Interim report of the HELCOM CORESET project

4.20. Mean zooplankton size
The indicator was meant to follow long-term changes in the zooplankton size as a response to changes in
food web (predation pressure) and eutrophication (hypoxia, altered phytoplankton species composition).
Larger zooplankton size indicates a better state of the environment.
No description of the indicator is available, but see the indicator for copepod biomass for some discussion
(below).
4.21. Zooplankton-phytoplankton biomass ratio
The indicator was meant to follow long-term changes in the biomass ratio of zooplankton and phytoplankton
as a response to changes in food web (predation pressure) and eutrophication (hypoxia, nutrient availability).
Bias to zooplankton indicates stronger top-down control and hence a better functioning food web
(piscivorous fi sh controlling planktivorous fi sh, releasing zooplankton from high predation.
No description of the indicator is available but see the indicator for copepod biomass for some discussion
(below).
Description and testing of zooplankton indicators: Introduction
In aquatic ecosystems, changes in species composition and abundance of small, rapidly reproducing organisms,
such as plankton, have been considered among the earliest and sensitive ecosystem responses to
anthropogenic stress (Schindler 1987). Zooplankton are integral to aquatic productivity, serving as primary
consumers of nutrient-driven primary producers, and as prey for fi sh. Despite their potential as indicators
of environmental changes and their fundamental role in the energy transfer and nutrient cycling in aquatic
ecosystems, zooplankton assemblages have not been widely used as indicators of ecosystem condition
(Stemberger and Lazorchak 1994), and zooplankton is not included as a relevant quality element for the
assessment of ecological status within Water Framework Directive. Nevertheless, changes in primary productivity
and physical conditions due to eutrophication and warming and the consequent reorganization
of zooplankton communities have been documented worldwide, albeit, more often in freshwater than in
marine systems. In the Baltic Sea, alterations in fi sh stocks and regime shifts received a particular attention
as driving forces behind changes in zooplankton (Casini et al. 2009).
Here, we consider a possibility of using zooplankton as indicators for eutrophication and fi sh feeding conditions.
With respect to eutrophication, it has been suggested that with increasing nutrient enrichment of
water bodies, total zooplankton biomass increases (Hanson and Peters 1984), mean size decreases (Pace
1986), and relative abundance of calanoids generally decrease, while small-bodied cyclopoids, cladocerans,
rotifers, copepod nauplii, and ciliates increase (Brook 1969; Pace and Orcutt 1981). With respect to fi sh
feeding conditions, the following properties of zooplankton assemblages are usually associated with good
food availability: high absolute or relative abundance of large-bodied copepods and low contribution of
small zooplankters.
Description of proposed indicators
Copepod biomass (CB; mg/m 3 ) –zooplankton-as-food indicator; calculated using abundance and individual
weights. Alternatively (or in addition), contribution of copepod biomass to total mesozooplankton
biomass (CB%) may be used. This is a state indicator; refl ects composition of zooplankton community and
food availability for zooplanktivorous fi sh. In the Baltic Sea, copepods contribute substantially to the diet on
zooplanktivorous fi sh, such as sprat and young herring, and fi sh body condition and weight-at-age (WAA)
have been reported to correlate positively to abundance/biomass of copepods (Cardinale et al. 2002,
Rönkkönen et al. 2004). Copepods included here are mostly herbivores, therefore, this indicator would be
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