Microbial activity in the Greenland Sea: Role of DOC lability, mineral nutrients and temperature

Abstract

Bacterial turnover of dissolved organic carbon (DOC) and the regulation of bacterial activity was investigated during 2 cruises in the Greenland Sea in June and August 1999, in order to provide information on the role of bacteria for the biogeochemical cycling of DOC in the northern North Atlantic. We measured the actual pool size of labile DOC (DOC-L) and in situ bacterial growth rate (production:biomass ratio; P:B) in vertical profiles in the Greenland Sea, covering a depth range from surface to 3662 m depth. In situ bacterial growth rates ranged from 0.0002 to 0.5 d-1, and significant bacterial growth was observed down to the deepest samples. Approximately half of the variation in in situ growth rate could be described by a model with DOC-L (μM) and temperature (°C) as variables: log(P:B) = 0.37 × log(DOC-L) + 0.059 × temperature - 2.41 (r2 = 0.47, p < 0.0001). DOC-L concentration ranged from 13.5 μM (surface waters) to <0.1 μM in some of the deep-water samples and showed a significant decrease with depth. The presence of DOC-L in samples from below 1000 m depth (average 0.28 ± 0.21 μM C (n = 12)) suggested an efficient transport of organic carbon from the productive layer, possibly by sedimenting particles. Growth experiments with bacterioplankton from surface waters and deep waters of the Greenland Sea showed significant influence of both temperature (°C) and DOC-L (μM) on bacterial growth rate (μ; d-1) as described by log(μ) = 0.25 × log(DOC-L) + 0.081 × temperature -1.14 (r2 = 0.73, p < 0.0001). Inorganic nutrient concentration did not affect bacterial growth rate. Our findings suggest that bacterial DOC uptake in the Greenland Sea is controlled by a combination of temperature and the concentration of DOC-L.