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
50 cm. 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 sediment elevation, harvested
plots were eroded (7.6 ± 0.4 and −15.8 ± 0.5mm yr−1 respectively, mean ± 95%
CI). Carbon concentration in the surface sediments was significantly reduced with a
mean carbon loss of 2.21Mg C ha−1 in the top 5 cm. Because sediment was lost from
harvested plots, with a mean difference in elevation of 3 cm, an additional carbon loss of
up to 2.54Mg 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 toward 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 led to rapid
changes in sediment dynamics and chemistry driven in part by significant alterations in
the faunal community.
