School of Engineering and Science - Jacobs University

CHAPTER III
ABSTRACT
Mesocosm experiments coupled with dilution grazing experiments were carried out
during the phytoplankton spring bloom 2009. The interactions between phytoplankton,
microzooplankton and copepods were investigated using natural plankton communities
obtained from Helgoland Roads (54°11.3’N; 7°54.0’E), North Sea.
In the absence of mesozooplankton grazers the microzooplankton rapidly responded to
different prey availabilities; this was most pronounced for ciliates such as strombidiids
and strobilids. The occurrence of ciliates was strongly dependent on specific prey.
Abrupt losses in their relative importance with the disappearance of their prey were
observed. Thecate and athecate dinoflagellates had a broader food spectrum and slower
reaction times compared to ciliates. In general, high microzooplankton grazing impacts
with an average consumption of 120% of the phytoplankton production (P p ) were
measured. Thus, the decline in phytoplankton biomass could be attributed to an
intensive grazing by microzooplankton. Copepods were less important phytoplankton
grazers consuming on average only 47% of P p . Microzooplankton in turn contributed a
substantial part to the copepods’ diets especially with decreasing quality of
phytoplankton food due to nutrient limitation during the course of the bloom. Copepod
grazing rates on microzooplankton exceeded microzooplankton growth. As a result of
selective grazing by microzooplankton less preferred diatom species bloomed both in
our mesocosms and in the field with specific species (Thalassiosira spp., Rhizosolenia
spp. and Chaetoceros spp.) dominating the bloom. This study demonstrates the
importance of microzooplankton grazers for structuring and controlling phytoplankton
spring blooms in temperate waters and the important role of copepods as top-down
regulators of the microzooplankton.
Keywords: microzooplankton, selective grazing, spring bloom, North Sea, mesocosm,
Temora longicornis
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