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Seasonal variability of inorganic and organic nitrogen in the North Sea

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Abstract

This study considers the cycling of nitrogen in the waters of the North Sea, particularly focussing on organic nitrogen. Dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and particulate organic nitrogen (PON) were measured in the North Sea over a one-year period (autumn 2004–summer 2005). The surface water concentrations of nitrate, ammonium, DON and PON during the present study ranged from <0.1–7.2 μM, <0.1–2.0 μM, 1.9–11.2 μM and 0.3–5.6 μM, respectively, with DON the dominant fraction of total nitrogen at all times. These nutrients concentrations were significantly lower compared to previous studies in the southern North Sea. The seasonal variations showed high mean surface concentrations of nitrate (4.7 ± 0.6 μM) and DON (8.9 ± 0.9 μM), low ammonium (<0.1 μM) and PON (0.8 ± 0.1 μM) in winter, shifting to low nitrate (0.3 ± 0.3 μM) and DON (4.2 ± 1.2 μM) in summer, with high ammonium (0.8 ± 0.4 μM) in autumn and PON (2.5 ± 1.2 μM) in spring. Highest mean surface DON concentration was measured in winter and may be due to resuspension of the organic matter from the bottom sediments. For autumn and spring, phytoplankton DON release was likely to be the most significant source of DON as shown by high concentrations of low molecular weight (LMW) DON and its positive correlation to chlorophyll a. Low total and LMW DON concentrations during summer were likely to be due to the uptake of the LMW DON fraction by phytoplankton and bacteria and the stratification of the water column. DON is therefore shown to be a potentially important source of nitrogen in shelf seas especially after the spring bloom has depleted nitrate to limiting concentrations.

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Acknowledgement

The authors would like to thank the officers and crew of the RV CEFAS Endeavour for their valuable help during the cruises. We are grateful to Dr N. Greenwood and Ms O. Andres for providing DIN and chl-a concentrations data. This research was supported by a Malaysian Government fund awarded to SS for his PhD study. This article also benefited from the comments of two anonymous reviewers.

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  1. Environmental Research Group (ERG), Department of Chemical Sciences, University Malaysia Terengganu, 21030, Kuala Terengganu, Terengganu, Malaysia

    Suhaimi Suratman

  2. Laboratory for Global Marine and Atmospheric Chemistry (LGMAC), School of Environmental Sciences, University of East Anglia, Norwich, Norfolk, NR4 7TJ, UK

    Tim Jickells & Keith Weston

  3. Centre for the Environment, Fisheries and Aquaculture Science (CEFAS), Pakefield Road, Lowestoft, Suffolk, NR33 0HT, UK

    Liam Fernand

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  1. Suhaimi Suratman
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Correspondence to Suhaimi Suratman.

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Suratman, S., Jickells, T., Weston, K. et al. Seasonal variability of inorganic and organic nitrogen in the North Sea. Hydrobiologia 610, 83–98 (2008). https://doi.org/10.1007/s10750-008-9424-y

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