Growth, biomass and productivity of the seagrass Thalassia hemprichii at Ashmore Reef, Australia

Ashmore Reef, located approximately 630 km north of Broome, Australia, is notable for its contribution to overall seagrass cover in the Timor Province bioregion, providing valuable ecosystem services including critical habitat for feeding and breeding green turtles (Chelonia mydas) and a small geographically isolated population of dugong (Dugong dugon). Yet, seagrass surveys at Ashmore Reef have not been completed since 2005. Seagrass was surveyed in 2019, with only two (Thalassia hemprichii and Halophila ovalis) of the five previously recorded species observed. Monitoring survey sites were established to determine density, biomass and productivity of the dominant species, T. hemprichii. Overall mean total biomass was 116.9 DW m−2 with an above- and below-ground biomass ratio of 1:2.5. Mean shoot density was 407 shoots m−2 (range 88.9–600 shoots m−2) with a mean canopy height of 42.8 mm (range 10–80 mm), and average blade length of 37.6 mm ( ± 1.1 SE) and blade width of 4.5 mm ( ± 0.2). Mean leaf growth was calculated as 2.2–2.4 mm day−1 and shoot growth 3.2–3.5 mm day−1. Canopy height increased by 5.3–5.7 % day−1 with a turnover rate of 17.5–21.3 days. Mean biomass of emergent leaves was 8 g DW m−2 with a grazing rate of 0.4 g DW m−2 day −1 or 123 kg DW ha−1 month−1. This study suggests that T. hemprichii at Ashmore Reef is productive and may support high turtle grazing rates. •This was the first monitoring study conducted at Ashmore Reef since 2005.•Only two seagrass species were observed (Thalassia hemprichii and Halophila ovalis).•Although considered relatively stunted, T. hemprichii appeared to be productive.•Growth and biomass measurements suggested exposure to high environmental stress.•Grazing rates suggested ability to support high feeding levels by green turtles.