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Chlorophyll concentrations | |
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Submerged aquatic vegetation | |
Direct effects
CHLOROPHYL CONCENTRATIONS
Since 1980 absolute concentrations of chlorophyll
a in Danish open sea areas have been >50% above
background concentrations given by OSPAR. At first chlorophyll a concentrations
seem to have decreased in open sea areas since 1980. However, year-to-year
variations have been substantial. Chlorophyll a index values
adjusted for variations in climatic conditions shows that chlorophyll
a values were very constant in the period 1987-2001.
In estuaries and coastal waters chlorophyll a concentrations have decreased since the late 1980s. The chlorophyll a index values adjusted for variations in climatic conditions for 1993–2001 were at a consistent and decreasing lower level than that found in the mid 1980s.
SUBMERGED AQUATIC VEGETATION
The colonisation depth of eelgrass reflects differences in water quality
and physical setting along estuarine gradients. In the deeper, more protected
waters, reductions in eelgrass abundance towards the lower depth limit
correlate with light attenuation and are therefore more directly coupled
to changes in eutrophication. During the period 1989-2000 eelgrass colonisation
depth showed no significant trend and did not reflect the slight amelioration
of water clarity observed through the same period. During the period 1989-2001
eelgrass cover deeper than 2 meters showed no significant trend, but the
cover of shallow populations was significantly reduced in inner estuaries
and along open coasts. There is no obvious explanation for this pattern.
Though eelgrass colonisation depth and abundance from intermediate depths
towards deeper waters are likely to be better response parameters to eutrophication
than the abundance of shallow populations.
In 2001 many Danish estuaries still had an abundant cover of nuisance macroalgae. While there seemed to be a decrease in the relative cover of nuisance species at 0-1 and 1-2 meter depths since 1997, there was an in-crease in cover from 1998 to 2001 at 2-4 meters. In general, there where no significant changes during the period 1993 to 2001. The tendency of a reduction in relative cover was not true for some estuaries where relative cover had actually increased.
Results from 2001 showed that total cover of upright vegetation on reefs
in open waters tended to increase according to the mean of the period
1994–2001. In general, algal coverage was low in years with relatively
high runoff and high in years of low runoff during the period. Accordingly,
there was no over all trend in the distribution on the monitored reef
stations.
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