Enhanced bacterioplankton production and respiration at intermediate salinities in the Mississippi River plume

Bacterial abundance and production (thymidine and leucine incorporation) were measured along a salinity gradient from the Mississippi Rlver (0 % o ) to the open waters of the Gulf of Mexico (36 %a) during July-August 1990 and February 1991. Bactenal production in surface waters was maximal at intermediate salinities (15 to 30 %o). Nutrlent enrichment experiments suggested that bacterial growth near the outflow of the river was C limited whereas bacteria in plume waters of intermediate salinities were P and N limited. Rates of plankton community oxygen demand measured during winter were also maximal at intermediate salinities indicating an area of increased heterotrophic activity. The oxygen demand associated with heterotrophic bacterioplankton activity during summer was an important factor leading to hypoxic conditions in bottom waters of the Louisiana continental shelf. In summer, bacterial abundance and production ranged from 0.25 to 3.34 X log cells 1.' and from 4 to 90 pg C 1-' d-l, respectively. In winter, the corresponding ranges were 0.36 to 1.09 X log cells I-' and 3 to 20 pg C 1-' d-' Depth-integrated bacterial production on the Louisiana shelf decreased from 443 k 144 mg C m-2 d' in summer to 226 f 124 mg C m-2 d-' in winter. Using empirically-derived bacterial growth efficiency values of 19 and 29 %, we estimated that bacterial production in summer could be supported by 10 to 58 % of phytoplankton production. In winter, the amount of carbon needed to support bacterial production exceeded phytoplankton production suggesting that bacterial growth during this season was heavily dependent on riverine sources of organic matter.