Input of excessive macronutrients (e.g., nitrogen or phosphorus) and organic matter results in a change in the trophic status of a body of water, and leads to eutrophication in the coastal sea areas. In response to these changes, the biological community, particularly the microbial community, will alter. Microorganisms are sensitive and easily influenced by eco-factors such as temperature, organic matter, etc, in which marine heterotrophic bacteria are able to utilize dissoluble organic matter to perform a "second production" coupling with the classic grazing food chain, and causing a cycle of matter. Multiple measurement methods indicate that the abundance and biomass of bacterioplankton are greater in eutrophic coastal areas than in normal and clean sea areas. Furthermore, with the intensifying eutrophication of seawater, the concentration of chlorophyll a increases in the water body, enabling rapid growth and resulting in significant biomass of bacterioplankton. To date, the population density and biomass of bacterioplankton have not been found to decline with the eutrophication of seawater. Bacterial physiological groups include species that carry out the same function but exhibit different morphological characteristics and belong to different taxa; these groups are closely associated with eutrophication and include nitrifying bacteria, denitrifying bacteria, coliform bacteria, anaerobic sulfate-reducing bacteria, and methanogens and so on. The processes of nitrification and denitrification intensify in a manner that is not in proportion with increasing eutrophication in coastal areas or bays; therefore, nitrifying and denitrifying bacteria are more abundant in aquatic, in particular benthic, habitats. The presence of sulfate-reducing bacteria and methanogens indicates an anaerobic habitat; in addition, their population densities directly respond to an anaerobic status to some extent. Eutrophication of a water body easily leads to an anaerobic habitat and a lower oxidation-reduction potential, favoring the growth and reproduction of sulfate-reducing bacteria. The methanogen population will subsequently increase in extremely anaerobic habitats, especially benthic sediments. Coliform bacteria are microbial indicators of the marine environment, and their population indicates the extent of eutrophication on the basis of leading factors resulting from land input in coastal areas. Therefore, almost all studies show that the population of coliform bacteria decreases from landward stations to seaward ones. Studies of bacterial community structure in seawater mesocosms differing in nutrient status reveal that the eutrophication of seawater decreases the diversity of bacterioplankton species, resulting in a simpler community structure, although the available organic and inorganic matter might greatly affect bacterial species in eutrophic sea areas, altering community structure on a spatial-temporal scale. Meanwhile, well-balanced bacterial function is also disturbed by seawater eutrophication. The production of bacterioplankton is higher in eutrophic sea areas than in oligotrophic ones. With the increasing eutrophication of sea water, the ratio of total dissolved organic carbon to microbial available organic carbon increases, the enzymatic decomposition potential of substrates and remineralization rates of matter rise, the ratio of protein to carbohydrate decomposing enzymes increases, and the ratio of oxygen to sulfate respiration and oxygen penetration depths in sediments decreases; on the other hand, nitrification and denitrification rates, fluxes of ammonia and phosphate, and emission of nitrous oxide and methane also increase. From these data, it is apparent that seawater eutrophication deeply influences bacterial community structure and function.