The Role of Seagrass Meadows in Gazi Bay, Kenya as Carbon Sinks

Abstract

Ongoing work on the ‘blue carbon’ has established that vegetated coastal ecosystems – mangroves, seagrasses and tidal marshes – are exceptionally powerful natural carbon sinks. Hence, there are important applied arguments for understanding the role of these ecosystems in climate change mitigation. Many gaps in knowledge remain, with seagrasses in particular being poorly understood. The overall aim of the present study was to understand the role of seagrass meadows of Gazi Bay, Kenya as carbon sinks. The specific objectives of the study were: (i) Carry out a comprehensive review on biomass and productivity studies of seagrasses in Africa. (ii) Estimate the carbon storage in the seagrass meadows of the bay (iii) Investigate the impact of seagrass canopy removal on sediment dynamics and on the benthic faunal communities (iv) Investigate productivity of the dominant seagrass species of the bay. (v) Highlight the implication of the knowledge and how it could be useful in the bundling of the ecosystem services of the seagrass meadows with that of adjacent mangrove ecosystem under the payment for ecosystem services (PES). The review work encountered 32 papers and 8 theses/reports on seagrass biomass and productivity at African sites, with the majority of the studies having been done along the E. African coast; however, there were no published reports of sedimentary Corg from Africa, revealing a major gap in knowledge. Results of carbon storage of the seagrass meadows of Gazi, Bay on biomass and sediment organic carbon (Corg) for the four dominant species, Thalassia hemprichii, Thalasodendron ciliatum, Syringodium isoetifolium and Enhalus acoroides, indicated that sediment organic carbon was highly significantly different between species, range: 160.7 – 233.8 Mg C ha-1 compared to the global range of 115.3 to 829.2 Mg C ha-1 . Vegetated areas in all species had significantly higher sediment Corg compared with un-vegetated areas and revealed a surprising degree of spatial consistency and longevity in relatively small patches of seagrass meadows and bare areas thus demonstrating an exceptionally powerful effect of seagrass on C sequestration. Through a seagrass removal experiment that simulated the impact of seagrass loss on biodiversity and ecosystem functions the study recorded positive elevation change of the sediment in the controls and negative elevation in the treatments with significant effect of treatment and time on surface elevation change. Similarly, there were significant weight losses on clod cards in seagrass removed areas compared to the controls, an indication of the role of seagrasses in the reduction of the speed of water current. Carbon density in surface sediment was significantly higher in the control areas as compared to the treatments while higher litter decay rates were observed in seagrass removed areas than in the controls. Seagrass removal areas had significant decline on the fauna. Productivity estimates of the dominant seagrass species of the bay through a combination of the leaf plastochrone, and ingrowth cores, showed inter-species variability with Thalassia hemprichii recording the highest above and below-ground productivity at 188.6±34.8 g DW m-2 yr-1 and 197.4±108.7 g DW m-2 yr-1 respectively. Knowledge on the role of seagrasses of the bay as carbon sinks is likely to open opportunities for bundling seagrass ecosystem services with that of the contiguous mangrove ecosystem as part of Payment for Ecosystem Services (PES); an approach that makes economic and ecological sense, given the strong connections between the two ecosystems, and could provide a useful buffer against environmental shocks such as sea level rise and increasing storm surges.