Ecological studies on seagrasses of SW Australia - Cockburn Sound - Field transplants of seagrass

Brief description

Two field experiments were carried out on seagrass in the Cockburn Sound area to assess the effect of water quality on seagrass growth. Leaf area was measured before and after transplanting.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: The first field experiment was carried out in summer 1977-78 (October - March) and the second in 1978 (November - December). These will be referred to as Experiments1 and 2 respectively. * FIELD EXPERIMENT1 * This investigation was undertaken to assess the suitability of water quality for seagrass growth along the eastern shores of Cockburn Sound following extensive dieback of seagrass after 1970. Three trial stations in Cockburn Sound along the eastern shore of Cockburn Sound, (1) offshore from Alcoa, (2) north of James Point and BP oil refinery, (3) CBH jetty, and one reference station at Mersey Point in Warnbro Sound, close to the seedling collection site, were established in 2-3 cm water depth (see thumbnail for locations, shown as open circles). Seedlings aged about ten months from the same seed crops of P. australis and P. sinuosa were collected from Mersey Point, Warnbro Sound during 26-28 October 1977. A solid-walled plastic tube, 20 cm in length and 6 cm in diameter, was pushed around each seedling. The base of each tube was capped with a plastic cap before it was removed to reduce root disturbance. Seedlings of species were transplanted to each of the four stations which were marked by steel pickets and subsurface floats. 9 or 10 seedlings of each species were transplanted at each site. The seedling leaf areas were measured before transplanting and the plants checked each month during the five month trial. Leaf area measurements were taken again at the end of the trial. Severe grazing of leaves by herbivores compromised the results of this first trial. The enclosed nature of the plastic tubes sealed with a tightly fitting plastic cap resulted in blackened roots, indicating that the sediment had become anoxic during the trial. * FIELD TRANSPLANT 2 * The second series of transplant trials had the same aim as the first trial, to assess the suitability of water quality for seagrass growth along the eastern shores of Cockburn Sound following extensive dieback of seagrass after 1970. The two locations chosen for the transplant stations (Cockburn and Warnbro Sound) are shown in the thumbnail as closed triangles. The second trial was limited to P. sinuosa, which was originally the dominant species in Cockburn Sound. 80 two-to-three-year-old seedlings of P. sinuosa were used. The technique was modified by enclosing the experimental plants in cages to prevent grazing of the leaves by herbivores. The plastic planter tubes were also larger, 30 cm long and 8 cm in diameter, and the bases were covered by terylene gauze held in place by a rubber band, allowing better aeration of the sediment. An air-lift was used to remove sediment from around the roots of each plant and any with a stunted, blackened root system were discarded. Each healthy seedling was then placed underwater (using local sediment to cover the roots) into a plastic tube described above. Care was taken to select comparable plants with an average of two to three shoots per plant, and comparable leaf area. Twenty plants were selected, replanted into tubes, measured for leaf area and the tubes resited on the same day. This operation was repeated on each of the four days giving a total of 40 plants transplanted to the Warnbro control station and 40 to the Kwinana station. At each station four tubes were placed in the centre of each of five protective cages, staged in a line at 5 m intervals. The 20 uncaged plants were placed along a line parallel to the cages, also in groups of four, each marked by a tagged picket. Each plant could be identified by a number and the sites were marked with both subsurface and floating markers. The cages were designed to prevent grazing of the leaves, which had occurred during Transplant 1. Each cage consisted of a frame 50 cm square and 35 cm deep. The sides were covered with 0.5 cm plastic mesh and the hinged lid with 1 cm chicken wire. The open base was placed over the plants and the sides sunk 5 cm deep into sediment and pegged down with 0.5 m length steel pins. The size of the cage was designed to minimise breaking of leaf ends owing to contact with the sides of the cage. The mesh size was chosen to prevent entry through the sides of the cages of sea urchins more than 0.5 in size. The leaf lengths and widths were measured once a week. The cages at the Warnbro Sound station were brushed clean of marine growth each week whereas those at the Kwinana station required cleaning twice a week. Irradiance measurements were made on three occasions inside and outside cleaned and uncleaned cages using a LI-COR 185 photometer. The general condition of the plants and the extent of epiphytic growth on the leaves were noted each week. Six weeks after transplanting and following the final measurement of leaf area, each plant was extracted from its core to examine the health of the root system. The epiphytic growth was gently scraped from the leaves and a 100 g sample of leaf tissue from each of the two stations was kept for analysis of hydrocarbons.

Notes

Credit
Department of Conservation and Environment, Cockburn Sound Study Group