Paul G Provost

Validation of a fish farm waste resuspension model by use of a particulate tracer discharged from a point source in a coastal environment

To validate a resuspension model of particulate material (salmonid farm wastes), a UV fluorescent particle tracer was selected with similar settling characteristics. Tracer was introduced to the seabed (water depth ≈30 m) and sediment samples taken on days 0, 3, 10, 17 and 30 to measure the horizontal and vertical distribution of tracer in sediments. A concentric sampling grid was established at radii of 25, 50, 100, 150, 200, 400, 700 and 1, 000 m from the source on transects 30° apart. The bulk of the deployed tracer was initially concentrated in an area 25 m radius from the release point; tracer was observed to steadily decrease to zero over a period of 30 days. In a 200 m region measured from the release point in the direction of the residual current, the redeposition of tracer was low. A Lagrangian particle tracking model was validated using these observed data by varying resuspension model parameters within limits to obtain the best agreement between spatial and temporal distributions. The validated model generally gave good predictions of total mass budgets (±7% of total tracer released), particulary where tracer concentrations were high near the release point. Best fit model parameters (critical erosion shear stress=0.018 N m−2, erodibility constan=60 g m−2 d−1) are at the low end of reported parameters for coastal resuspension models. Such a low critical erosion shear stress indicates that the frequency of resuspension and deposition events for freshly deposited material is high.

HOMER: early results from a novel seabed-resident water column profiler

The HOMER deep water vertical profiler (HOMing Environmental Recorder), originally devised at the Proudman Oceanographic Laboratory, has recently been further developed and tested at the Scottish Association for Marine Science (SAMS). The HOMER system comprises a seabed resident winch which repeatedly releases and recovers a buoyant, internally-recording, 0.25 m diameter spherical sensor module. The sensor sphere performs CTD (conductivity, temperature, depth) profiles of the water column from the seabed to a pre-programmed altitude. Other sensors will be added in due course, for example current meters and shear micro-structure sensors. In its present configuration the instrument is capable of a total of approximately 200 profiles to a height of 400 m above the bed in water depths of up to 4000 m. Deployments of up to 2 years duration are possible. Water column profiles are taken at pre-programmed time intervals, and winch control is performed through an embedded microcontroller. The pre-programmed microcontroller instructs a pair of oil filled brushless DC motors; one motor drives the main take-up spool and the other drives a capstan which controls the ascent and decent rate. Power is supplied through a bank of standard lithium D-cells. Sensor sphere and winch are connected via a non-conducting mono-filament line. An infra-red link transfers data from the sensor sphere to the sea-bed frame control sphere between profiles to minimise the risk of data loss. The entire instrument remains on the seabed between profiles, thus minimising problems associated with fishing damage and biofouling.