Biogeochemical impacts of flooding discharge with high suspended sediment on coastal seas: a modeling study for a microtidal open bay

Yasuhiro Hoshiba,Hiroyasu Hasumi,Sachihiko Itoh,Satoshi Nakada,Yoshimasa Matsumura

doi:10.1038/s41598-021-00633-8

Yasuhiro Hoshiba, Hiroyasu Hasumi + Show 3 more

Open Access

https://doi.org/10.1038/s41598-021-00633-8

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Abstract

Freshwater, suspended sediment matter (SSM), and nutrients discharged from rivers into the ocean have large impacts on biological production. In particular, during floods, coastal areas are greatly stirred up and large amounts of nutrients are supplied to the sea surface. We investigate the biogeochemical impact of flooding river discharges containing a large amount of SSM by conducting numerical simulations for a specific flooding event of the Yura River, Japan. Parameters are varied over wide ranges of SSM properties and nutrient content in riverine water. Two qualitatively different regimes of the riverine plume, hypopycnal and hyperpycnal, appear within realistic parameter ranges. Compared with the reference case without SSM, the surface salinity (nutrients) within the riverine plume becomes lower (higher) in hypopycnal cases and higher (lower) in hyperpycnal cases within a few days after the flooding discharge. These results suggest the necessity of properly taking into account the effect of SSM in assessing the influence of high river discharges on coastal biogeochemistry. It is the case not only for the specific river and event we are dealing with but also for other flooding events and other rivers and connecting coastal seas.

Highlights

Freshwater, suspended sediment matter (SSM), and nutrients discharged from rivers into the ocean have large impacts on biological production
If SSM has a relatively long residence time, homopycnal-mode processes[5] work as follows: (1) SSM spreads over a larger area due to the smaller particle sinking rate; (2) the vertical density difference between the surface and sub-surface layers is reduced as a consequence of the higher density in the whole surface layer resulting from residual SSM; and (3) the salinity in the surface layer increases owing to enhanced vertical mixing (Fig. 1b)
Two qualitatively different regimes of riverine plumes appeared within these parameter ranges: hypopycnal and hyperpycnal processes

Summary

Introduction

Freshwater, suspended sediment matter (SSM), and nutrients discharged from rivers into the ocean have large impacts on biological production. Compared with the reference case without SSM, the surface salinity (nutrients) within the riverine plume becomes lower (higher) in hypopycnal cases and higher (lower) in hyperpycnal cases within a few days after the flooding discharge. These results suggest the necessity of properly taking into account the effect of SSM in assessing the influence of high river discharges on coastal biogeochemistry. A series of mechanisms by which SSM affects hypopycnal plumes has been demonstrated[4]: (1) the horizontal density difference between the riverine and offshore waters is reduced by the discharged SSM content near the river mouth; (2) the estuarine circulation is weakened due to the reduced density difference; and (3) the weakened vertical estuarine circulation suppresses vertical water exchange between the surface and sub-surface layers (Fig. 1a). SSM–physical interactions through the hypopycnal and hyperpycnal processes can substantially change nutrient-supply environments; to the best of the authors’ knowledge, no previous study has investigated how riverine water containing SSM affects the supply of nutrients to the sea surface

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