SPATIAL-TEMPORAL DISTRIBUTION OF SEA URCHINS AND SEAGRASS ALONG DIANI-CHALE LAGOONAL REEFS, MOMBASA

Abstract

Overgrazing by herbivores has been postulated as a potential threat to seagrass in Kenya. Indirectly, fishing may result in increase in sea urchin population caused by removal of sea urchin predators. This study determined the spatial and temporal distribution of sea urchins and seagrass along Diani lagoonal reefs.The objectives of this study were (i) To determine seasonal abundance and diversity of sea urchins and seagrass in Diani Beach (ii) To determine spatial variation of density of sea urchins in healthy and degraded seabeds in Diani Beach (iii) To evaluate the influence of sea urchin abundance on benthic seagrass cover in Diani beach. Three sites of Mvuleni, Chale, and Mwaepe, and three seasons of northeast monsoon, intermonsoon, and southeast monsoon were selected.Healthy and degraded sections were identified in each of the selected sites and 40 samples were obtained from each study site and for each season using one-meter square quadrats (N = 360). Seagrass cover was computed using percentages, density of sea urchins was calculated using frequencies, and the diversities of both were determined using Simpson’s diversity index. In the analysis of data, the study used t-test, two-way analysis of variance and post hoc test in comparing differences among sites and seasons. Ultimately, Pearson’s correlation and regression analysis were used to establish the nature of relationships and the influence of sea urchins on seagrass cover. Research findings indicated that the abundance and diversity of sea urchins and seagrass varied according to seasons. The density of sea urchins was highest during northeast monsoon and lowest during southeast monsoon.Correlation outcome demonstrated that the density of sea urchins was statistically significant negative predictor (r = -0.699) of the proportion of seagrass cover and accounted for 48.8% of variation. Canonical Correspondence Analysis suggests that seasons, study sites, and sea urchins influence distribution of seagrass species and nature of seabed because they account for 7.87% in the first dimension and 86.9% in the second dimension. However, other studies are necessary to establish the role of other biotic factors and abiotic factors in the growth of seagrass in seabed. This study recommends sustainable use of ocean resources for economic growth,improved livelihoods and jobs while preserving the health of ocean ecosystem.