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The northern Red Sea, with its northernmost extension the Gulf of Aqaba, is an oligo- trophic marine ecosystem, for which the growth and substrate dynamics of the heterotrophic bacteri- oplankton have not yet been studied. In 1999, we carried out a comprehensive investigation of bac- terioplankton growth dynamics in early spring (February/March), a time of year when the Gulf waters are deeply mixed, while permanent stratification prevails in the northern Red Sea. Most of the parameters measured yielded low values (bacterial numbers: 0.5 to 12.8 × 10 5 ml -1 ; production: 0.9 to 56.8 ngC l -1 h -1 ; growth rates: <0.01 to 0.15 d -1 ; turnover rates of dissolved free amino acids, DFAA: 0.008 to 1.35 d -1 ; glucose: 0.001 to 0.14 d -1 ; and concentrations of dissolved free neutral monosaccha- rides: <2 to 86 nM). Glucose was the only monosaccharide detected in 93% of the samples. Concen- trations of DFAA ranged from 13.2 to 176 nM, and those of dissolved combined amino acids and neutral monosaccharides from 0.42 to 3.69 µM and 0.05 to 3.31 µM, respectively. Uptake of glucose (as percent of bacterial production) in 93% of the samples was < 50%, whereas that of DFAA was much higher, often exceeding 100% (the latter result may have been due to methodological biases). Most of the parameters measured were more variable and covered a wider range in the northern Red Sea than in the Gulf, where vertical patterns were more homogenous. Only concentrations of dis- solved free neutral monosaccharides were systematically lower in the northern Red Sea. Bacterial production in this area was significantly correlated with turnover and uptake rates of DFAA, and with glucose turnover rates below 100 m. Aminopeptidase and β-glucosidase activities were fairly similar in both study areas, with means ranging from 19.4 to 29.9 nmol l -1 h -1 . Whereas aminopeptidase hydrolysis rates were in the same range as values found in other oceanic regions, β-glucosidase hydrolysis rates were much higher, possibly due to the persistence of active β-glucosidase in the dis- solved phase, as has been reported for alkaline phosphatase in this area. Analysis of the data indi- cates that in the northern Red Sea bacterioplankton growth dynamics were mainly controlled by bio- logical processes, whereas in the Gulf hydrographic processes were more important.

Bacterioplankton dynamics in the Gulf of Aqaba and the northern Red Sea in early spring