School of Engineering and Science - Jacobs University

CHAPTER III
General development of the spring bloom
Starting with 44 µgC L -1 phytoplankton biomass, the spring bloom evolved immediately
and reached its maximum at 269 µgC L -1 within the first eight days of the experiment
(24.03.09) (Figure 1). At this exponential phase phytoplankton had a maximal net
growth rate of 0.48 d -1 (18.-20.03.09) and a mean net growth rate of 0.23 d -1 (16.-
24.03.09). During the following three weeks the biomass decreased at a mean rate of -
0.05 d -1 to a final value of 84 µgC L -1 .
Microzooplankton, starting with 14 µgC L -1 , followed the phytoplankton bloom with a
delay of roughly a week and peaked on the 30.03.09 with 124 µgC L -1 (Figure 1).
Microzooplankton reached growth rates of up to 0.27 d -1 (23.-24.03.09) but grew at a
mean rate of 0.16 d -1 . Until the end of the experiment microzooplankton biomass
decreased at a mean rate of -0.15 d -1 to 12 µgC L -1 , i.e. close to the starting value.
Phytoplankton composition
The spring bloom was dominated by diatoms and small flagellates (five size classes of 5
– 25 µm length) see Figure 3a+b. While flagellates contributed 34% to phytoplankton
biomass at the start they played only a minor role during the bloom phase. The bloom
itself was dominated by diatoms (96%) of the genera Chaetoceros (C. danicus and other
Chaetoceros spp. of different size classes), Thalassiosira (T. rotula and T.
nordernskjoeldii) and Rhizosolenia (R. stylisformis/hebetata group and R. pungens),
each genus contributing roughly one third. In the later bloom phase Thalassiosira
became more dominant and represented up to 49% of the phytoplankton carbon. The
category ‘other diatoms’ (Pseudonitzschia spp., Navicula spp., Asterionellopsis
glacialis and others) contributed only 1-4% to the phytoplankton biomass. Flagellate
biomass peaked four days earlier than the diatoms and showed a steeper decline. Along
with decreasing diatom shares, it increased again to around 20% of the total
phytoplankton carbon in the last week of the experiment.
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