Macroalgae and phytoplankton as indicators of ... - Naturstyrelsen

tial variation would give a better spatial description and reduce the random
variation between sites.
Coupling algal variables to water quality
The variation in water quality variables was initially analysed using a
model similar to the algal model. The model describes water quality
variables with respect to area-specific variation, site-specific variation,
seasonal variation and year-to-year variation among hydrological years,
i.e. July-June. For each water quality variable we calculated area-specific
marginal means.
Algal variables were related to physico-chemical variables through multiple
regression analysis using backward elimination. First we introduced
all the potential independent variables in the regression, and then excluded
variables one by one until only the significant variables remained.
The analyses were conducted on a spatial basis to explain differences in
algal parameters between various coastal areas/sub-areas.
Testing the "Spanish index" on Danish data
A test of the "Spanish index" on Danish data demands some adjustment
since Danish and Spanish algal data are collected differently and species
composition and depth distribution differ. A first adjustment regards the
definition of the three components of the index which also affects the
data range and thus the scoring system. However, the principle of the
scoring system, i.e. its depth and area dependence may also need adjustment
for the index to be applicable under Danish conditions.
Our approach for testing the "Spanish index" is the following:
• Verify whether the individual components of the index reflect nutrient
gradients in Danish coastal waters since this is a prerequisite for
including them in the index.
• Identify whether each component is depth dependent and whether its
level varies between areas, e.g. depending on salinity. This part of the
test will tell us whether the principle of the Spanish scoring system
can be transferred to Danish conditions.
Below we explain how the "Spanish index" is defined and translated to
Danish conditions and how the scoring system of the "Spanish index"
operates.
Definition of the three components of the "Spanish index"
The first component of the index is the cover of characteristic species. In
Spain, this variable is assessed by estimating the percentage of the stable
substratum of the sample area which is covered by 'characteristic species'
as defined for Spanish areas. Characteristic species are those which are
not opportunistic. We translate the Spanish 'characteristic species' to the
Danish 'late-successional species'. The first component of the Spanish index
is therefore approached by the Danish variable 'cumulated cover of
late-successional species'. The data range is 1-100% for the Spanish variable
but may exceed 100% for the Danish variable.
26