Altered nutrient cycling functionality in seagrass meadows under a simulated future marine heatwave event

Area of Science:

• Marine Ecology
• Biogeochemistry
• Climate Change Impacts

Background:

• Seagrasses enhance marine nutrient cycling and water quality by absorbing nitrogen.
• Marine heatwaves (MHWs) pose a significant threat to these vital ecosystems.
• Understanding MHW impacts on seagrass nutrient cycling is crucial for predicting ecosystem service changes.

Purpose of the Study:

• To investigate the effects of a simulated 30-day marine heatwave (+5°C) on nitrogen cycling and transformation rates in the tropical seagrass Halophila ovalis.
• To assess changes in seagrass physiological responses and sediment microbial functions under heat stress.
• To evaluate the implications for ecosystem services like water quality improvement and ocean acidification buffering.

Main Methods:

• Experimental simulation of a 30-day marine heatwave with a +5°C temperature increase.
• Measurement of nitrogen assimilation, respiration, survival, and growth rates in Halophila ovalis.
• Quantification of sediment denitrification rates and seawater total alkalinity.

Main Results:

• Seagrass leaves showed increased ammonium assimilation and respiration rates under MHW conditions.
• Survival and growth of Halophila ovalis were highly variable under heat stress.
• Sediment denitrification rates decreased significantly, indicating reduced nitrogen removal capacity.
• Seawater total alkalinity declined, suggesting a diminished capacity to buffer ocean acidification.
• No significant difference in N cycling rates was observed between vegetated and unvegetated sediments at this timescale.

Conclusions:

• Simulated marine heatwaves can significantly alter nitrogen cycling in seagrass meadows.
• Reduced denitrification under MHWs diminishes the water quality benefits provided by seagrass.
• Compromised ocean acidification buffering capacity is a further consequence of MHWs on seagrass habitats.
• Future MHWs may lead to substantial reductions in seagrass ecosystem functions and services.