The standing crop (weight of above-ground material) of seagrass at different locations from Cockburn Sound was measured in December 1977.
Standing crops were measured at 21 sites (see thumbnail) in 1977 on the three remaining extensive meadows of seagrass, from a few deteriorating stands in Cockburn Sound, and from a nearby embayment, Warnbro Sound, which had no industry but was in a residential area. The areas were chosen from aerial photographs. Harvests were made in early December, near the time of maximum standing crop.
The day before sampling, divers set a steel stake in the substrate to mark the corner of two sampling transects. To sample the transects a ten metre rope was attached to the central stake and pegged to run due north. A wire frame (quadrat) enclosing an area of 0.33m2 was placed at six, regularly spaced intervals marked on the rope. The seagrass in each quadrat was harvested by cutting level with the sand surface and then placed in labelled plastic bags, N1-N6.
When harvesting a mixed seagrass community no attempt was made to separate species. Mussels was attached to seagrass, were included in the harvest and when no seagrass was present within the quadrat the labelled bag was closed and returned. When the northern transect was harvested, the rope was swung to run due west and the harvesting repeated for another six quadrats, W1-W6. Bags were sealed to prevent drying, but slits in the bags allowed water to drain away.
At a field laboratory, the seagrass in each bag was sorted into species, extraneous material such as mussels and heavy growths of epiphytes were removed and the seagrass wet weighed.
The contents of two bags from each station were retained for decalcification in dilute hydrochloric acid (0.1M) and dried to allow a wet to dry weight conversion. Wet weights of Amphibolis spp. included stems and epibiota.
Department of Conservation and Environment, Cockburn Sound Study Group