Abstract
Rising sea water temperature will play a significant role in responses of the world's seagrass meadows to climate change. In this study, we investigated seasonal and latitudinal variation (spanning more than 1,500 km) in seagrass productivity, and the optimum temperatures at which maximum photosynthesis and net productivity (for the leaf and the whole plant) occurs, for three seagrass species (Cymodocea serrulata, Halodule uninervis, and Zostera muelleri). To obtain whole plant net production, photosynthesis, and respiration rates of leaves and the root/rhizome complex were measured using oxygen-sensitive optodes in closed incubation chambers at temperatures ranging from 15 to 43°C. The temperature-dependence of photosynthesis and respiration was fitted to empirical models to obtain maximum metabolic rates and thermal optima. The thermal optimum (Topt) for gross photosynthesis of Z. muelleri, which is more commonly distributed in sub-tropical to temperate regions, was 31°C. The Topt for photosynthesis of the tropical species, H. uninervis and C. serrulata, was considerably higher (35°C on average). This suggests that seagrass species are adapted to water temperature within their distributional range; however, when comparing among latitudes and seasons, thermal optima within a species showed limited acclimation to ambient water temperature (Topt varied by 1°C in C. serrulata and 2°C in H. uninervis, and the variation did not follow changes in ambient water temperature). The Topt for gross photosynthesis were higher than Topt calculated from plant net productivity, which includes above- and below-ground respiration for Z. muelleri (24°C) and H. uninervis (33°C), but remained unchanged at 35°C in C. serrulata. Both estimated plant net productivity and Topt are sensitive to the proportion of below-ground biomass, highlighting the need for consideration of below- to above-ground biomass ratios when applying thermal optima to other meadows. The thermal optimum for plant net productivity was lower than ambient summer water temperature in Z. muelleri, indicating likely contemporary heat stress. In contrast, thermal optima of H. uninervis and C. serrulata exceeded ambient water temperature. This study found limited capacity to acclimate: thus the thermal optima can forewarn of both the present and future vulnerability to ocean warming during periods of elevated water temperature.
Highlights
Rising sea surface temperature caused by global climate change threatens coastal and marine communities around the world
Maximum Pgross(AG) of C. serrulata was lower than H. uninervis and it did not vary among latitudes in summer, but was lower in winter
C. serrulata and H. uninervis have higher thermal optima that did not exceed ambient water temperature at the study sites; they inhabit northern Australia and the Indo-Pacific, including the Coral Triangle (Waycott et al, 2004), where water temperature can approach the thermal optima of H. uninervis and C. serrulata for months, and exceed thermal optima over tidal cycles as outlined further below. These thermal optima of net plant productivity were calculated from temperature-productivity responses of leaf photosynthesis, leaf respiration, and below-ground respiration and each of these processes has a maximum efficiency at a particular optimum temperature, thermal optimum (Topt) (Table 1)
Summary
Rising sea surface temperature caused by global climate change threatens coastal and marine communities around the world. Multi-decadal variation in climate-driven changes in temperature and rainfall were associated with changes in seagrass meadow area and abundance in far northern Australia (Rasheed and Unsworth, 2011). These emerging climate pressures exacerbate the “global crisis” for seagrass ecosystems caused by localized stressors such as water quality (Orth et al, 2006; Waycott et al, 2009) and are predicted to induce functional extinction of some seagrass meadows within the few decades (Jorda et al, 2013). Temperature-induced seagrass loss may compromise the socioecological functions of seagrass meadows: habitat for fisheries species; food for herbivores including dugong, manatees and turtle; shoreline protection; a globally significant carbon stock; and, removal of potential pathogens (Heck et al, 2008; Marsh et al, 2011; Pergent et al, 2014; Dewsbury et al, 2016; Lamb et al, 2017)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
