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Profiling mooring in the Gotland Basin

The Gotland Deep Environmental Sampling Station (GODESS) is a profiling mooring, meaning that a profiling body with the payload (the instruments making measurements) is ascending and descending through the water column at predefined times or intervals. During a deployment of the mooring (typically between 3 and 6 months) repeated profiles of the measured parameters are registered so that we gain an insight of the changes and dynamics during this deployment period. A special interest for this station is the redoxcline in the Gotland Basin between the oxygenated surface layers and the anoxic deep layer.

GODESS consists of a bottom weight, holding the station in place, the acoustic releaser with recovery line, the underwater winch and the profiling instrumentation platform.

underwater winch

The underwater winch is an Automatic Elevator System Type 3, built by Nichiyu Giken Kogyo Co. Ltd., Japan. At pre-programmed times or intervals a set length of the Kevlar line is paid out; since the profiling platform has a net buoyancy of about 9 kgf it ascends through the water column. When the set length is paid out the winch stops for a set time and starts reeling the line back in. The underwater winch was delivered with 350 m of line which limits the reach. In the Gotland Basin with a maximum depth of 250 m this is no limitation. In our application the winch is moored in a depth of about 180 m, held in place by the bottom weight pulling downwards and the net buoyancy of the winch system (about 36 kgf) pulling upwards.

profiling instrumentation platform

The profiling instrumentation platform (PIP) was designed at the IOW, based on the titanium frame for the CTD of Sea & Sun Technology GmbH. This frame was modified and fitted with syntactic foam plates for buoyancy. Currently the instrumentation of the PIP consists of a Sea & Sun Technology CTD 90 M with the following sensor suite:

An additional fast oxygen optode (JFE Advantech Co. Ltd., Japan) is also installed on the platform and connected to the CTD that logs all the data and also supplies all sensors by it's internal batteries.

 

A new platform, PIP2, is currently build and will accommodate more instruments and a central data logger with rechargeable power supply. A drawing of the PIP2 can be seen below.

data from a 55-day winter deployment

Data from the third GODESS deployment from 16. November 2010 to 11. January 2011, one profile every 8 hours.

Dissolved oxygen and turbidity (left), temperature (middle) and salinity (right). Lighter curves show preceeding profiles.

The profiling mooring in the Gotland Basin is one of the field observatories of the EU project HYPOX.

Datasets of the first deployments are stored in the Pangaea data base:

http://doi.pangaea.de/10.1594/PANGAEA.777607
http://doi.pangaea.de/10.1594/PANGAEA.777608
http://doi.pangaea.de/10.1594/PANGAEA.777609

 

Mooring sketch
Sketch of mooring with profiler, underwater winch, releaser and recovery line and bottom weight (from top to bottom)