School of Engineering and Science - Jacobs University

CHAPTER III
The mesocosms were stirred by a propeller (107.5 rpm, 15 minutes on, then 15 minutes
off) to ensure the continuous mixing of the water column and to avoid sedimentation of
the plankton. Light was provided by computer-controlled light units (Profilux II, GHL
Groß Hard- and Software Logistics, Kaiserslautern, Germany) operated via an external
control computer (Programme ‘Prometheus’, GHL, modified version ‘Copacabana’).
The light units were equipped with two different fluorescent tubes to obtain full light
spectra (‘Solar Tropic’ and ‘Solar Nature’, JBL, Neuhofen, Germany), enabling the
simulation of a daily triangular light curve (see Sommer et al. (2007) for details). The
light cycle and intensity was adjusted daily to account for changes in the photoperiod
during the experimental run according to the geographical position of Helgoland
following the model by Brock (1981).
In order to initiate the phytoplankton spring bloom after filling a light intensity of 60%
of surface irradiance was chosen, simulating the intensity of light at 1.50 m water depth
with a light attenuation coefficient of 0.34 (5 m Secchi depth) under in situ conditions.
Calculation of the light intensity was done via equations given by Tyler (1968) and
Poole and Atkins (1929).
Stocking with natural inocula
During early seasonal succession many plankton organisms hatch from cysts, resting
eggs or other resting stages. To ensure the same successive patterns of the plankton in
the enclosed mesocosms like in the field, including those organisms hatching from
cysts, resting stages etc., we introduced a small inoculum of natural seawater from
Helgoland Roads on a weekly basis. Five litres of 200 µm screened seawater were
added to each mesocosm. An additional 15 L of filtered seawater (0.2 µm) were added
to the mesocosms to compensate for evaporation and water removal due to the sampling
for monitoring and experiments.
Sampling the mesocosms
Daily measurements
Daily measurements of temperature, pH and in vivo fluorescence (chlorophyll a) (Algae
Analyser, BBE Moldaenke, Kiel, Germany) were conducted between 8 and 9 am.
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