Temporal and Spatial Patterns in Recruitment of Juvenile Corals on Lagoon Reefs in Kenya Coast

Abstract

Measuring recruitment patterns and mortality of corals is important for understanding mechanisms that regulate their populations and mediate species coexistence. However, there is limited data on coral recruitment dynamics in Eastern Africa and much of the Western Indian Ocean (WIO) region. I studied the spatial and temporal patterns of coral recruitment, mortality and growth in four lagoonal reefs in Kenya from May 2006 to June 2007. The aim was to determine the population dynamics of juvenile corals < 10 cm diameter) in four reef sites of varying environmental conditions and different protection level. Juvenile coral abundance, distribution, densities, recruitment, mortality, size frequency distribution. and growth were compared between the four lagoonal reef sites and seasons. Each study site had two replicate stations of 25 x 8m, within which six 1 m 2 quadrats were randomly placed. Juvenile corals were sampled monthly in each quadrat, recording the maximum diameter coral colony and identifying to lowest taxonomic level possible. Benthic substrate type, water temperature, water flow and light intensity were recorded. There was spatial variation among sites in benthic substrate characteristics. The temporal pattern in water temperature was similar at all sites, but the spatial pattern showed significantly higher temperatures and greater extremes at Kanamai (maximum' of 36.23 degree C). Mombasa Marine Park (MMP) had significantly higher water flow and light intensity (at 18.2 cm s super(-1) and 2117 photons m super(-2) s super(-1), respectively). A total of 16 genera were , recorded with Porites, Favia, Favites, Pocillopora and Pavona the most abundant at all sites. Mombasa Marine Park (MMP), a protected site, and the nearby Nyali site (partially protected) showed the highest genus richness (13 and 12, respectively) which may be due related to the presence of tidal channels and being deep lagoons. The relative abundance. of coral genera was site specific; Porites was most abundant at Vipingo (48 %) possibly as a result of coral resistance and adaptation to unfavorable environmental conditions. Juvenile coral density was highest at the protected MMP and the unprotected Kanamai sites (7.5 and 7.4 juveniles m super(-2), respectively) which could be related to high coral cover at these sites. There was spatial and temporal variation in recruitment rate among genera, for example, Porites had the highest recruitment rate at Vipingo site (0.29 recruits m super(-2) month super(-1)) during the South East (SE) monsoon season. Coral mortality was patchy at sites and did not show a random pattern in space or time. Size frequency distribution of the coral genera was site specific, for example, small sized Pocillopora were highest at Nyali site. There was no significant difference in the absolute growth rates of juvenile coral genera between the four study sites. A stepwise multiple regression analysis showed a positive effect of hard coral cover on juvenile coral density whereas coral rubble and water temperature had a negative effect. These findings are important in explaining the factors that influence spatial and temporal variation in juvenile coral density and recruitment rate on Kenyan lagoonal reefs.