The Role of non-phytoplankton food for zooplankton in the North Sea

The relationship between phyto- and zooplankton in the North Sea
is characterized by an apparent mismatch in abundance. During the
phytoplankton spring bloom, zooplankton abundance is low, however
it reaches high abundances after the decline of the phytoplankton
bloom.
This high zooplankton biomass is maintained for a considerable period
of time. It has therefore been speculated that zooplankton might feed
on non-phytoplankton food sources during this period. Recent findings
on the occurrence of 'micro-particles' such as transparent exopolymer
particles and spontaneous aggregation of dissolved organic matter in
the sea further support the hypothesis that zooplankton cover their
carbon and energy demands to a considerable extent from these sources.

Moreover, distinct plumes of silt and clay originating from the
English Coast are transported towards the Frisian Front, which is
characterized by high primary production and a rich benthic
community. Since silt and clay particles efficiently adsorb dissolved
organic matter, we will also test whether zooplankton are capable of
utilising the particle adsorbed biofilm. In laboratory experiments,
radiolabled organic colloidal matter will be produced and offered
either separately or in combination with phytoplankton to selected
zooplankton species. This will allow us to determine whether the
dominant zooplankton species are actually using this potential food source.