Light climate and energy flow in the seagrass canopy of Amphibolis griffithii (J.M. Black) den Hartog
Thirty 3 m diameter plots were established in Warnbro Sound, Western Australia in early 1991, within an Amphibolis griffithii meadow, and the shoot densities manipulated to 0, 25, 50, 75 and 100% (control) by pruning. These experimental manipulations were monitored and maintained on a monthly basis until March 1993. The effects of these density changes on sediments were investigated.
Extensive seagrass meadows of the species Amphibolis griffithii at a depth of 3 m were chosen in Warnbro Sound, south of Perth. - Treatments - Dense stands were selected and nominated as "100%" cover. Densities were reduced to give the following shoot densities: 75% 50%, 25% 0%, relative to the unmanipulated controls. These were checked monthly and if necessary, adjusted to retain the required treatment density. All treatments were significantly different from each other over the time period of the experiment. - Experimental units - 3 m diameter (7.13 m2 area) plots were pegged out (a central star picket) within a uniform area of seagrass, i.e. with similar depth, wave exposure, and seagrass density throughout. Six replicates were established for each density, randomly within the area, making a total of 5 (densities) x 6 (replicates) = 30 (3 m diameter) plots. Samples were collected from experimental plots quarterly. Four cores of 10 cm depth and 4.5 cm diameter were taken randomly from each plot and pooled into a plastic bag. On returning to the lab, each pooled sample was mixed to homogenise the core samples. Approximately 20 g of sediment was then wet sieved through 2 mm and 63 um sieves. The 2 mm sieve removed the larger organic material and shells which have non-uniform shapes and thus non-standard settling rates; while the 63 um sieve removed the finest sediments, considered too small to be measured accurately, and forming <1% of the total dry weight of a sediment sample. The sediment fraction collected in the 63 um sieve was then dried at 75 degrees Celsius and ashed at 550 degrees Celsius for 3-4 hours to remove all remaining organic matter. After ashing, the samples were dry sieved again through a 63 um sieve to remove any ash. Samples weighing approximately 3.0 g were then dropped through a 2 m setting tube, and the settling rates calculated, using an IBM computer program. Sediment heights within the plots were assessed by placing 2 stakes (30 cm of 10 mm diameter steel, which were later replaced with PVC) into each plot, 1 m south-west and north-east from the central starpicket in each plot. These were then measured monthly to record whether variations in each plot were related to seagrass shoot density. To assess whether there was a strong 'rippling' effect at scales less than 20 cm within a mature A. griffithii bed, small scale variation in sediment height was assessed by placing out a 5 m and then a 30 m transect and measuring the sediment height at 10 and 50 cm intervals; respectively. Using a measuring tape and compass, two star pickets were placed in the seagrass bed at a distance of approximately 5 m to form a NE to SW transect. This was assumed to be the direction of dominant swell influencing the bay and hence the direction of sediment processes. Stakes were then placed into the sediment at approximately 10 cm intervals along the transect and differences between each successive stake were calculated using trigonometry.