Carbon cycling in seagrass beds: anthropogenic and natural drivers

Mona A. Andskog

doi:10.25918/thesis.584

Seagrass beds are one of the most productive ecosystems in the world, with a net primary productivity of 6.7 ± 0.6 t C ha−1 yr−1. This production is vital for their role as ecosystem service providers, including carbon cycling and carbon sequestration. However, their capacity to function as carbon sinks depends on the complex interaction between plant performance, microbial metabolism, and environmental variability. The location of seagrass beds in a highly dynamic coastal environment means that they are particularly vulnerable to natural and anthropogenic changes that have the potential to impact the cycling and storage of carbon. Understanding how natural and anthropogenic drivers shape these processes is important for accounting for the role of seagrass beds in global carbon cycles. This thesis explores the mechanisms underlying carbon uptake, allocation, loss and storage in seagrass beds, with a particular focus on how anthropogenic nutrient enrichment and climatic disturbances alter carbon cycling pathways. Stable isotope labelling (13C) was used to trace carbon uptake, and the transfer of newly fixed carbon into seagrass leaves, roots, epiphytes, sediment and dissolved carbon pools. In Chapter 2, I quantified carbon cycling through a seagrass bed over approximately 5 weeks. Carbon cycling was compared in seagrass beds that were receiving a moderate nutrient treatment of nitrogen and phosphorus (~2 months) to beds under ambient conditions. Moderate nutrient enrichment stimulated aboveground carbon uptake, at the cost of belowground transfer, and the additional carbon taken up was exported rather than staying in situ. Chapter 3 explored pathways of methane (CH4) production, i.e. methanogenesis. Methane is an important greenhouse gas, and its production is hypothesised to be tightly linked to seagrass primary production. Using stable isotope labelling with 13C-enriched dissolved inorganic carbon (13C-DIC), I identified CH4 production in seagrass beds in the absence of primary production. Actually, primary production was found to suppress methanogenesis by slowing the transformation rate of 13C-DIC to 13C-CH4, highlighting natural ecosystem-level controls on methanogenesis. Chapter 4 examined the effects of a natural extreme weather event, Cyclone Alfred, on seagrass carbon cycling. While the cyclone removed epiphytes and increased seagrass primary production, it also severely disturbed the sediment, increasing CH4 emissions and causing substantial losses of sediment organic carbon. This led to a reduced capacity for this seagrass bed to act as a blue carbon sink. Together, these studies show that carbon cycling responds rapidly to external stressors and drivers. Transient nutrient enrichment and extreme weather have pathway-specific effects, stimulating primary productivity while simultaneously driving carbon loss through export, sediment disturbance, and greenhouse gas emissions. However, high seagrass productivity also plays a key role in limiting further carbon loss by slowing down greenhouse gas release. Given the rapid changes in coastal environments, with increasing magnitude of extreme events and more widespread nutrient input, highlights the need to include greenhouse gas emissions, carbon export pathways, episodic carbon loss and future climate scenarios into blue carbon accounting. While seagrasses remain robust primary producers and resilient inhabitants of highly dynamic environments, this thesis highlights that their climate-mitigation potential is highly complex and variable, and sensitive to environmental change.

Carbon cycling in seagrass beds: anthropogenic and natural drivers

Files and links (1)

Metrics

Abstract

Details

Carbon cycling in seagrass beds: anthropogenic and natural drivers

Files and links (1)

Metrics

Abstract

Details

Southern Cross University Social media