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Inshore Site Investigation - Benthic & PSD Surveys

The scope of work is usually to define the physical and biological conditions of the seabed at representative stations along an existing or proposed outfall corridor.

The grab survey is undertaken along the outfall using a suitable seabed sampler. Inshore operations using small grabs, such as Van Veen and Shipek, are sometimes not successful. In these cases we use a hydraulic clamshell grab which is considered the most suitable for achieving representative samples although a Hamon grab is also utilised, in particular for benthic work, as this instrument is recommended by CEFAS for this type of operation.

Sampling of surface sediments (the top 5cm) is normally required on and off the pipe-route at regular intervals.

Samples collected are retained and sub-sampled into double-lined plastic bags prior to chilling within a cool chest. Samples are transported back to the laboratory for sieve analysis in this fashion.

Additional samples are collected for benthic fauna analysis using the Hamon grab which has been specifically designed to minimise washout of surface sediment as the grab is raised from the seabed. This means that the sample is completely enclosed within the grab. Samples are representative of around 0.1m² of the seabed. The recovered material is initially described and subsequently sieved through a 1mm mesh.

Organisms retained on the sieve are fixed immediately in 10% buffered saline formalin solution (4% formaldehyde), the volume of fixative used exceeding the volume of the sediment sample by a factor of at least two.

Large stones are returned to the sea following in situ inspection by the scientist onboard, recording the retention of attached fauna.

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