Objectives and scientific background 
The North Sea is a shallow shelf sea with a deep trough along the Norwegian coast with 
depth exceeding 700 m locally. Its physical status, primarily characterised by temperature 
and salinity, is to a large extent determined by the exchange of water masses with the 
Atlantic at its open northern boundary. There is also a link to the Atlantic via the English 
Channel which is important for the shallow southern North Sea. The Baltic Sea is linked to 
the North Sea via Skagerrak, Kattegat, Great and Little Belt, and The Sound. The Baltic 
outflow with its low saline water influences significantly the oceanographic conditions of the 
Skagerrak and Norwegian Coastal Current. Other drivers are inter alia continental river run- 
offs, the ocean-atmosphere heat exchange, and the rate of precipitation to evaporation. 
All parameters exhibit a strong seasonal and/or inter-annual variability. Seasonal heating 
leads to the establishment of a seasonal thermocline between spring and end of August or 
midst of September with vertical gradients exceeding 3 K/m in most of the years. Strength 
and depth of the thermocline vary locally and from year to year. Near-bottom tidal mixing and 
wind induced mixing at the surface suppress stratification in areas shallower than 25 to 30 m. 
Stratified and vertically mixed areas are separated by so-called tidal mixing fronts. 
In order to assess the physical and chemical state of the North Sea during summer the BSH 
started its North Sea Summer Surveys (NSSS) in 1998. They cover the entire North Sea with 
seven coast to coast east-west sections between 54° and 60°N and additional stations 
vetween 54°N and the entrance of the English Channel. The surveys were realised at a time 
when thermal stratification is expected to be at its maximum and phytoplankton production 
has passed its maximum. With the exception of the first survey in 1998 all surveys served a 
fixed grid of vertical CTD casts (see station without an A, B, or S in Fig. 1). Between the 
CTD-stations ship-mounted temperature-, salinity- and optical sensors provided data at 
about 4 m depth. 
For the monitoring of artificial radio nuclides additional stations in the English Channel, in the 
Skagerrak, and in the Northern Minch are served alternately every three years. 
The objective of the NSSSs is the assessment of the oceanographic and chemical state of 
the North Sea, the calculation of heat and salt budgets, and the identification of changes due 
to climate change. The data are also used for the validation of operational and climate 
models and for the calibration of satellite-based ocean colour data and downstream products 
(Secchi depth, turbidity, CDOM, chlorophyll-a) which are used for assessments and MSFD 
reporting. All NSSSs are listed in Table 1. Most of the data are available via the German 
Oceanographic Data Centre (DOD) and the MEris MAtchup In-situ Database MERMAID.' 
' http://www.bsh.de/en/Marine data/Observations/DOD_Data_Centre/index.jsp 
http://hermes.acri.fr/mermaid/home/home.php