Interactive temperature and ocean acidification effects on tropical seagrasses

The effects of increasing atmospheric carbon dioxide (CO2) on the marine environment are two-fold. Firstly, increased seawater pCO2 lowers pH (Ocean Acidification, OA) and secondly, water temperature is set to rise. Both OA... [ view full abstract ]

The effects of increasing atmospheric carbon dioxide (CO2) on the marine environment are two-fold. Firstly, increased seawater pCO2 lowers pH (Ocean Acidification, OA) and secondly, water temperature is set to rise. Both OA and rising temperature will affect coastal and reef ecosystems over the course of this century, but combined effects of OA and increases in temperature on seagrasses are not fully understood. Therefore, we tested the interactive effects of OA and temperature on the photoacclimation, metabolism (photosynthesis (P) and respiration (R)), growth and abundance of three tropical species of seagrass (Cymodocea serrulata, Halodule uninervis and Zostera muelleri). Each species was exposed to four temperatures (20°C, 25°C, 30°C and 35°C) and three pCO2 levels (400ppm, 1000ppm, 1600ppm) for seven weeks in a computer-controlled climate-dosing facility. Temperature, in particular, had strong effects on each of the species studied. Photosynthesis-irradiance curves demonstrated photoacclimatory responses to treatments including maximum photosynthetic rate (Pmax) and compensation irradiance (Ic) but responses to temperature and pCO2 varied among species. The metabolic balances of seagrasses (net primary production) were also strongly affected by temperature and OA. Net production and leaf growth increased with temperature from 20-30°C (being 3 times faster at 30°C compared to 20°C for H. uninervis and C. serrulata) but declined sharply at 35°C. Surprisingly, Z. muelleri, a species found in cooler subtropical and temperate water maintained growing shoots at 35°C after seven weeks albeit at reduced density, compared to cooler temperatures. Overall, this study has demonstrated that temperature strongly affects seagrass productivity and growth, and there are interacting effects of pCO2 and temperature.