Introduction
The Danish and Swedish
"Authorities Control- and Monitoring Programme for the
Fixed Link across Øresund" applies aerial photography as
a means of detecting changes in eelgrass
communities during the construction of the
fixed link.
In the following, we refer to
the Autorities Monitoring Programme and show the most
recent results. (Published in Krause-Jensen et al. 1999).
Air photography
Air photography was carried out in august 1996-1998 from an
altitude of 3000 ft in calm weather. The water transparency was generally very good.
Images were recorded from a twin-engine aeroplan, Partenavia 68
observer.
Partenavia 68 observer
The plane was equipped with a camera, Rolleiflex 6008, with a 50-mm
special lens. Slides of the brand Ectachrome 200 Professional were used. Images were
recorded at a resolution of 15-30 cm per pixel. In all, approximately 1500 pictures were
taken to cover the Øresund area. The overlap of the
images was approximately 40%: The position of the plan was established by DGPS with an
accuracy of 5-10 m. The position, speed, altitude and banking was logged
continuously on the flight computer by using the programme Bio
Flight. Image orthoretification and image analyzing was done as descript under
post-processing.
Analysis of vegetation area coverage and biomass
Coverage of Zostera marina was mapped on the basis of
the factual information on coverage obtained by paravane diving along
transects.
(henvisning
til sampling of vegetation for remote sensing analyses)
Results
Occurrence
Zostera marina is widely distributed in the central part of Øresund
inside the 6 m depth curve. The accuracy of the
analysis is lowest closest to the depth limit where coverage and biomass and, hence,
reflection from the vegetation is lowest. because the transparency of the water was high
when the air photographs were taken, however, it was possible to map the Zostera
marina vegetation until the 6 m depth curve and sometimes even deeper. This means
that it was possible to map the vegetation approximately until the depth limit of main
occurrence.
In 1998, the distribution area of Zostera marina was 139,7 km². The
distribution area increased from 1996 to 1997 and decreased again from 1997 to 1998.
Area coverage
The area coverage of Zostera marina was analysed on the
basis of the individual photos and supplemented with factual information on
coverage obtained by paravane-diving along the transects. Using interpolation, maps where
then drawn, showing the coverage of Zostera
marina. In
general, highest coverage is observed in the depth interval 1-4 m; in deeper water the
distance between individual shoots increases, resulting in a decrease
in total coverage.
Area biomass
Based on the key map of the coverage of Zostera marina and
the relation between coverage and coverage-corrected
biomass, a key map of the distribution of Zostera marina biomass
in the central part of Øresund has been produced, figur
x. In 1998, the total
Zostera biomass in the total area surveyed in Øresund was 8,944 t. The assessments of Zostera
marina biomass are only approximate since some inaccuracy is contained in
distribution areas, interpolation of coverage as well as in the conversion of coverage to
biomass.
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