Edinburgh Napier University

  Nutrient manipulation experiments were conducted on a natural planktonic community in outdoor mesocosms. Inorganic
nitrogen (N) and silicon (Si) were added to achieve N: Si ratios of 1:1 and 4: 1. Total particulate carbon (PC) biomass of
the microbial assemblage was determined by elemental analysis. Cell volume measurements by microscope on individual
components of the community (bacteria, diatoms, photosynthetic nano¯agellates, heterotrophic nano¯agellates,
dino¯agellates and ciliates) were also made. We applied published C:volume relationships to determine the volume
estimated C content (CBV) of these microbial groups and hence of the total assemblage. The total CBV and total PC were
compared to test the applicability of C:volume relationships under different nutrient regimes both before and after nutrient
exhaustion. For initial N: Si ratios of 1: 1, prior to nutrient exhaustion, the relationship between CBV and PC was linear
with a gradient of approximately 1, (0±99³0±06), indicating that the published C:volume relationships accurately predicted
the C content of the microbial assemblage. For N: Si ratios of 4: 1, a linear relationship was again evident between CBV
and PC (slope : 1±36³0±08). However, statistical comparison using a general linear model indicated that the gradient of this
relationship differed signi®cantly from that when the N: Si ratio was 1: 1, and hence CBV overestimated elemental C. For
both N: Si ratios, subsequent to nutrient exhaustion (N or Si), and hence when the diatom fraction of the microbial
assemblage was in yield-limited post-exponential phase, the two measures of biomass were not well correlated. This
indicated that measured cytoplasmic cell volume was a poor indicator of C biomass within the microbial assemblage in
nutrient-deplete conditions.