Diel variability of heterotrophic bacterial production and underwater UV doses in the eastern South Pacific

Abstract

Diel variability of heterotrophic bacterial production (BP) was investigated in the eastern South Pacific from October to December 2004. Three sites differing by their trophic status were studied: Marquesas Islands, the center (GYR) and the eastern South Pacific Gyre. By using a Lagrangian approach and high frequency measurements, an important increase (2-to 4-fold) in BP was observed at the 3 sites during the afternoon-sunset period within surface layers. To evaluate the impact of solar UV radiation on this variability, we determined, from in situ optical measurements, the mean UV-B (at 305 nm) and UV-A (at 380 nm) doses received within the mixed layer at a daily scale. At GYR, the doses were as high as 0.3 and 11 kJ m -2 nm -1 for the whole day, respectively due to high surface irradiances and very low light attenuations in the water column. The UV-B/UV-A tri-hourly dose ratios (Q) displayed substantial variations during the daytime, with highest values recorded during the periods 9:00 to 12:00 h or 12:00 to 15:00 h. The negative linear correlation observed between Q and BP in the surface waters of GYR suggests that changes in the balance between DNA damages and photorepairs (reflected by changes in the Q-ratio) could have a significant influence on the diel variability of BP in open oceans. However, assessing the effects of UV radiation on diel variability of BP through an in situ measurement approach, independently from other causes like availability of resources, is not so evident, even in these clearest waters of the world ocean. KEY WORDS: Eastern South Pacific · Heterotrophic bacterial production · Diel variability · Ultraviolet radiation · UV-B and UV-A doses · Mixed layer · Leucine Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 387: [97][98][99][100][101][102][103][104][105][106][107][108] 2009 ural assemblages and bacterial cultures exposed to artificial UV-B after secondary irradiation with UV-A and PAR. It appears that UV-A is more efficient than PAR in inducing photoreactivation (Kaiser & Herndl 1997, Joux et al. 1999. Therefore, the rates of CPDs production and PERs in marine microbial assemblages may be related to the UV-B and UV-A doses (irradiances integrated over time) received by cells (Boelen et al. 2001). In the surface ocean, the intensity of UV-B and UV-A doses for a given time period depends on several parameters: (1) the level of surface UV-B and UV-A irradiances, (2) the attenuation of these surface irradiances in the water column, which is mainly controlled by chromophoric dissolved organic matter (CDOM) (Diaz et al. 2000) and (3) the depth of the mixed layer (Z m ), i.e. the depth at which cells can be transported within the upper water column (Boelen et al. 2001).The French project BIogeochemistry and Optics SOuth Pacific Experiment (BIOSOPE) was dedicated to a multidisciplinary exploration of the eastern South Pacific including the hyper-oligotrophic South Pacific Gyre (SPG) as well as its western (Marquesas Islands) and...

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