Macroalgae and phytoplankton as indicators of ... - Naturstyrelsen

3.4.5 Reference levels and class borders sensu WFD for macroalgal
variables
For each of the estuaries/coastal areas we entered the TN-levels defining
reference levels and class boundaries (Table 2.4 in chapter 2) and annual
mean salinities (shown in Figure 3.7G) in the empirical relationships (defined
in Table 3.9). We thereby calculated the level of algal variables defining
reference conditions and class boundaries (Appendices 1-6). The
level of the boundaries, the variability associated with them and the
width of the status classes differ widely between variables and between
areas. The applicability of the algal variables for assessment of water
quality thus also varies between variables and areas.
In order for an algal variable to be applicable for assessment of ecological
status, the class borders should be defined with low variability and the
status class bands should be relatively broad. This demand was best fulfilled
for the variables which were associated with a low degree of stochastic
variability and a strong response to changes in TN concentrations.
The variables 'total cover', and 'number of late-successional species' generally
showed relatively low stochastic variability and a strong response
to TN concentration at high salinities so these indicators had class borders
with relatively low variability in high salinity areas (Table 3.6). The
'fraction of opportunists' also had relatively limited stochastic variability
with weak responses to changing TN concentrations at high salinities. So
class borders of this variable generally showed high variability and narrow
status class bands in high salinity areas. By contrast, in low salinity
areas where responses to TN were stronger, class borders were defined
with less variability.
The cumulated algal cover variables were associated with more stochastic
variability than the algal variables mentioned above and therefore tended
to have class borders connected with relatively large variability.
The response of TN concentration to changes in TN loading in a given
area also affects the width of the status class. Thus, areas which demonstrate
a strong response in TN upon changes in N-loading, i.e. areas having
a large regression slope for the regression of TN-concentration upon
TN-input (chapter 2, Figure 2.6) have broader status class bands than areas
showing a weak response to TN inputs. Many open areas, such as Bornholm,
show weak responses in TN to changing TN inputs and, therefore,
have narrow status class bands. By contrast, areas with significant nutrient
sources and low water exchange such as inner Odense Fjord, several
Limfjord basins, Randers and Nissum Fjords show strong responses in
TN concentration to changes in TN inputs and therefore have broader
status class bands. The demand for the indicators to have low stochastic
variability and high sensitivity to TN is intensified if the area in question
has narrow status class bands for TN.
For the Sound, Nivå Bay, Limfjorden west of Mors and Roskilde Fjord
we have illustrated the class boundaries and their associated variability
together with actual levels of the algal indicators (Figures 3.9 - 3.12).
These figures illustrate that status class bands are narrow in the Sound
and Nivå Bay – areas with a weak response of TN concentrations to TN
44