State of the North Sea

Assessment of the current status and identification of long-term changes

Related to its oceanographic state variables such as surface and volume temperature, salinity and stratification, the North Sea is a highly variable marine area. Part of this variability is brought about by the Atlantic Ocean, the Baltic outflow into the Skagerrak and the atmosphere. In order to reliably assess the North Sea status, the entire North Sea must be sampled quasi-synoptically in three dimensions. Therefore, since 1998 the Federal Maritime and Hydrographic Agency (BSH) carried out annual oceanographic-chemical surveys of the entire North Sea between 51° and 60° latitude north during summer at the time of maximum stratification.

Aim of surveys and use of the data obtained

The aim of the surveys is to describe and assess the current oceanographic and chemical status of the North Sea. This includes the calculation of the heat and salt budgets (how much heat and salt is present in the entire North Sea) and the distinction between long-term climate-induced signals and the strong natural inter-annual to multi-decadal variability of the North Sea.

The data sets obtained also serve for validation and plausibility checks of operational and climate models, as well as for the validation of satellite-based optical remote sensing data (such as chlorophyll, Secchi depth, yellow substance, turbidity), which are increasingly used for current area-wide state assessments. Following the processing, quality control and analysis of the data and samples obtained at sea, the data are transferred to the German Oceanographic Data Centre (DOD) and are used for current state assessments of the North Sea.

In addition, the BSH carries out further regular surveys in the North Sea and Baltic Sea. The Exclusive Economic Zone (EEZ) of both seas is sampled on about 5 to 6 surveys per year. Within the framework of national and international projects, climate-relevant measurements are also carried out in the deep North Atlantic. During these cruises, ship-bound measurements are carried out and deep-sea moorings with recording instruments for temperature, salinity and currents are deployed for a period of about one year.

BSH North Sea surveys at the time of maximum stratification (1998-2018)

Period of recordingVessel and voyage numberDistance [sm]Cruise report
24.06.1998 - 16.07.1998R/V Gauss 317ca. 2600
02.07.1999 - 22.07.1999R/V Gauss 335ca. 2600
09.08.2000 - 23.08.2000R/V Gauss 353ca. 2600
11.07.2001 - 02.08.2001R/V Gauss 370ca. 2600
16.07.2002 - 31.07.2002R/V Gauss 385ca. 2600Fahrtbericht 2002 (PDF, 1MB, Not barrier-free file.)
28.07.2003 - 13.08.2003R/V Gauss 405ca. 2600Fahrtbericht 2003 (PDF, 260KB, Not barrier-free file.)
05.08.2004 - 20.08.2004R/V Gauss 425ca. 2600Fahrtbericht 2004 (PDF, 1MB, Not barrier-free file.)
10.08.2005 - 29.08.2005R/V Gauss 446ca. 2600Fahrtbericht 2005 (PDF, 2MB, Not barrier-free file.)
02.08.2006 - 20.08.2006R/V Gauss 463ca. 2600Fahrtbericht 2006 (PDF, 2MB, Not barrier-free file.)
03.08.2007 - 17.08.2007R/V Pelagia 273 ca. 2600Fahrtbericht 2007 (PDF, 1MB, Not barrier-free file.)
21.07.2008 - 05.08.2008R/V Pelagia 2932715Fahrtbericht 2008 (PDF, 3MB, Not barrier-free file.)
20.08.2009 - 09.09.2009R/V Pelagia 3113610Fahrtbericht 2009 (PDF, 5MB, Not barrier-free file.)
04.08.2010 - 22.08.2010R/V Pelagia 3233310Fahrtbericht 2010 (PDF, 1MB, Not barrier-free file.)
08.08.2011 - 28.08.2011R/V Celtic Explorer 110103220Fahrtbericht 2011 (PDF, 3MB, Not barrier-free file.)
07.08.2012 - 30.08.2012R/V Celtic Explorer 120113500Fahrtbericht 2012 (PDF, 1MB, Not barrier-free file.)
10.08.2013 - 04.09.2013R/V Celtic Explorer 130124090Fahrtbericht 2013 (PDF, 1MB, Not barrier-free file.)
01.08.2014 - 25.08.2014R/V Celtic Explorer 140123470Fahrtbericht 2014 (PDF, 1MB, Not barrier-free file.)
07.08.2015 - 30.08.2015R/V Celtic Explorer 150133580Fahrtbericht 2015 (PDF, 3MB, Not barrier-free file.)
03.08.2016 - 26.08.2016R/V Celtic Explorer 160114000Fahrtbericht 2016 (PDF, 3MB, Not barrier-free file.)
11.08.2017 - 03.09.2017R/V Celtic Explorer 170133600Fahrtbericht 2017 (PDF, 4MB, Not barrier-free file.)
28.08.2018 - 13.09.2018R/V Celtic Explorer 180193100Fahrtbericht 2018 (PDF, 4MB, Not barrier-free file.)

Station Grid

Research vessels (Gauss, Pelagia, Celtic Explorer) Research vessels. Left: Gauss, center: Pelagia, right: Celtic Explorer

The historical core program consists of about 50 CTD stations (Conductivity, Temperature, Depth) on a fixed station grid. The stations are used to record the vertical profiles of the most important oceanographic parameters such as temperature, salinity, oxygen, turbidity and chlorophyll. This is done with a so-called CTD, which measures conductivity, temperature and depth via pressure.


From 1998 to 2014, also towed CTD systems were used on the transits between the CTD stations which were able to oscillate between surface and bottom. Due to technical problems and changed requirements, the towing systems were abandoned in 2015. To compensate for this, additional stations were added to the station grid.

Since 2009, the station grid has been extended by additional transit stations for the determination of artificial radionuclides in seawater. At these stations the ship did not stop, but seawater samples were taken from the seawater pipe during the cruise. From 2015 on, the ship also stopped at these stations and they were sampled by the other working groups like a regular CTD station.

For a better understanding of the physical exchange processes in the transition area between North Sea and Atlantic, the station grid was extended by stations north of 60°N in the years 2010 to 2015. Since 2016, the 60° north section has been extended to 8° W to include the sampling of the Eastern Boundary Current at the shelf edge. Its water masses mix with Atlantic Water entering the North Sea at its northern edge and thereby also modify the physical status of the North Sea.

From 1998 to 2006, the North Sea surveys were carried out by BSH's own research vessel Gauss. After the Gauss was decommissioned, BSH chartered the Dutch research vessel Pelagia operated by NIOZ from 2007 to 2010 and the Irish Celtic Explorer operated by the Marine Institute in Galway from 2011 on.

Development of the station network

The red stations form the "core grid" which has been sampled annually since 1998. Later, the grid was successively expanded and adapted to current monitoring requirements. The stations north of 60°N were sampled in the years 2010-2015 in the context of the projects "RACE I and II - Regional Atlantic Circulation and Global Change" in order to investigate more closely the exchange processes between North Sea and Northeast Atlantic. Starting in 2016, the 60°N section was extended to 8°W to include the eastern boundary current at the shelf edge, because its varying water mass properties also influence the oceanographic conditions in the North Sea. The stations in the English Channel and in the Skagerrak (green dots) are currently sampled bi-annually in alternation, primarily to record artificial radionuclides. The stations in the Northern Minch between Scotland and the Outer Hebrides are also part of the radioactivity monitoring network.

CTD probe

One of the most important devices on board a research vessel is the CTD probe. CTD stands for the three most important measured variables sampled by this device, Conductivity, Temperature, and Depth, which is determined by the pressure. These three parameters can also be used to calculate salinity; the more salt there is in seawater, the higher its conductivity.

Usually, the CTD probe is combined with a rosette water sampler. Depending on the size of the bottles used in the sampler, this allows a certain number of water samples to be taken at selected depths. Once the probe is back on board, water samples can be taken from these bottles for the analysis of chemical and/or biological substances.

CTD and water sampler are attached to a special cable which supplies both devices with electricity via an inner insulated conductor and simultaneously transmits the data to a computer on the ship. At the measuring positions the ship stops and CTD and water sampler are slowly lowered into the water via a winch to just above the seabed. As a rule, all bottles of the rosette sampler are open at top and bottom. Only a few bottle types for special analyses open at depth prior to the proper sampling. The scientists on board see on a screen how temperature and salinity change with depth. The CTD is often also equipped with sensors for chlorophyll, yellow substance, turbidity or oxygen, and these parameters are also displayed on the screen. On the basis of this display, the scientists decide at which depths they want to take their water samples. When the probe is lifted back on board, the bottles of the rosette sampler can be closed at these selected depths.

Additional ship-bound sampling methods

Until around 2012/2013, BSH also used special disposable probes to measure a temperature or a temperature and salinity profiles from a moving ship. These relatively inexpensive "Expendable BathyThermographs" (XBTs) or "Expendable Conductivity, Temperature, Depth" (XCTDs) consist of an approximately 35 cm long rocket-shaped measuring probe that is electrically connected to a small launching device via a very thin wire. If the probe falls into the depth, this wire unwinds both from a coil inside the probe and from a second coil inside the launcher.

Reaching its maximum sample depth defined by the length of the wire, the wire breaks, the connection to the ship is interrupted and the probe falls to the seabed. A computer connected to the launching unit stores the profile data, which are transmitted to a land station via satellite after the measurement and from there fed into a worldwide data network as real-time data.

XBTs are not only used on research ships, but also on other ships that have agreed to do so. These so-called ‘ships-of-opportunities’ are, for example, merchant ships or ferries sailing on fixed routes. Today, these measurements are being replaced on a large scale by data from profiling floats from the operational ocean observation program Argo.

The ‘North Sea System’

The North Sea is a relatively shallow shelf sea. Its physical state – primarily characterized by the spatial distribution of salinity and temperature – is largely determined by the exchange of water masses with the Atlantic across the open northern edge of the North Sea. The southwestern North Sea is connected to the Atlantic by the shallow English Channel and the Strait of Dover. The influence of this connection upon the entire North Sea is significantly smaller, but essential for the shallow southern North Sea. The Baltic Sea is connected to the North Sea via the Skagerrak and Kattegat and via Great and Little Belt and Øresund. The Baltic outflow with its low salinity waters shapes the oceanographic conditions above the Norwegian Channel. Other impacts include continental freshwater runoff, heat exchange with atmosphere (global radiation) and the ratio of precipitation to evaporation. Due to these many interactions, one also speaks of the "North Sea System".

All impact factors show strong seasonal as well as inter-annual fluctuations. Due to seasonal warming, a warm upper mixed upper layer builds up in spring, causing a thermal stratification of the North Sea until about the end of September. Thermocline strength and thickness of the upper mixed layer can vary considerably both regionally and from year to year. In areas with water depths of less than 25-30 m, tidal friction close to the ground and wind-induced mixing at the surface prevent stratification and the water body remains predominantly vertically mixed. Both areas are separated by a so-called tidal mixing front.