Maps of some of the variables measured by the CTD and other sensors on the ‘Meetvis’ and the rosette sampler are presented here: salinity, fluorescence and transmission (25 cm sensor) in the surface layer (4 m depth). The data collected by the ‘Meetvis’, mainly in operation during the night, were averaged over 10 minute periods, and pooled with the (4 m depth) data from the rosette sampler stations by day. Values by the rosette sampler transmissometer were disregarded since these were not sufficiently compatible to those of the ‘Meetvis’. The data were split in two periods to detect possible changes and to account for the difference in coverage: from 30 Aug. to 5 Sep. the core of the study area was surveyed, whereas from 5 till 10 Sep. a much larger area was visited.
Acknowledgement - The data were processed by Swier Oosterhuis (NIOZ), who also constructed the maps.
Salinity
All 4 major water masses in this part of the North Sea seemed to be present, with Continental Coastal Water (salinity < 34) in the east, in the regions closest to the Dutch coast. In the north, Central North Sea Water (> 34.5) was found during the first period. In the second period, when this northern region was not visited, a clear tongue of Channel Water (> 35) was found in the centre of the Southern Bight. Most of the water had intermediate salinity (34-34.5), indicating a predominance of English Coastal Water.
Fluorescence
In the area surveyed during the first period, high levels of fluorescence were very rare. A ‘green curtain’ (see Introduction) corresponding with chlorophyll a concentrations > 5 mg.l-1 (probably > 15 arbitrary units in the present maps) at or just north of the 30 m contour depth line was not found. The fluorescence in the Continental Coastal Water was low except for the area west of Texel. During the second period, no clear signs for the development of a ‘green curtain’ at the Frisian Front were found either. Relatively high fluorescence was encountered in corners of the study area with lower salinity: some rosette sampler stations at 6o East, the Rijkswaterstaat Noordwijk section, and close to the coast of Norfolk. Except for the latter site, the whole English side of the study area had a low fluorescence.
Transmission
On 9 Sep., when the ship sailed a zigzag pattern of ‘Meetvis’ sections from east to west over the Frisian Front, it was noticed that a pronounced dip in transmission occurred west of the Frisian Front. Afterwards, during the processing of the data, it became obvious that this ‘turbidity plume’ (= transmission minimum) was even more outspokenly present during the first period. During the operations with the Triple-D at the English sections in between and adjacent to the Norfolk Banks at 2o10’ and 3o East, it was discovered that the Brown Bank Clay lies at the very surface of the bottom at many places. This formation from the previous interglacial, some tens of metres thick, is apparently eroded during each tidal cycle, producing a clear plume of silt and clay in the direction of the Frisian Front. So it can be concluded that besides the winter erosion of the Norfolk coast, there is an all-year source from the bottom for the silt plume in addition! SeaWiFS images from 3 and 5 Sept. nicely show a plume of ‘chlorophyll’ off Norfolk all the way towards the German Bight. From the above it is obvious that we think that the western part of this plume is ‘non-chlorophyll’. As soon as the seatruth data on Suspended Particulate Matter and chlorophyll a will become available and have been processed, it is planned to present maps of these variables (comprising only the rosette sampler stations, though) on these web pages. This will give some insight into the quantitative contribution of the Brown Bank Clay to the ‘chlorophyll’ plume. Samples of water and bottom will also be analysed on clay minerals, which will hopefully give a qualitative picture of the silt composition (clay fingerprints) in different water masses and in top layer sediments of different areas.
