Seasonal variation in standing crop, density and leaf growth rate of the seagrass, Heterozostera tasmanica, in western Port and Port Phillip Bay, Victoria, Australia

Abstract

Standing crop, density and leaf growth rate of Heterozostera tasmanica (Martens ex Aschers.) den Hartog along with light, temperature, nutrient and sediment characteristics were determined monthly for fifteen months at three study sites in Western Port and one site in Port Phillip Bay, Victoria, Australia. Erect vegetative stems of H. tasmanica were frequently branched, were present throughout the year and accounted for 25–60% of the above-sediment biomass, with the stem proportion higher during winter than summer. At three of the four sites there was a unimodal seasonal pattern in which minimum leaf standing crop (27–61 g dry wt. m −2), density (600–2000 leaf cluster m −2) and leaf productivity (0.34–0.77 g dry wt. m −2 day −1) generally occurred during winter (June–August) and maximum leaf standing crop (105–173 g dry wt. m −2), density (2700–5000 leaf cluster m −2) and leaf productivity (2.6–4.2 g dry wt. m −2 day −1) occurred during summer (December–February). A bimodal seasonal pattern with minimum standing crop and density during midsummer occurred at one site. This anomalous seasonal pattern may be due to exposure and desiccation stress during spring low tides. At the site receiving the lowest irradiance, standing crop, density and annual leaf production also were lowest, but length and width of leaves, shoot height and leaf growth rate per leaf cluster were the highest of the four study sites. On average, each leaf cluster at any one of the study sites produced 30–31 leaves per year with mean leaf turnover rates of 1.3–1.7% day −1. Annual leaf production of H. tasmanica ranged from 410 to 640 g dry wt.m −2 at the four sites.