Edinburgh Napier University

  A nutrient-manipulation mesocosm study was conducted in Norwegian coastal waters to determine the effect
on a nitrogen-limited microbial food web of changes in the relative concentrations of dissolved organic and
inorganic nitrogen (N). Four replicated treatments were studied: no N addition, inorganic N, organic N, or
a 50 : 50 mix of inorganic and organic N. Comprehensive abundance, biomass, and rate measurements were made
over the 20-d experiment. The form of N available influenced species composition, succession, and the efficiency
of carbon (C) incorporation. Inorganic N generated a bloom of the diatom Leptocylindrus danicus, even though
silicon concentrations were below 1 mmol L21. The c-proteobacteria initially competed effectively with diatoms
for inorganic N, exhibiting high productivity and escaping grazing control. Organic N, when available alone, was
utilized more slowly by the microbial community, with diatom growth, fueled by regenerated N, achieving a lower
peak abundance. A more diverse diatom species assemblage was also evident. The c-proteobacteria were less
prevalent and net bacterial productivity was initially lower when the N source was organic and, in contrast to the
response following inorganic N addition, the bacterial community was initially grazer controlled. A rapid
succession of heterotrophic grazers occurred but with differences in species and their contribution to biomass
between treatments. The efficiency of C-biomass production was lower when the N source was purely organic.
However, when both inorganic and organic N were available, biomass yield was greater than the mean of the two single N source treatments