Macroalgae and phytoplankton as indicators of ... - Naturstyrelsen

The statistical analysis was carried out on log-transformed variables (Figure
4.2), and the common factor (b) was estimated as 0.55 (±0.05). The sitespecific
factors varied from 0.16 to 2.96, typically lower for coastal sites
with the exception of Dybsø Fjord and increasing as the estuaries/
coastal sites become more enclosed (Figure 4.3). Two sites, Nissum Fjord
and Ringkøbing Fjord before change of sluice practice, had very high factors
giving unrealistically high chla yields compared to TN when using
Redfield ratios and a carbon:chla ratio of 50 for conversion. This indicates
that additional nitrogen is introduced to these systems. In fact, both
systems have been dominated by cyanobacteria (ca. 33% and 80% for
Nissum Fjord and Ringkøbing Fjord, respectively) and this additional nitrogen
input may have come from nitrogen fixation. Thus, the consequence
of this is that chla-TN relationships as described here are not applicable
to coastal systems with a large proportion of nitrogen-fixing
cyanobacteria, particularly if the proportion of cyanobacteria varies
greatly from year to year.
Figure 4.2 Summer chla versus winter-spring TN mean concentrations for 42 different sites. Solid line shows the estimated
relationship averaged over all sites, and dotted lines show relationship with the lowest factor (Dybsø Fjord) and the highest
factor (Ringkøbing Fjord before change of sluice practice).
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