A Halodule-dominated community in a subtropical embayment: physical environment, productivity, biomass, and impact of dugong grazing

Physical variables, standing crop and dugong activity were monitored over 14 months in a subtidal community dominated by the tropical seagrass Halodule uninervis and the green alga Penicillus nodulosus in a small cove in subtropical Shark Bay, Western Australia. Water temperature ranged from 14.5 (June) to 30.5°C (February), salinity from 48 (August) to 62‰ (March). Attenuation coefficients were 0.18–0.32 m −1 in February and 0.10 m −1 in June. Mean daily PAR at the sea bed was 200 μmol m −2 s −1 in September and 400 μmol m −2 s −1 in January. Visually, Halodule appeared dominant, but Penicillus biomass exceeded Halodule biomass by ≈3% on ridges and averaged ≈ nine times higher in gullies. Total Halodule biomass on ridges was 46.5 g m −2 in May and 69.8 g m −2 in March, rhizome biomass (40–65 g dry weight m −2) was four to six times leaf biomass (7–16 g dry weight m −2). Productivity on the ridges, measured over 4–6 week intervals was 0.12 g dry weight m −2 per day in August–October and 1.56 g dry weight m −2 per day in March–April. Growth persisted throughout the year, and was not limited to temperatures of ≥21°C. Productivity was estimated as 295 g dry weight m −2 per year. The plastochrone interval for rhizomes (PIR) was 12.3 days in May and June and 5.9 days in February. The temporal peak in productivity did not coincide with peak insolation, but did coincide with high temperatures and high and continuous dugong activity. Statistical analysis indicated that light and temperature influenced leaf productivity more than they did rhizome, root, or total productivity. Dugongs ( Dugong dugon) rooted in the community from January through April, raising levels of suspended sediments and attenuation coefficients and reducing PAR. Halodule biomass in dugong exclosures at the end of the grazing season was 1.8 times that in adjacent unprotected areas. Dugongs departed when autumn temperatures fell below 19°C. During the grazing season loss of biomass resulting from dugong activity exceeded 50% of production.