Abstract
Seagrass habitats are important natural carbon sinks, with an average of ~14 kg C m−2 buried in their sediments. The fate of this carbon following seagrass removal or damage has major environmental implications but is poorly understood. Using a removal experiment lasting 18 months at Gazi Bay, Kenya, we investigated the impacts of seagrass loss on sediment topography, hydrodynamics, faunal community structure and carbon dynamics. Sediment pins were used to monitor surface elevation. The effects of seagrass removal on water velocity was investigated using Plaster of Paris dissolution. Sediment carbon concentration was measured at the surface and down to 50cm. Rates of litter decay at three depths in harvested and control treatments were measured using litter bags. Drop samples, cores, and visual counts of faunal mounds and burrows were used to monitor the impact of seagrass removal on the epifaunal and infaunal communities. Whilst control plots showed elevation, harvested plots were eroded (7.6 ± 0.4 and -15.8 ± 0.5 mm yr-1 respectively, mean ± 95% C.I). Carbon concentration in the surface sediments was significantly reduced with a mean carbon loss of 1.13 Mg C ha-1 in the top 5cm. Because sediment was lost from harvested plots, with a mean difference in elevation of 3cm, an additional carbon loss of up to 22.9 ± 2.4 Mg C ha-1 may have occurred over the 18 months. Seagrass removal had rapid and dramatic impacts on infauna and epifauna. There was a loss of diversity in harvested plots and a shift towards larger bodied, bioturbating species, with a significant increase in mounds and burrows. Buried seagrass litter decomposed significantly faster in the harvested compared with the control plots. Loss of seagrass therefore lead to rapid changes in sediment dynamics and chemistry driven in part by significant alterations in the faunal community.
Highlights
Seagrass beds are critical marine habitats with a wide global distribution
The current study was confined to areas of monospecific stands of Thalassia hemprichii and Enhalus acoroides since these seagrass species are most abundant in the accessible intertidal regions
The mean (± 95% CI) C densities in the top 5 cm of sediment were 0.0085 (± 0.0027) and 0.004 (± 0.0005) gC/cm3, translating to a mean difference between treatments of 2.21 Mg C ha−1 in the top 5 cm. This difference was limited to the surface, with no significant differences found over the whole depth profiles
Summary
Seagrass beds are critical marine habitats with a wide global distribution. Their dense canopies and organically enriched sediment are habitat and refuge for a large community of resident and transient fauna including commercially important fish species, crustaceans and molluscs (Howard et al, 2014). Seagrasses are estimated to have the greatest spatial extent of the three blue carbon ecosystems (ranging between 164,000 and 500,000 km, with a best approximation of 177,000 km−2; Green and Short, 2003) They probably suffer the fastest rates of destruction; one estimate of 7% area lost yr−1 may be the worst trend for any global habitat (Waycott et al, 2009). In their study of the global impacts and costs of carbon (C) emissions from the degradation of blue carbon habitats, Pendleton et al (2012) assumed that between 25 and 100% of C in the top meter of sediment or soil is oxidized following habitat destruction This large range, and the concomitant uncertainty in climate change impacts, underlines the need for further research on the temporal dynamics of sediment C following habitat loss
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