Seagrass, sediment and infauna - a comparison of Posidonia australis, Posidonia sinuosa and Amphibolis antarctica, Princess Royal Harbour, South-Western Australia I. Seagrass biomass, productivity and contribution to sediments

Brief description

The above- and below-ground biomass of three seagrass species, Posidonia australis, Posidonia sinuosa and Amphibolis antarctica, were measured within Princess Royal Harbour. Three sites were used of varying water movement and eutrophication. Sediment samples (to measure grain size and composition) were taken from below seagasses and from adjacent non-vegetated areas for comparsion and productivity measurements were carried out in situ.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Sites were chosen where monospecific seagrass meadows >2 m diameter could be sampled at similar depths (1-2 m). In order of increasing distance from the entrance to Princess Royal Harbour, and hence decreasing water movement and increasing eutrophication, the sampling locations were Bramble Point, Quaranup Jetty and the Yacht Club (see thumbnail). Samples for all measurements were taken from the same area, within a radius of 50 m. Non-vegetated sediment samples were taken from >2 m diameter sand patches within seagrass meadows except at Bramble Point, where seagrass cover was 100% and samples were taken just inshore of the seagrass. A 0.1 m2 circular metal quadrat with serrated "teeth" was placed non-selectively within the seagrass meadow, and forced into the substratum to a depth of 10 cm, cutting through the rhizome mat with minimum disturbance. The above-ground plant material was clipped and collected for later exaamination of epifauna, and then the quadrat excavated using an air-lift, with animals >1 mm trapped by a mesh bag (Well et. al., 1985). Six samples were taken for each species/site combination. encrusting bryozoans, hydroids, sponges or colonial ascidians were not quantified. Seagrass below-ground biomass samples were extracted from the material retianed on a 1 mm mesh. The method of sampling allowed full extracted of below-ground material, and so dead rhizome mat was also separated. This was compressed and the volume measured by displacement, so that the volume of the quadrat occupied by the rhizome mat could be calculated, indicating how much space below the seagrass canopy was occupied by fibre and therefore unavailable for fauna. Above ground seagrass biomass was sampled separately as clipping leaves led to losses. six 20x20 com quadrats samples were taken for each site/species, sorted and measured, scraped of epiphytes, dried to constant weight (24 hours at 110 degrees Celcius) and then ashed at 550 degrees Celcius for one hour to give ash-free dry weights (organic weight), and then at 1000 degrees Celcius for 2 hours to determine calcium carbonate content. Productivity measurements were carried out in situ within a two week period, offset by one day for each site. Amphibolis leaf production was measured using the technique of Walker (1985), and Posidonia spp. using a modified Zieman (1975) leaf marking technique (Walker and McComb, 1998). Sediment samples were taken in perspex 5 cm diameter cores to a depth of 10 cm, from below seagrasses and from adjacent non-vegetated areas for comparison. These sediments were sub-sampled and a sample of 100 g dry weight, wet sieved through 2.0, 1.0, 0.5, 0.25, 0.125 and 0.063 mm sieves. The size fractions were then dried to constant weight (110 degrees Celcius for 24 hours), weighed and ground. Known weights of each size fraction were ashed and weighed to determine organic content (550 degrees Celcius for 1 hour) and calcuim carbonate content (1000 degree Celcius for 2 hours). Sediment production by seagrass leaves and their epiphytes was calculated by multiplying the "calcimass" (i.e. total weight of calcium carbonate on seagrass leaves in 1 m2) by the turnover, derived from production measurements.