The  Role  of  Intertidal  Macrophytes  on  Sediment  and  Coastal  Stabilization  at  Bofa  Beach,  Kilifi  – Kenya.  

Randall  MABWA1,  Bernerd  FULANDA2,  1 Department  of  Biological  Sciences,  2  School  of  Pure  and  Applied  Sciences;  Pwani  University,  Kenya

mabwa_anjili@hotmail.com 1

Results

Conclusion-­‐ The  study’s  results  support  the  hypothesis;  Intertidal  macrophytes  have  a  significant  influence  on  sediment  stability.  

-­‐ High  erosion  rates  were  noted  in  low  cover  areas  comparative  to  the  low  rates  in  highly  covered  areas.  

-­‐ The  role  in  sediment  and  coastal  stabilization  is  evident  due  to  the  positive  impact  of  leaf  height  in  settling  sediments  (deposition).  Leaf  width  aids  in  reducing  resuspension  of  sediments  and  further  reducing/  preventing  erosion;  hence  stabilizing  the  intertidal  area  and  coastline.  

-­‐ It  is  imperative  that  intertidal  macrophytes  not  be  overlooked  in  ecosystem  management  efforts  with  respect  to  stabilizing  intertidal  zones.  

-­‐ Future  research  focusing  on  a  single  macrophyte  species  would  be  essential  to  prove  the  role  clearly.    

-­‐ It  is  possible  to  adopt  the  use  of  effective  macrophytes  in  intertidal  erosion  management.  This  is  essential  to:

1. The  conservation  of  beaches  and  their  natural  profiles.  

2. Adopting  seawall  design  that  considers  sediment  and  flora  -­‐ role  interactions.  

-­‐ A  positive  influence  between  leaf  height  and  deposition  (r2 =  0.7378)  was  noted;  Thalassia  hemprichii  and  Thalassodendron  cilitium  had  greater  influence  on  sediment  deposition.

-­‐ The  lowest  influence  was  noted  in  Macro  algae  (Chlorophyta)  

-­‐ At  r2=  0.4066,  leaf  width  had  a  comparatively  lesser  influence  on  sediment  deposition  than  leaf  height.

-­‐ Increase  in  leaf  width  led  to  a  reduction  in  erosion  by  species  despite  a  lesser  coefficient  of  correlation  (r2=  0.4643);  lesser  coefficient  is  due  to  use  of  multiple  species  as  compared  to  one  specie  in  the  study.    

-­‐ There  was  minimal  to  no  relationship  between  leaf  height  and  reduction  of  erosion  (r2=0.0086)

• 7 macrophytic species were identified; 5 in seagrass families while 2 comprised macroalgae.

• Thalassia hemprichii was most the dominant followed by Syringodium isoetifolium, Cymodocea rotundata,

Thalassodendron cilitium, ENS value-­‐ 4.7 (the exponent of the Shannon wiener index value; 1.546).

• There was variable habitat characterization; seagrass, sand, coral; macroalgae and coral with variable cover along

percentage ranges. (100% to less than 40%).

• A correlation test of erosion and deposition rates by leaf size (leaf height and leaf width).

Introduction

• The study site was located at the Bofa beach, north of the Kilifi creek; -­‐3.633⁰ S 39.850⁰ E, straddling over 1.5km of

the intertidal area. The area is characterized by a steep descent with sandy, coral reef flats at the intertidal areas;

sedges on the terrestrial side.

Methodology• A 1.5km transect was divided into 3 sites with 10 stations each, with Sampling occurring from January to April 2017,twice weekly during low Neap and Spring tide periods.

• Calibrated plastic cups were attached to pegged poles; An empty one at a height of 10cm for sediment collectionand one filled with sand to the brim to measure erosion at ground level twice during the sampling day.

• Species data was sampled via use of a 1m2 string quadrat and all macrophytes classified up to the lowest levelpossible.

• The leaf blade width (mm) and leaf height (mm) measurements; (denoting leaf size) were recorded via use ofVernier calipers.

• Correlation between the leaf size and erosion/ deposition rates was determined and an inference made.

ReferencesChristianen, M. J., van Belzen, J., Herman, P. M., van Katwijk, M. M., Lamers, L. P., van Leent, P. J. & Bouma, T. J. (2013). Low-canopy seagrass beds still provide important coastal protection services. PloS one, 8(5), e62413.

Friend, P. L., Ciavola, P., Cappucci, S. & Santos, R. (2003). Bio-dependent bed parameters as a proxy tool for sediment stabilization in mixed habitat intertidal areas. Continental Shelf Research, 23(17), 1899-1917.

Gacia, E. & Duarte, C. M. (2001). Sediment retention by a Mediterranean Posidonia oceanica meadow: the balance between deposition and resuspension. Estuarine, coastal and shelf science, 52(4), 505-514.

Koch, E. (1999) Sediment resuspension in a shallow Thalassia testudinum banks ex.Koning bed. Aquatic Botany 65, 262–280.

Mwakumanya, M. & Tole, M. P. (2003). Coastal Erosion at Mombasa Beaches Hydrodynamic and Morphological Interactions. Journal of Geography and Regional Planning Vol. 2(12), p. 299-309.

Mwanje, J. I. (1997). Socio-Economic Impacts of Coastal Instability (Erosion) in Kenya: A Case Study. Ocean Documents Repository. UNESCO, p. 26-40

Ondiviela, B., Losada, I. J., Lara, J. L., Maza, M., Galván, C., Bouma, T. J. & van Belzen, J. (2014). The role of seagrasses in coastal protection in a changing climate. Coastal Engineering, 87, 158-168.

Chlorophyta

Thalassodendron cilitium

Syringodium isoetifolium

Cymodocea rotundata

Thalassia hemprichii

Halophila ovalis

Phaeophyta

Leaf  Height  and  Width  versus  Deposition  and  Erosion  

Habitat  Characterization