Seasonal changes in specific growth rates, production and biomass of a bacterial community in the water column above a Mediterranean seagrass system

Bacterial abundance, biomass and production rates were determined at 3 depths (5, 10 & 15 m) in the water column above a Mediterranean seagrass bed in the Gulf of Calvi (west coast of Corsica, France) from 1988 to 1990.We used dialysis bags for in s ~t u incubation of 2 pm prefiltered seawater sampled from the respective depths to determine bactenal growth parameters and conducted light-and dark-bottle incubations to estimate planktonic primary production by O2 measurements.Bacterial density and biomass was subject to marked seasonal changes.Bacterial density varied clearly over the seasons and between the 3 depths, with maximum values being recorded in Aug and Oct 1988 at the 10 and 15 m depths.Differences in bacterial biomass and density patterns were mainly attributed to changes of abundance and biovolume of rod-shaped bacteria.Highest carbon values were recorded during the summer months in 1989 and 1990 at the 3 depths and ranged from 32 to 65 pg C I-' Bacterial growth rates were closely correlated to temperature, with highest specific growth rates (0.075 to 0.125 h-') found in summer, when chlorophyll a concentrations were at a minimum during this season.Dur~ng Jan and Feb 1989 and 1990, when chl a concentrations were at a maximum, bacterial growth rates were below 0.001 h-' Doubling times (g) ranged from 5.2 to 23 h in summer, being lowest at the 5 m depth.Highest g values were recorded in Jan 1989 at 10 m (259 h).Dunng this period we observed an increase bacterial numbers within the dialysis bags, but a decrease in biovolume of the 4 morphotypes.We hypothesize that the observed growth strategy is necessary for bacteria to resist starvation and to obtain a competitive advantage for nutrient scavanging under oligotrophic conditions.In Jan, bacterial production corresponded to 7.6 % of gross primary production.In summer, bacterial production ranged from 18.5 to 48.4 % of gross primary production.Carbon requirements of the bacterial population in the water column were discussed in view of various carbon conversion efficiencies.The range of our bacterial production values is compared with values from other systems and seen in the context of the n~ethodological approaches.