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

Abstract

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.

Morphology and DNA content of bacterioplankton in the northern Gulf of Mexico: analysis by epifluorescence microscopy and flow cytometry

“…In agreement with the present study, maximum bacterial abundance at low salinities near the river mouth was reported by Chin-Leo & Benner (1992). Maximum bacterial production sometimes occurs further downstream the salinity gradient than maximum abundance (Chin-Leo & Benner 1992, Pakulski et al 2000.…”

“…Knowledge of bacterioplankton in the open Gulf of Mexico is scarce (Biddanda & Benner 1997) because most studies concentrated on the Mississippi River plume and Louisiana shelf waters (Chin-Leo & Benner 1992, Amon & Benner 1998, Pakulski et al 2000. Vertical and regional patterns in bacterioplankton diversity were addressed by quantification of morphologically distinguished bacteria using epifluorescence microscopy and of subpopulations with different apparent DNA content using flow cytometry.…”

Photogeneration of singlet oxygen and free radicals in dissolved organic matter isolated from the Mississippi and Atchafalaya River plumes

“…al., 1995 . Other investigations, however, focus on photochemical and microbial transformations of terrigenous DOM, which may contribute to the produc-Ž tivity in these nearshore systems Moran et al, 1991;Chin-Leo and Benner, 1992;Bushaw et al, 1996; . Opsahl and Benner, 1997 .…”

Nutrient cycling in the water column of a subtropical seagrass meadow

“…Respiration due to bacterioplankton, at this time, was assumed to be 70% of the total plankton respiration. This assumption was based on previous measurements of bacterial respiration ranging from 40 to 100% of plankton respiration for different marine environments (Williams 1981, Chin-Leo & Benner 1992, Biddanda et al 1994). …”