Primary productivity of Zostera muelleri Irmisch ex Aschers. In Westernport Bay (Victoria, Australia)

Abstract

From simultaneous measurements of photosynthetically active radiation (PAR) and gas exchange under both exposed and submerged conditions in situ in Westernport Bay it is shown that the response of photosynthesis by Zostera muelleri ex Aschers. to light can be approximated by a Blackman limiting response curve. The maximum rate of net photosynthesis under submerged conditions is ca. 230 μg CO 2 m −2 leaf area s −1, this rate being reduced by about one-third on exposure. Under submerged conditions the light compensation point corresponds to a photon flux density of 25 μmol m −2s −1, and photosynthesis becomes light-saturated at a photon flux density of about 120 μmol m −2s −1. Estimates of net primary production for each month of the year were calculated from the light response curve and solar radiation records. Photosynthesis is shown to be light saturated for the greater part of each day, and the seasonal variation in net primary production is therefore influenced strongly by day length. Annual net primary production by Z. muelleri amounts to 4.3 g dry matter g −1 dry wt. of plant (17.2 g dry matter g −1 dry wt. of leaf), indicating a turnover of about four times per year. For the whole of Westernport Bay, annual net primary production by Z. muelleri, which covers an area of 38 km 2, is estimated to be 16.3 × 10 9 g dry wt. Taking into account primary production by other species of seagrass and benthic algae, which together cover an area of 181 km 2, it is evident that Westernport Bay is indeed a highly productive ecosystem.