Macroalgae and phytoplankton as indicators of ... - Naturstyrelsen

3 Macroalgae as indicators of water
quality
3.1 Introduction
Eutrophication is a major threat to submerged plant communities. Increased
nutrient richness stimulates the growth of planktonic algae and
thereby reduces water clarity and shades the benthic vegetation (e.g.
Nielsen et al. 2002a). The shading effect may be further accentuated by
epiphytic algae which also tend to proliferate under eutrophic conditions
(Borum 1985). Lack of light reduces the depth penetration of benthic
vegetation (Duarte 1991; Nielsen et al. 2002b) and also reduces vegetation
abundance in the deeper, light limited waters (Duarte 1991; Dahl & Carstensen
2008).
Opportunistic and perennial macroalgal species may respond differently
to changes in nutrient and light levels. Nutrient enrichment tends to
stimulate the growth of opportunistic algae which then shade the perennial
species (Littler & Littler 1980; Steneck & Dethiers 1994; Duarte 1995;
Pedersen 1995). The abundance of opportunistic algae is therefore likely
to increase at the expense of perennial algae as a function of increased
nutrient input. Moreover, the number of algal species may decline along
a nutrient gradient (Middelboe et al. 1997).
In our previous work for the Danish EPA we tested the response of Danish
coastal macroalgal communities to eutrophication and found that it
to some extent followed the patterns outlined above (Carstensen et al.
2005, Krause-Jensen et al. 2007a & b). The abundance of the macroalgal
community as a whole as well as the abundance of perennial and opportunistic
algae at given depths decreased significantly along a eutrophication
gradient. By contrast, the relative abundance of opportunists did not
respond to changes in nutrient level, but instead responded to changes in
salinity, being largest in the most brackish areas. These results indicate
that at large geographical scales the marked salinity gradient of the Danish
coastal waters overrules possible effects of nutrients on the relative
abundance of opportunists.
Our previous studies of the coastal Danish macroalgae thus strongly
suggested that cover of the total macroalgal community and cover of
perennial macroalgae are useful indicators of water quality. However,
there is a need to develop these indicators to become even more sensitive
to changes in water quality, to define boundaries between ecological
status classes and to describe precisely how to use the indicators for assessing
water quality according to the WFD.
For Spanish coastal waters it has been identified that not only the cover
of characteristic algal species declines along a nutrient gradient, the
number of the characteristic species also declines and the fraction of the
total algal community made up by opportunistic algae increases along
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