Long time series – monitoring networks and moorings
The BSH’s marine monitoring network in the North and Baltic Sea
To obtain long time series at fixed positions BSH uses two methods. Firstly, BSH operates its marine environmental monitoring network MARNET in the Exclusive Economic Zones (EEZ) of the North and Baltic Sea with fixed measuring positions mounted on buoys, bottom mounted platforms and unmanned light vessels. Under these ‘carrier units’ cables equipped with measuring instruments at different water depths are hanging in the water column reaching down just above the seabed. The instruments record temperature, salinity and other parameters which are transmitted to the BSH hourly. These data enable the BSH to assess permanently the current state of the German waters in the North and Baltic Sea. Detailed information can be found in the chapter Messnetz-MARNET.
Another way to obtain long time series is to equip a mooring cable which is anchored to the seabed with recording instruments for current, salinity, temperature and other parameters at different depths. The duration of such deployments varies between a few days and weeks up to long-term deployments of up to two years. There are two different designs for such moorings: shallow water moorings for water depths of up to about 100 m and deep-sea moorings for depths of up to several 1000 m. At the end of the measurement campaign, the moorings are recovered and the measurement data can be read from the internal memories of the instruments.
From the shallow shelf sea...
In a shallow water mooring, the anchor weight of the measuring chain is connected by a 100 to 200 m long ground cable with the anchor of a marker buoy. In order to keep the instruments of the measuring chains vertically in the water column, in both mooring designs buoyancy elements are mounted at the upper end of the chain and also between the instruments.
Depending on water and measuring depths, these consist of steel or plastic spheres (up to 100 m depth) or of plastic-coated glass spheres, so-called "cornings", which can be used in water depths of up to 6000 m. The uppermost buoyancy package of a shallow water mooring is about 15 m below the water surface in order to decouple the measuring chain from the swell.
When the anchorage is recovered, the marker buoy will be picked up first, then the anchor weight of the marker buoy can be recovered by the anchor cable of the buoy. If the anchor weight – and thus also the one end of the ground cable – is on board, the anchor of the measuring chain can also be pulled on board by the ground cable. Thereby, the measuring chain surfaces and instruments and buoyancy elements can be recovered.
...into the deep-sea
The much longer deep-sea moorings have no marker buoy. Instead, two acoustic releasers are mounted between the anchor weight at the sea bottom and the measuring chain.
To recover the mooring, an acoustically coded signal is emitted from the ship which triggers the opening of a hook at the lower end of the releaser. This frees a short steel chain between the releaser and the anchor weight at the sea bottom and the whole measuring chain can surface. For safety reasons usually two releasers are used, but the opening of one releaser hook is sufficient to release the mooring.
The top buoyancy of the mooring should be at least 300 m below the water surface for protection against deep-sea trawling. However, there are also short deep-sea moorings for measurements close to the bottom where the top buoyancy is more than 4000 m below the surface.
In order to facilitate the recovery of the measuring chain in bad weather and/or darkness, the top buoyancy is equipped with a flag, a direction finder transmitter and a satellite transmitter. The satellite transmitter also reports via Internet if the mooring appears unintentionally, for example due to material defects, corrosion or deep-sea trawling.