Optimum Temperatures for Net Primary Productivity of Three Tropical Seagrass Species

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 (T_opt) for gross photosynthesis of Z. muelleri, which is more commonly distributed in sub-tropical to temperate regions, was 31°C. The T_opt 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 (T_opt 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 T_opt for gross photosynthesis were higher than T_opt 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 T_opt 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.

Keywords: Cymodocea serrulata; Halodule uninervis; Zostera muelleri; climate change; net primary productivity; sea temperature; thermal stress; tropical seagrass.

Figure 1

Photosynthesis and respiration were measured in the laboratory from seagrasses collected from Green Island in the northern Great Barrier Reef and from Moreton Bay. Seagrass distribution is shown in green from McKenzie et al. (2014) and Roelfsema et al. (2014).

Figure 2

Expected temperature-dependence of gross photosynthesis and respiration rates for seagrass. Biological rates with this temperature-dependence can be fitted to the empirical model shown in Equation (1) to obtain maximum metabolic rates (P_max), thermal optima (T_opt), and maximum temperature (T_max).

Figure 3

Gross photosynthesis (P_gross(AG), Top), and respiration in above-ground tissues (R_AG) (Middle) and below-ground tissue (R_BG) (Bottom) at water temperatures ranging from 15 to 43°C, for the seagrass species H. uninervis, C. serrulata, and Z. muelleri. These were measured at Green Island in summer and Moreton Bay in summer and winter. Data points shown for photosynthesis and net respiration (n = 6). Modeled fits for photosynthesis, and above-ground respiration and below-ground respiration, are shown by colored lines, and associated shaded error bounds indicate 95% CI in the model fits. Note the different scales on the y axes.

Figure 4

Net productivity of above-ground (P_net(AG), Top) and above and below-ground tissues together (P_net(AG+BG), based on mean below-ground to above-ground biomass ratio, Bottom) of Halodule uninervis, Cymodocea serrulata, and Zostera muelleri. These were measured at Green Island in summer and Moreton Bay in summer and winter. Ribbons are 95% CIs.

Figure 5

P_max (Left) and T_opt (Right) calculated from the relationship between P_net(AG+BG) and temperature using a range of below-ground to above-ground biomass ratios measured in situ (Table S3). Ribbons are 95% CIs.

Figure 6

Modeled net plant productivity of the above-ground and below-ground tissues together (P_net(AG+BG)) vs. temperature for Z. muelleri. P_net(AG+BG) was calculated from P_gross(AG), R_AG, and R_BG for three different below-ground to above-ground biomass ratios: the minimum (Left), mean (Middle), and maximum (Right) below-ground to above-ground biomass ratio measured for Z. muelleri at the Moreton Bay site in summer. Ribbons are ±SD.