Effects of shading on seagrass morphology and thermal optimal of productivity

Abstract

Light and temperature are important factors affecting seagrass primary productivity. Acclimatisation to reduced light availability may affect the optimal temperature at which seagrasses photosynthesise, potentially causing synergistic effects between increasing water temperatures and decreasing light levels on coastal productivity. This study investigated the effects of reduced light availability on the morphology (leaf size, shoot density) and thermal optimal of net productivity in Halophila ovalis (R.Br.) Hook. A 12-week in situ shading experiment was conducted at Chek Jawa Wetlands, Singapore, testing high (68% shading), low (49%) and control (0%) shadings. Every 4 weeks, photosynthetic and respiration rates of H. ovalis leaves and the root-rhizome complex were measured in closed incubation chambers at temperatures from 22 to 42°C (at 4°C intervals). A fitted temperature-response model of net photosynthesis was used to estimate the thermal optimal for each shading treatment. High shading reduced shoot density (mean ± s.e.) 87.06 ± 7.86% and leaf surface area 31.72 ± 24.74%. Net productivity (6 mg O2 g-1 DW h-1) and its thermal optimal (28-30°C) were not significantly different among shading treatments throughout the experiment. Light levels appeared to have minimal influence on the thermal dependency of H. ovalis net productivity.