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/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 summer state of the North Sea 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 between 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 red dots in Fig. 1). Between these fixed stations a towed CTD-system (1998-2008 the 
3SH Delphin, since 2009 an EIVA MK2 ScanFish) was deployed which oscillated between 
surface and bottom to record the distribution of relevant oceanographic parameters with high 
resolution in space and time (24 Hz). Both CTD-systems are recording temperature, salinity, 
fluorescence (chlorophyll-a, yellow substance), and oxygen concentration. Additionally, ship- 
mounted temperature-, salinity- and optical sensors provided data at about 4 m depth. In 
order to sample the transition area between North Sea and Atlantic the survey was expanded 
northwards to 62.5°N since 2010. 
Due to technical problems and staff deficit there are no ScanFish measurements in 2015. 
This was compensated by about 40 additional vertical CTD stations marked with S in Fig. 1. 
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 
io 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