Environmental Factors and Coral Bleaching in Kenya.

Abstract

Elevated seawater temperature in March and April 1998 caused by the El Nino Southern Oscillation (ENSO) caused mass coral bleaching along the Kenya coast. The ENSO event had an effect on the climate of the study area and the region in general, with heavy rains starting in October 1997, and continuing to July 1998. Seawater temperature in March and April rose to an average 1.5 °C above values measured in the same period in 1997, with daytime low-tide highs of over 32°C. Coral bleaching is the whitening of corals resulting from the loss of symbiotic zooxanthellae and/or a reduction of the photosynthetic pigment concentrations in zooxanthellae. Bleaching was first observed in October 1997 in Kanamai during and after the El Nino rains, most likely due to sedimentation and seawater dilution in the shallow lagoon. In November 1997 bleaching was recorded in Malindi Marine National Park (MNP) and was mainly caused by sediment discharged from the Sabaki River. Later, extensive temperature induced bleaching and subsequent coral mortality was observed in mid March 1998 covering the entire coast of Kenya. Over 90% bleaching and mortality was recorded in Mombasa MNP, Malindi MNP and Kanamai. Bleaching and mortality was highest in the shallow lagoons such as Kanamai and areas where corals are exposed to sediment influence such as the north reef of Malindi MNP. The coral species most susceptible to bleaching and mortality were Porites nigrescens, Porites lutea, Acropora spp, Pocillopora spp and Stylophora pistillata. There was a distinct relationship between temperature and bleaching. As temperature increased to a maximum 32°C the number of normal coral colonies decreased due to bleaching and mortality. Temperature vs. bleaching relationship indicated a strong positive correlation for normal Porites nigrescens (r2 = 0.779), Porites lutea (r2 = 0.781) and Pocillopora sp (r2 = 0.803). The density of zooxanthellae ranged between 0.7 x 106 and 4.5 x 106 (mean, 2.5 x 106 ± 1.2 x 106, n = 11) per cm 2 for normal corals and 0.02 x 106 and 0.2 x 106 per cm 2 (Mean, 0.2 x 106 ± 0.02 x 106, n = 6) for bleached corals; Total loss of zooxanthellae and pigment was recorded in a few coral fragments. Zooxanthellae densities showed significant differences between species ((P<0.001) and between normal and bleached decussata had the highest zooxanthellae densities while Acropora sp and Stylophora pistillata had the lowest. The concentration of Chlorophyll-a ranged between 0.176 mg/cm2 and 0.795 mg/cm2 (Mean, 0.498 ± 0.287 mg/cm2 , n = 11) for normal corals and 0.002 mg/cm2 and 0.284 mg/cm2 (Mean, 0.0765 ± 0.01 mg/cm2, n = 6) for bleached corals. Chlorophyll-a concentrations differed significantly between bleached and normal colonies (p<0.005) and between species (p<O.OOI). In normal corals Porites nigrescens and Pavona decussata had the highest chlorophyll-a concentrations while Acropora sp and Stylophora pistillata had the lowest. Chlorophyll-a concentration per zooxanthellae was not significantly different between species but was significant between bleached and normal fragments (p<0.05). Bleached corals had higher values of chlorophyll-a per zooxanthellae compared to normal corals as can be observed in Acropora sp, porites nigrescens, porites lutea and Pavona decussata. Only in Slylophora pistillata that bleached fragments had a lower value of chlorophyll-a per zooxanthellae compared to the normal fragments. Overall this study indicates that coral bleaching and consequent mortality was caused by two sets of stressors; sediment/rainfall stress in Kanamai and Malindi in 1997 and elevated sea surface temperatures in all the sites in MarchiApri11998.