PenP*Product

In 6 land based mesocosms the effects of eutrophication and
stratification on algal species composition and productivity
were studied during the period 1994-1998. Results indicate that
microzooplankton grazing was a significant factor determining
phytoplankton species composition, in addition to the effects
of the nutrient loading and water column mixing rates, resulting
in a tendency towards blooms of Phaeocystis sp. under eutrophic
conditions in a well mixed water column and dinoflagellate blooms
under eutrophic conditions in a stratified water column.

The present situation in the North Sea coastal zone is
characterized by decreased Phosphorous loadings and a strong Nitrogen
surplus and thus high N:P ratios with possible consequences on food
quality. Copepods may graze selectively on certain algal species
and microzooplankton. There is uncertainty about the effect of
interactions between nutrient loadings and zooplankton grazing on
phytoplankton composition and productivity. High grazing pressure
may result in changes in phytoplankton composition for which several
mechanisms are possible: 1)Copepod grazing on microzooplankton will
lead to a smaller grazing pressure on nanoflagellates; in situations
with a surplus of nutrients, or because nanoflagellates may take up
nutrients faster, this may lead to blooms of nanoflagellates and
2) Poor food quality, inedibility or toxicity of phytoplankton may
eliminate mesozooplankton grazing pressure and will thus lead to a
competitive advantage for microzooplankton.

The 1999 and later experiments addressed the following questions:
1) Does mesozooplankton promote the development of inedible or
toxic algal blooms under the present circumstances (high N-load)
2) What is the influence of shifts in plankton structure on
mesozooplankton productivity
3) What is the influence of shifts in N:P ratio on food quality?