Lateral Border of a Small River Plume: Salinity Structure, Instabilities and Mass Transport

Abstract

The interfaces between small river plumes and ambient seawater have extremely sharp horizontal and vertical salinity gradients, often accompanied by velocity shear. It results in formation of instabilities at the lateral borders of small plumes. In this study, we use high-resolution aerial remote sensing supported by in situ measurements to study these instabilities. We describe their spatial and temporal characteristics and then reconstruct their relation to density gradient and velocity shear. We report that Rayleigh–Taylor instabilities, with spatial scales ~5–50 m, are common features of the sharp plume-sea interfaces and their sizes are proportional to the Atwood number determined by the cross-shore density gradient. Kelvin–Helmholtz instabilities have a smaller size (~3–7 m) and are formed at the plume border in case of velocity shear >20–30 cm/s. Both instabilities induce mass transport across the plume-sea interfaces, which modifies salinity structure of the plume borders and induces lateral mixing of small river plumes. In addition, aerial observations revealed wind-driven Stokes transport across the sharp plume-sea interface, which occurs in the shallow (~2–3 cm) surface layer. This process limitedly affects salinity structure and mixing at the plume border, however, it could be an important issue for the spread of river-borne floating particles in the ocean.

Response of the Coastal Phytoplankton Community to the Runoff from Small Rivers in the Northeastern Black Sea

“…The disappearance of freshwater species confirms the importance of the salinity boundary at 4-7 as a critical point of physiological stress for freshwater species [61]. Seaward with smoother spatial gradients in salinity from 15 to 17 and a decrease in current velocities at the plume periphery of about three times (0.6-0.8 m s −1 ) [60], and the penetration of seawater into the plume zone [62] can bring marine phytoplankton into it and stimulate the development of diatoms. During each survey in April and May-June, on a small spatial scale (less than 1 km), we revealed an increase in photosynthetically active cells of diatoms as the dominant group at a salinity of about 15.…”

Riverine Litter Flux to the Northeastern Part of the Black Sea

“…For the Don, Kuban and small rivers of the study area, the total yearly input is much smaller and is equal to 3 t/year. Further transport of marine litter in the Black Sea is governed by physical processes associated with spreading and mixing of river plumes [26][27][28][29][30][31][32] and further processed at the sea [33,34]; however, this issue is beyond the scope of the current work.…”

Influence of the Coriolis Force on Spreading of River Plumes

“…Deflection angles analyzed in this study were calculated as differences between wind direction (obtained from atmospheric reanalysis) and plume spreading direction (obtained from satellite imagery) using the following methodology. First, the outer up-wind plume front was detected by satellite imagery using the Canny edge method described in detail in [10]. Second, the tangent vector was calculated for all pixels at the reconstructed plume front curve starting from the seashore.…”

“…Despite the relatively small volume of total freshwater runoff to the World Ocean, river plumes occupy up to 1/5 of shelf areas of the World Ocean [2] and substantially influence global fluxes of buoyancy, heat, terrigenous sediments, nutrients, and anthropogenic pollutants, which are discharged into the coastal sea with continental runoff [3][4][5][6]. River plumes generally constantly receive a freshwater discharge from their sources in river mouths and, at the same time, are constantly mixing with the ambient sea at their bottom and lateral boundaries [7][8][9][10]. Moreover, river plumes have energetic internal dynamics.…”