Macroalgae and phytoplankton as indicators of ... - Naturstyrelsen

Figure 3.8 continued Cumulated cover of opportunists (E), and fraction of opportunists (F) in relation to concentrations of total
nitrogen. Left panels show transformed (Tr.) data whereas right panels show backtransformed data. Algal variables are modelled
for a water depth of 7 m and it is indicated which salinity range data represent. Regression lines describing empirical relationships
between algal variables and TN-levels are shown for 3 salinities.
3.4.4 The "Spanish Index" tested on Danish macroalgal data
A prerequisite for the "Spanish index" is that it reflects negative antropogenic
influence on the marine environment. As it appears in the previous
section, the cover of late-successional species, the fraction of opportunists
as well as the number of late-successional species were all related to concentrations
of total nitrogen (Table 3.9, Figures. 3.8B, E and F). The criterion
for including these variables in the index was thus fulfilled for Danish
coastal areas.
The "Spanish index" operates with just two depth intervals. We found,
not surprisingly, that each of the three components were depth dependent
(Table 3.5) and showed marked changes with depth (Figure 3.6). Hard
substrate in Danish waters is made up of boulders and stones nearly everywhere
with changing depth distribution from site to site. Moreover,
hard bottom is generally found on deeper waters in open and less polluted
waters compared to sheltered fjords. This depth dependence and
distribution pattern of hard substrate are not compatible with a scoring
system with just two depth ranges. Rather, the depth dependence of the
algal components must be better integrated in the scoring system.
We also found that the level of algal components in the index varied
markedly between areas (Table 3.5) and that salinity was an important
variable affecting the algae (Table 3.9, Figure 3.7). The salinity will then
have a strong influence in the index scores.
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