Temperature responses of two sympatric seagrasses, Amphibolis antarctica and Amphibolis griffithii, in relation to their biogeography

Brief description

Seedlings of the viviparous seagrasses, Amphibolis antarctica (Labill.) Sonder & Aschers. and Amphibolis griffithii (Black) den Hartog, were collected from near Dongara in 1990 and grown in seawater cultures at temperatures of 10-30 degrees Celsius. This was to test whether any differences in temperature responses (survival and growth) existed between these two species under controlled conditions, and if so, whether these could account for the observed differences in distribution.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Seedlings were collected on 30 July 1990 from the reef at Cliff Head, near Dongara, Western Australia(29 18'S, 114 53'E), approaching the northern limit of the distribution of A. grzsthii. They were transported in a large bin of seawater and kept aerated until the experiment was established, within 48 hours of collection. Growth rooms were maintained at 10, 15, 20, 25 and 30 degrees Celsius and maximum-minimum thermometers used to check on variations in air and water temperature. Air temperatures varied by a maximum of +-2 degrees Celsius weekly, and more usually +-1 degree Celsius, resulting in a < 1 degree Celsius change in water temperature. Twenty seedlings of each species were grown at each temperature. At the beginning of the experiment, each seedling was wet weighed, and the number of leaves counted. Two seedlings were placed in a pot with acid-washed quartz sand and labelled. Five pots were placed in each of two aquaria, filled with 25 litres of filtered seawater, with airstones to provide aeration and circulation. No nutrients were added to the water as previous experiments have shown that serious epiphyte growth can occur. The seawater was changed completely every week for the 4 months of the experiment. The seedlings were grown under 12:12 light:dark cycle at a light intensity of 140 uE m-2 s-1. Light intensities varied slightly between temperatures (+-10 uE m-2 s-1), but were always above saturation irradiances for the species. Seedlings were removed from their pots every week and the leaf number counted. Observations were also made on the condition of the leaves and whether any new shoots or roots had formed at the base of the seedling. Any leaves that had been shed were 'attributed' to their parent seedling, removed and dried to constant weight. Seedlings were assessed as ?dead? when either all leaves had been shed, or when all leaves were fully blackened (Biebl & McRoy, 197 1) and no new leaves were emerging at the centre of the leaf cluster. Any seedlings that had died were removed and dried and weighed. At higher temperatures (>25 degrees Celsius), seedlings were prone to attack by a fungal pathogen, probably an Oomycete of the genus Pythium. To eliminate this problem, seedlings were dipped in a 2% solution of a proprietary antiseptic (Milton) containing sodium hypochlorite and sodium chloride, which has been used effectively as a surface steriliser for macroalgae (Kirst, pers comm., 1988). The solution was made up with sterile seawater and the seedlings rinsed in sterile seawater before returning to the aquaria. All seedlings were treated the same way, whether or not fungal activity had been noted. This restrained fungal activity, but did not affect the high rates of mortality at higher temperatures. Survival of seedlings was calculated by plotting the % remaining alive at each time interval. The percentage survival was calculated for each temperature. Regression values were calculated for absolute seedling survival against time along the linear part of the curve (Zar, 1980). Data for leaf production per surviving seedling were checked for normality, and found to be not normally distributed. They were then transformed but this and other transformations still did not achieve normality and so a non-parametric range analysis was carried out (Zar, 1980).