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 River (0 parts per thousand) to the open waters of the Gulf of Mexico (36 parts per thousand) during July-August 1990 and February 1991. Bacterial production in surface waters was maximal at intermediate salinities (15 to 30 parts per thousand). Nutrient 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 10(9) cells l-1 and from 4 to 90 mug C l-1 d-1, respectively. In winter, the corresponding ranges were 0.36 to 1.09 x 10(9) cells l-1 and 3 to 20 mug C l-1 d-1. Depth-integrated bacterial production on the Louisiana shelf decreased from 443 +/- 144 mg C m-2 d-1 in summer to 226 +/- 124 mg C m-2 d-1 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.