Macroalgae and phytoplankton as indicators of ... - Naturstyrelsen

prevail at high salinities, while green algae which contain many opportunists
prevail at low salinities (Nielsen et al. 1995). Other variables than
salinity do, however, also affect the gradients of macroalgal distribution
across the Baltic Sea (e.g. Middelboe et al. 1997).
3.5.2 Variability of algal variables
Another important characteristic of a good indicator is that it can be assessed
precisely, i.e. with low stochastic variability.
All algal variables were found to be associated with considerable variability
(Table 3.6). The variables showing least stochastic variability were
total cover, fraction of opportunists (in areas with low salinity) and number
of late-successional species.
The variables expressing cumulated cover showed much more variability
– especially the cumulated cover of late-successionals and the cumulated
cover of opportunists. The higher variability of cumulated cover levels
relative to total cover levels is most likely due to the fact that the cumulated
levels are sums of cover observations with associated variability
and thus also accumulate this variability. The reason why cumulated
cover of opportunists and of late-successional species are more variable
than cumulated cover of the entire algal community may be that there is
more natural variability connected with the specific algae which colonise
the substrates in a given area than with the total colonisation of the substrates.
This would imply that sub-samples differ more with respect to
the cumulated cover of perennials and opportunists than with respect to
cumulated cover of all species.
The power analyses confirmed the patterns indicated by the stochastic
variability but also demonstrated that the number of observations needed
in order to assess ecological status with sufficient accuracy strongly depends
on the type of the estuary/coastal area in question. Thus, areas
with weak responses of TN concentration to TN input demanded more
observations than areas with strong TN responses.
A further reduction of the stochastic variation of the algal variables requires
that diver variation is reduced and that effects of site-specific features
such as currents and bottom topography are adequately described.
Biologic interaction like space competition between macroalgal vegetation
and blue mussels (Mytilus edulis) or sea squirts (Ciona intestinalis) are
also likely to contribute to the observed stochastic variation.
3.5.3 Choice of algal indicators
The best algal indicators are those which reflect nutrient gradients as
clearly as possible and which are associated with low variability. Based
on these criteria and the discussions of the previous two sections total
algal cover and number of perennial algal species together with the fraction
of opportunists in areas of low salinity were the most promising
among the potential algal indicators. However, it is important to keep in
mind that the indicator total algal cover needs deeper sampling stations
to provide useful data compared to indicators using cumulative cover.
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