Effects of shading and simulated grazing on carbon sequestration in a tropical seagrass meadow

Abstract

Summary 1.There is an ongoing world-wide decline of seagrass ecosystems, one of the world's most efficient carbon sink habitats. In spite of this, there is a clear lack of studies experimentally testing the effects of anthropogenic disturbances on carbon sequestration of seagrass systems. 2. We assessed the effects of two disturbances of global concern on the carbon sink function in a five-month in situ experiment within a tropical seagrass (Thalassia hemprichii) meadow by testing the impacts of shading and simulated grazing at two levels of intensity using shading cloths and clipping of shoot tissue. We measured the effects of these disturbances on the carbon sequestration process by assessing the net community production (NCP), carbon and nitrogen content in tissue biomass, and organic matter and THAA (total hydrolysable amino acids) in the sediment down to 40 cm depth. 3. Treatments of high-intensity shading and high-intensity clipping were similarly impacted and showed a significantly lower NCP and carbon content in the below-ground biomass compared to the seagrass control. No significant effects were seen in organic carbon, total nitrogen, C:N ratio and THAA in the sediment for the seagrass treatments. However, both clipping treatments showed different depth profiles of carbon and THAA compared to the seagrass control, with lower organic carbon and THAA content in the surface sediment. This can be explained by the clipping of shoot tissue causing a less efficient trapping of allochthonous carbon and reduced input of shredded seagrass leaves to the detritus sediment layer. In the clipping plots, erosion of the surface sediment occurred, which was also most likely caused by the removal of above-ground plant biomass. 4. Synthesis. Our findings show that during the course of this experiment, there were no impacts on the sedimentary carbon while the high-intensity disturbances caused a clear depletion of carbon biomass and reduced the seagrass meadow's capacity to sequester carbon. From a long-term perspective, the observed effect on the carbon biomass pool in the high-intensity treatments and the sediment erosion in the clipping plots may lead to loss in sedimentary carbon.

Sediment Properties as Important Predictors of Carbon Storage in Zostera marina Meadows: A Comparison of Four European Areas

“…The continuous loss of seagrass areas [6] leads to a decline in natural carbon sinks [74,75], and to ensure efficient management, factors for high carbon storage capacity should be evaluated. Several environmental and seagrass-related factors have shown to be of importance, i.e.…”

High midday temperature stress has stronger effects on biomass than on photosynthesis: A mesocosm experiment on four tropical seagrass species

“…At a larger scale perspective, the loss of seagrass biomass from coastal waters would affect a range of ecosystem services (Cullen‐Unsworth et al., ; Orth et al., ) such as sediment stabilization (Newell & Koch, ), the nursery habitat function (Heck, Hays, & Orth, ), and fisheries productivity (Nordlund, Unsworth, Gullström, & Cullen‐Unsworth, ). As seagrass beds are considered a major sink for atmospheric CO 2 (Kennedy & Björk, ), such loss of seagrass cover would decrease the carbon sequestration capacity of coastal seas (Dahl et al., ; Deyanova et al., ), eventually resulting in a decrease in the long‐term carbon storage (Mcleod et al., ). Furthermore, a loss and decay of biomass, especially the belowground parts, has also been suggested to promote the production of sulfide and methane gas emissions from the affected meadow (Lyimo et al., ).…”

Seagrass Removal Leads to Rapid Changes in Fauna and Loss of Carbon

“…Macreadie et al (2015) provide an example from Australia, where seismic testing in the 1960s damaged a series of Posidonia australis beds, leading to a loss of 72% of sediment C. Such "natural experiments" have the advantages of temporal and spatial scale, but do not allow the controlled understanding of causal drivers that a manipulative experiment permits. There are few experimental studies that explore the impacts of seagrass loss on C; the two most relevant (Macreadie et al, 2014;Dahl et al, 2016) found no C losses following small scale disturbances, which is in contrast to the findings of the natural experiment in Macreadie et al (2015). This suggests that disturbance may need to be large scale (in time and/or space) before an effect is found, or that there are site-specific factors not yet understood.…”