Macroalgae and phytoplankton as indicators of ... - Naturstyrelsen

4 Assessment of ecological status using
chlorophyll a
4.1 Boundaries for chlorophyll a
Summer primary production in Danish coastal waters is nitrogen limited
due to the exchange with phosphate-rich open waters. Therefore, the
summer phytoplankton biomass is considered related to the nitrogen
levels. Dissolved inorganic nitrogen (DIN) in winter has previously been
considered a good indicator for the phytoplankton biomass potential,
especially in deeper open waters where mixing depths exceed the photic
depths until around March when the spring bloom occurs. This mechanism
does, however, not apply to shallow coastal waters where primary
production and phytoplankton biomass can be considerable even in winter.
Secondly, DIN is a highly fluctuating variable and the relatively few
observations over a winter period does not give sufficient precision for
estimating the mean level. Consequently, total nitrogen is a more robust
variable that is directly related to nitrogen inputs from land and a large
fraction of TN is bioavailable, particularly if a large part of TN has terrestrial
origin. TN is considered composed of a bioavailable and a refractory
part, and it is believed that there is a generic functional relationship between
phytoplankton biomass and bioavailable nitrogen, whereas the fraction
of bioavailable nitrogen depends on site-specific features such as influence
of terrestrial nitrogen, retention time and exchange with Baltic
Sea water that has a relatively high refractory part of TN.
Let us assume that we can describe the generic relationship between
phytoplankton biomass, proxied by chlorophyll a (chla), and bioavailable
TN by means of a power function:
chla = a ⋅TN(
bioavailable)
= a ⋅ p(
bioavailable)
b
b
⋅TN
b
where p(bioavailable) is the bioavailable proportion of TN. Using a log-transform
on this equation yields
log( chla)
= log( a)
+ b ⋅ log( p(
bioavailable))
+ b ⋅ log( TN)
which is a linear relationship between log(TN) and log(chla) with an intercept,
log(a), that is site-dependent. The site-specific relationships were estimated
to analyse commonalities in the slopes (Figure 4.1). A total of 19
sites had significant slopes with 2 of these actually showing negative relationships
between chla and TN (Mariager Fjord and Ringkøbing Fjord
after change of sluice practice). Mariager Fjord is the only true fjord in
Denmark with a sill, permanent stratification and hypoxia/anoxia below
the pycnocline. Mussels in high densities are present along the shores
above the pycnocline, but grazing pressure in the deeper part, where the
monitoring station is located, is limited because the poor oxygen conditions
in the deep water gives little zooplankton recruitment. Consequently,
other mechanisms than bottom-up control may be more important for
the summer phytoplankton biomass. Ringkøbing Fjord experienced a re-
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