## 2 Material and Methods
### Experimental Design and Setup  
We conducted a fully crossed two-factorial experiment (Figure 1) to study the effects of temperature, CO\({}_{2}\), and their interaction on the trait responses of _E. acoroides_ seedlings. We ran the experiment using an indoor flow-through system with 24 aquaria (10 \(\times\) 29 \(\times\) 50 cm) in a temperature-controlled room set at 26 \({}^{\circ}\)C (MAREE, ZMT, Bremen, Germany). In each aquarium, three seagrass seedlings were carefully planted by hand. Each seedling had leaves and roots fully developed with an average (\(\pm\)standard error, SE) of 1.93 \(\pm\) 0.06 cm in leaf length, 6.27 \(\pm\) 0.22 cm of root length, 1.10 \(\pm\) 0.02 cm of seed diameter, and 0.9-5.5 g fresh weight (FW). Each aquarium was previously filled with pre-cleaned silicate sediments (-7 cm depth) and 10 L of natural seawater (NSW, North Sea). The NSW was pre-filtered before storage to remove plankton and avoid organic matter remineralization. A pump was placed in each aquarium to ensure the water aeration and  
Figure 1: Experimental setup in this study. Experimental tanks (ETs) were used as a water bath (white box) set at either regimes of 27 or 31 \({}^{\circ}\)C. Twenty-four small clear glass aquariums (striped box) were placed inside the water baths under the selected temperatures and were independently supplied with seawater either enriched with CO\({}_{2}\) or at ambient CO\({}_{2}\) using a flow-through system attached to a peristaltic pump (black oval). Water was pre-conditioned with CO\({}_{2}\) from water reservoirs (striped or dotted circles) of the desired CO\({}_{2}\) concentrations. In addition, CO\({}_{2}\) bubbling was also added directly in the small aquariums to help maintain CO\({}_{2}\) treatments within the ETs. homogeneous mixing by moving water from the bottom to the top, reducing the diffusive boundary layer. We applied two levels of temperature: low and high (27 and 31 \({}^{\circ}\)C) and two levels of CO\({}_{2}\) concentrations (ambient, 400 \(\upmu\)atm, and enriched, 700 \(\upmu\)atm) and all the possible combinations, yielding in four different treatments: low temperature and ambient CO\({}_{2}\) concentrations, low temperature and enriched CO\({}_{2}\) concentrations, high temperature and ambient CO\({}_{2}\) concentrations, and high temperature and enriched CO\({}_{2}\) concentrations. Each of the 24 aquaria were independent and served as replicates (n = 6) of the four treatments. Experimental temperature levels were selected based on lower and higher average temperatures in seagrass meadows in the region. The levels of CO\({}_{2}\) concentrations were selected according to scenarios forecasted by the Intergovernmental Panel on Climate Change (IPCC). Therefore, the selection of experimental temperature and CO\({}_{2}\) concentration levels was based on obtaining realistic future trait responses rather than to obtain stress-related responses. The experimental temperatures were obtained by placing aquaria in eight experimental tanks (ET) (42 \(\times\) 87 \(\times\) 58 cm) of 250 L (three aquaria in each ET), which served as water baths maintaining a constant water temperature. The water temperature was continuously controlled in each ET using heaters (EHEIM GmbH, Germany) connected to a digital controller and probe (\(\pm\)0.2 \({}^{\circ}\)C). A pump was also placed in each ET to ensure that the water was mixed to keep the temperature constant. Each of the two CO\({}_{2}\) concentrations, termed ambient and enriched, were applied to two 115-I water reservoirs. We achieved the desired concentration in each water reservoir by constantly bubbling NSW with CO\({}_{2}\) from the gas cylinder's main tank. Each water reservoir was used to provide an equal and constant water flow to aquaria at a rate of \(\sim\)4 mL min\({}^{-1}\) by using a 24-channel peristaltic pump (ISMATEC, Cole-Parmer GmbH, Germany) ensuring total water renovation inside each aquarium every \(\sim\)1.5 days. Therefore, each water reservoir pumped NSW to half of the total aquaria.