Moschonas, GrigoriosGowen, Richard J.Paterson, Ruth F.Mitchell, ElaineStewart, Brian M.McNeill, SharonGlibert, Patricia M.Davidson, Keith2021-07-162021-07-162017-06-15Moschonas, G., Gowen, R. J., Paterson, R. F., Mitchell, E., Stewart, B. M., McNeill, S., Glibert, P. M. and Davidson, K. (2017) ‘Nitrogen dynamics and phytoplankton community structure: the role of organic nutrients’, Biogeochemistry, 134(1–2), pp. 125–145. doi: 10.1007/s10533-017-0351-8.0168-25631573-515Xhttps://doi.org/10.1007/s10533-017-0351-8Publication history: Accepted - 7 June 2017; Published online - 15 June 2017.Dissolved organic nitrogen (DON) is recognised as an important N source for phytoplankton. However, its relative importance for phytoplankton nutrition and community composition has not been studied comprehensively. This study, conducted in a typical Scottish fjord, representative of nearpristine coastal environments, evaluates the utilisation of DON and dissolved inorganic nitrogen (DIN) by different microbial size fractions and the relationship of phytoplankton community composition with DON and other parameters. The study demonstrated that DON was important in supporting phytoplankton throughout the yearly production cycle. The higherthan-expected urea uptake rates and large fraction of the spring bloom production supported by DON suggested that organic N not only contributes to regenerated production and to the nutrition of the small phytoplankton fraction, but can also contribute substantially to new production of the larger phytoplankton in coastal waters. Multivariate statistical techniques revealed two phytoplankton assemblages with peaks in abundance at different times of the year: a spring group dominated by Skeletonema spp., Thalassiosira spp., and Pseudo-nitzschia spp. group delicatissima; and a summer/autumn group dominated by Chaetoceros spp., Scrippsiella spp., and Pseudonitzschia spp. group seriata. The multivariate pattern in community composition and abundance of these taxa was significantly correlated with the multivariate pattern of DON, urea, dissolved free amino acids, DIN, temperature, salinity, and daylength, with daylength and urea being particularly important, suggesting both physical and chemical controls on community composition.enOpen Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.Dissolved organic nitrogenPhytoplanktonNitrogen uptakeCommunity compositionArticle