This paper introduces some results of evaluation for organic matter production in plankton communities of coastal marine ecosystems in Red river delta before and after the Hoa Binh hydroelectric dam construction. The results showed: 1) Because of water regulation of Hoa Binh hydroelectric dam, the amount of water flowing into the sea through 09 estuaries and river mouths of the Bach Dang estuary system and Red river delta has been changed significantly, associated with the depletion of dissolved inorganic nutritions (P, N, Si) in Red river delta coastal waters from 33 to 47 percent compared to the time before dam construction
2) Compared with the period before dam construction, the characteristics of the organic matter production process in plankton communities in the waters has been reduced 38-80 percent and 34-70 percent in wet season and dry season, respectively.