Assessment of The Distribution, Abundance and Carbon Stocks in Seagrass Meadows Within Eastern and Western Creeks of Gazi Bay, Kenya

Abstract

Seagrass meadows are one of the most important blue carbon ecosystems within the seascape environment providing both ecological and economic benefits. They act as breeding and feeding grounds for fish and other organisms; perform carbon sequestration, nutrient cycling and other ecosystem services. Through their carbon capture and storage ability, seagrass can be incorporated in carbon offset schemes. However, they are highly degraded from both anthropogenic and natural factors. Carbon stocks assessment is required in understanding dynamics of seagrass meadows. In Gazi Bay, Kenya, carbon storage in the seagrass meadows has been determined in the open waters of the Bay. The present study aimed at contributing to carbon dynamics of Gazi bay by assessing the distribution, abundance and carbon storage in seagrass within the mangrove fringed creeks. The objectives included assessing distribution and abundance of seagrass in the creeks, determining above and below ground seagrass biomass and comparing sediment carbon stocks between vegetated and un-vegetated sites. Stratified random sampling strategy was used in collecting data within 80 square plots of 0.25m by 0.25m. Five species formation viz; Thalassia hemprichii, Cymodocea rotundata, Cymodocea serrulata, Enhalus acoroides, and Thalassidendron ciliatum were encountered as either single or mixed stand. A total of 480 samples were collected for sediment and biomass determination in the laboratory. The results showed a higher seagrass diversity in the Eastern creek, (H = 1.71), than Western creek, (H = 1.67). There was also a significant difference in the total biomass between the creeks (t= -8.44, df. = 53, p < 0.0001) and among species (F = 14.6, df = 79, p < 0.0001) with a mean of 7.25 ± 4.2 Mg C ha-1, (range: 4.1 - 12.9 Mg C ha-1). Sediment carbon varied between species within the 1.2 km2 creeks area; with a range from 97.6 to 302.4 Mg C ha-1, (mean: 183.4 ± 100.5 Mg C ha-1). This is lower than 236 ± 24 Mg C ha-1, reported in the open bay but within the global range. In all the species, vegetated areas showed significantly higher carbon values than the un-vegetated sites (t = 12.02 p < 0.0001). Based on this study, the total seagrass carbon stocks can be estimated at 21,118.8 Mg C. Using the IPCCC emission value of 7.9 tonnes of C ha-1, values for organic soils for wetlands, conservation of seagrass in these two mangrove fringed creeks will prevent emission of 2,682.13 Mg of CO2 equivalent yr-1 to the atmosphere. The avoided emission could be bundled with the existing offset scheme in the bay involving mangroves. Inclusion of seagrass in carbon offset scheme has a long term benefits of climate, community livelihood and biodiversity conservation.