Benthic Macrofauna Survey
The St. Helena Bay Water Quality Trust conducts "State of the Bay" assessments to determine the overall environmental health of St. Helena Bay and tracks effects that may be caused by human activities.
These assessments focus on the benthic environment in terms of sediment quality and abundance and distributions of the macrofauna living in and on the sediments in the bay.
The Benthic Macrofauna Survey is commissioned at approximitely 5-year intervals. The first of these assessments was carried out in 2001 (CSIR 2001), 2007 and then again in 2012. The next Benthic Macrofauna Survey is planned for 2017.
Definition of Benthic zone
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some sub-surface layers. Organisms living in this zone are called benthos. They generally live in close relationship with the substrate bottom; many such organisms are permanently attached to the bottom. The superficial layer of the soil lining the given body of water, the benthic boundary layer, is an integral part of the benthic zone, as it influences greatly the biological activity which takes place there. Examples of contact soil layers include sand bottoms, rocky outcrops, coral, and bay mud.
Benthos are the organisms which live in the benthic zone, and are different from those elsewhere in the water column. Many are adapted to live on the substrate (bottom). In their habitats they can be considered as dominant creatures. Benthic organisms can be divided into two categories based on whether they make their home on the ocean floor or 25mm - 50mm into the ocean floor.
Soft-bottom benthic macrofauna (animals larger than 1 mm) is the component of the marine biota most widely used in environmental impact studies (Warwick and Clarke 1993). The long generation time (years rather than months) of the macrofauna and its non-mobile mode of life means that community structure reflects the environmental conditions integrated over a long period of time.
The purpose of these assessments is to build on the survey data base in establishing the severity and extent of any discernible ecological impact associated with human activities in and around St Helena Bay.
Sediment samples are collected from stations within the study area, using a multi-corer device deployed from a boat or ship. The multicorer takes several tubes of sediment from the seafloor, and works primarily by vacuum. The multicorer is lowered slowly to the bottom, and once the legs come in contact with the bottom, the tension on the cable is reduced, and the slender piston in the center of the corer together with several kilograms of lead weight ensure that the tubes are slowly lowered into the sediment.
The tubes penetrate about 30 or 40 cm into the sediment, then the corer is lifted with the cable, and the tubes are slowly withdrawn. Just as the tubes begin to withdraw from the sediment, the capping devices spring onto the tops of the tubes and seal the sediment sample and overlying water inside the tube. This creates a powerful vacuum which holds the muddy sediment and water in place. When the tubes are completely removed from the sediment, the swing arms place a bottom plate underneath each tube to help keep the sample in place for its journey to the surface.
While the benthic organisms are collected using a van Veen grab.
On retrieval of the grab a subsample of the sampled sediment is taken for sediment texture and granulometry analysis and the balance of the sediment is then washed through a 1mm aperture stainless steel sieve. The benthic macrofauna retained on the sieve is washed into labeled sample bottles, fixed in ~ 4% formaldehyde solution, and transferred to shore laboratories for processing.
2012 Benthic Survey
The 2012 Benthic Survey has been completed.