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

Abstract. Diel variability of heterotrophic bacterial production (BP) was investigated in the South East Pacific from October to December 2004 during the BIOSOPE cruise. Three sites differing by their trophic status were studied: Marquesas Islands (MAR; 08° S, 141° W), the centre of the South Pacific Gyre (SPG) (GYR; 26° S, 114° W) and the eastern part of the SPG (EGY; 32° S, 91° W). At the three sites, diel variability of BP ranged from 17 to 40% and from 13 to 22% for volumetric surface (5 m) and integrated (to Z_e and Z_m) data, respectively. The main feature we observed was at 5 m, an abrupt increase (×2 to ×4) in leucine activity during the afternoon-sunset period (12:00–18:00 at the site MAR and 15:00–21:00 at the site GYR) and lowest activities recorded between 10:00 and 14:00. To assess the potential influence of solar ultraviolet radiation (UVR: 280–400 nm) on this BP diel variability, we determined, from in situ optical measurements, the mean tri-hourly ultraviolet B (UVB, 305 nm) and ultraviolet A (UVA, 380 nm) doses (irradiances integrated over time) within the mixed layer (H_m(UVB) and H_m(UVA), respectively). The wavelengths 305 nm and 380 nm were used as biologically effective wavelengths for the induction of DNA damages (cyclobutane pyrimidine dimers: CPDs) and photoenzymatic repairs (PERs), respectively. In the SPG, daily H_m(UVB) and H_m(UVA) were 0.6 and 14 kJ m⁻² nm⁻¹, respectively. The latter were probably the highest daily doses ever measured in the marine environment. The H_m(UVB)/H_m(UVA) ratio (Q) increased by 58, 117 and 46% from 06:00–09:00 to 12:00–15:00, and decreased by 36, 26 and 16% from 12:00–15:00 to 15:00–18:00 at the sites MAR, GYR and EGY, respectively. The relationship between Q and BP suggested a significant influence of UVR on the diel variability of BP (BP decreased when Q increased) at the site GYR from the surface waters to Z_m, likely in relation with its hyper-oligotrophic status. Therefore, possible alternance of CPD and PER periods attributed to Q ratio, as well as a strong lags between process of autotrophic production with their associated dissolved organic carbon (DOC) release and heterotrophic utilization of organic matter could explain such diel variations.