Species Composition, Distribution, and Organic Carbon Stocks in the Subtidal Seagrass Meadows of Gazi Bay, Kenya

Abstract

Seagrass beds occur in both intertidal and subtidal zones within shallow marine environments such as bays and estuaries globally. This important ecosystem support fisheries production, attenuate strong wave energies, human livelihoods and sequester large amounts of CO2 that may help mitigate the effects of climate change. At present, there is increased interest globally to understanding how these ecosystems could help alleviate the challenges likely to face humanity and the environment in future. Unlike other blue carbon ecosystems; mangroves and saltmarshes, seagrasses are less understood especially on their contribution to the carbon dynamics. This is particularly true in regions with less focus and limited resources. Paucity of information is even more on the subtidal meadows that are less accessible. In Kenya much of the available information on seagrasses, is from Gazi Bay, where the focus has been on the extensive intertidal meadows. As such, like other regions there remains paucity of information on subtidal meadows. This limits our understanding of the overall contribution of seagrasses in carbon capture and storage. This study provides the first assessment of the species composition and variation in carbon storage capacity of subtidal seagrass meadows of the bay. Sampling was done in forty quadrats of 0.25m2 placed randomly along eight transects in each of the four zones in the subtidal area of the bay. In each sampling point, canopy cover and species composition were determined. Above-ground samples were obtained using the harvesting technique whereas below-ground and sediment samples were obtained through coring. Organic carbon content in biomass and sediment was determined using the Loss on Ignition (LOI) technique. Two-way ANOVA was used to test for significant differences in above, belowground biomass as well as sediment carbon among the dominant species as well as among the zones in subtidal zone. T-test was used to determine if there was variation in sediment carbon stocks between mono-specific and multispecific meadows. xvi Relationship between canopy cover, canopy height, percentage cover and biomass were tested using spearman’s rank correlation. Nine seagrass species were encountered in the subtidal area of the bay with Thalassia hemprichii, Thalassodendron ciliatum, Cymodocea serrulata and syringodium isoetifolium being the most dominant species. Aboveground biomass stocks were significantly lower than belowground biomass (t (43) =-6.817, p<.05) within the sub-tidal seagrass meadows of the bay. Aboveground biomass was significantly different among the dominant species (F (3,16) = 4.967, p <.05), while belowground carbon stocks was not significantly different among species (F (3,16) = 1.108, p >.05). Mean sediment carbon stock in sub tidal seagrass area was 113. ±8 Mg C ha-1. Sediment carbon was not significantly different among the four zones (F (3,39) = 0.35, p= 7.90), as well as among the four dominant species (F (3,16) = 0.958, p= .437). Shoot density (F (3,16) = 24.708, p<.05) and shoot height (F (3,16) =13.592, p<.05) showed significant variations among species whereas canopy cover did not vary among species. There was a positive relationship between canopy height and aboveground biomass (r (38) = .71, p< .001), as well as between belowground biomass and total biomass (r (38) = .98, p<.001). Conversely, shoot density and canopy height were negatively correlated (r (38) = -.34, p=.036). Salinity, depth, turbidity, temperature and pH were significantly different among the four zones in subtidal area with Zone A being the shallowest (2.2 ±0.6 m), highest temperature (28.9 ±0.4°C), most saline (35.8±0.5 ؉), most turbid (35340.5±370 mg/l) and had the highest pH (7.8 ±0.1) while zone D was the deepest (4.9±1.3 m), coolest (28.2±0.4°C), less saline (35.1±0.1 ؉), least turbid (34661.3±46 mg/l), and had the least pH (7.7±0.1). The total seagrass ecosystem carbon in the bay is about 196,721 Mg C with the intertidal seagrasses storing about 119,790 Mg C (61%), followed by the subtidal seagrasses 55,742 Mg C (28%) and seagrasses in the mangrove creeks storing 21,189 Mg C (11%). These results demonstrate the importance of seagrass meadows in storing carbon and provide a xvii wealth of information on the significance of blue carbon ecosystems in mitigating climate change, which highlights the need to preserve these ecosystems.