Don River -Delta-Taganrog Bay). Remote sensing included Landsat and Sentinel images for upward surges episodes for the same period.  We use three combined models:  a hydrological model of the Don estuary area (DonDeltaHECRAS) for simulation of the river flow and water levels during surges; model of suspended matter dynamics (DonDeltaBalanceModel), which allows us to calculate the suspended matter dynamics in the Don estuary area; and model of vegetation and soil dynamics (DonDeltaEcoModel), which is aimed at estimating the carbon accumulation in vegetation and soil in the delta. We found out that with the recent frequency of surges on average 20% of organic chemicals transported with the river runoff is deposited in the delta. Thus, marine factors affect accretion of soil within the delta and change both the carbon pools and fluxes in the delta and the estuary.