Interim report of the HELCOM CORESET project

190
Notes for the GES boundary
The copepod biomass is an indicator of fi sh feeding conditions and, accordingly, the reference period is
selected on the basis of the fi sh growth status. As we know, moderate eutrophication is actually benefi cial
for fi sh nutrition, and therefore, it is not surprising that highest growth of zooplanktivorous fi sh coincides
with some (but not the highest!) eutrophication (expressed as Chl a or Secchi depth). It would be against
the theoretical expectations to observe the best feeding conditions in an oligotrophic system.
The example that is used in this example is for a relatively pristine area (Askö) and in 1980-ties the Secchi
and Chl values were in a better status than in the next 20 years. Therefore, the highest copepod biomass
did not coincide with the heavy eutrophication. One has to remember that all reference periods should be
area-specifi c.
Existing monitoring data
Zooplankton data required for this analysis are collected on a regular basis within national and HELCOM
monitoring programs. Laboratories that follow HELCOM methodology for sampling and sample analysis,
should possess all data necessary for indicator development and use. Depending on the sampling frequency,
a specifi c period for zooplankton stocks should be considered and used consistently; this period
may vary between different areas/countries/laboratories, because sampling frequency is not uniform.
Unfortunately, in some areas, sampling coverage is low and not all sea areas are equally well represented
(see also Weaknesses and Concerns).
Data interpretation
A dialogue with experts (and data holders) responsible for establishing GES values for eutrophication and
fi sh stocks would greatly facilitate selection of the reference periods for zooplankton indicators and estimating
GES boundaries.
Although the empirical relationships between zooplankton abundance and eutrophication status are common
in scientifi c literature, the underlying mechanisms are not well understood. For zooplankton indices to have
relevance to management, it is necessary to postulate and test (through research and/or data analysis) hypotheses
that explain the response of zooplankton to water quality. Similarly, it is necessary to identify fi sh species
individually or feeding groups for which the zooplankton-as-food indicator is relevant. It is also necessary to
include other aspects of habitat quality, particularly for coastal fi sh, and the zooplankton-as-food indicator
can serve as one element in a more comprehensive index of habitat condition.
Zooplankton communities include herbivores, predators and omnivores, i.e. organisms with different
trophic roles in the food web, but from an ecological viewpoint, all of them are intermediate players, i.e.,
subject to bottom-up pressures as well as top-down demand. Therefore, zooplankton information is most
useful within the framework of a broader, multi-trophic-level monitoring providing indicators of ecosystem
functioning (i.e., MSFD Descriptor 4).
Weaknesses and concerns
Presently, we do not have a good overview on south-north and east-west variability in zooplankton community
structure, population stocks and seasonal fl uctuations in the Baltic Sea. This complicates development
of indicators applicable in different areas and may further hamper between-area comparisons;
Due to difference in zooplankton compositions between the Baltic Sea areas, some modifi cations may be
required for taxa composition in each specifi c indicator;
Data are generally noisy, owing to multiple factors affecting zooplankton growth and mortality;