Zooxanthellae Densities are Highest in Summer Months in Equatorial Corals in Kenya

J Mwaura,D Obura,N Amiyo,J Kilonzo,G Grimsditch

doi:10.4314/wiojms.v8i2.56980

Abstract

—Coral bleaching (loss of zooxathellae) is an increasing problem for the health and persistence of corals, but the phenomenon can not be fully comprehended without understanding seasonal fluctuations in the field. Seasonal dynamics of coral zooxanthellae (population density and mitotic indices) of eleven scleractinian coral species (Acropora sp., Echinopora gemmacea, Favia sp., Galaxea fascicularis, Hydnophora microconos, Montipora aequituberculata, Pavona decussata, Pocillopora damicornis, Pocillopora eydouxi, Porites cylindrica and Porites lutea) were monitored in Mombasa Marine Park from 1998 to 2006. Direct tracking of mapped corals provided evidence that zooxanthellae densities were highest during the North-East Monsoon (NEM) season and displayed highest mitotic indices during transition periods directly preceding this season. The higher densities found during the northeast monsoon (when temperatures and light radiation levels are higher) are surprising as they are contrary to trends found at higher latitudes. It is possible that at higher latitudes seasonal variability of temperatures and light is so great that it dictates zooxanthellae density dynamics, while corals closer to the Equator are less influenced and other factors may have greater influence on zooxanthellae dynamics. The present study highlights the degree of variability of zooxanthellae dynamics that may exist among coral species and compares sites from widely different geographic locations.Keywords: Zooxanthellae, Symbiodinium, Mitotic Index, Coral bleaching, Kenya

Highlights

Tropical reef-building corals contain microalgae known as zooxanthellae (Symbiodinium spp.) within their tissue with which they exist in an obligate symbiosis that underpins the very existence of coral reef ecosystems (Muscatine and Porter, 1977)
Mass bleaching events in the last decade have prompted increased research into zooxanthellae population dynamics, and zooxanthellae density counts are useful in quantifying bleaching responses (Fagoonee et al, 1999)
Different zooxanthellae densities in different species peaked in different months, all species sampled displayed highest densities at some point during the overall northeast monsoon season (1 November to 30 April) and most displayed highest mitotic indices during the transition period directly preceding the northeast monsoon season

Summary

Introduction

Tropical reef-building corals contain microalgae known as zooxanthellae (Symbiodinium spp.) within their tissue with which they exist in an obligate symbiosis that underpins the very existence of coral reef ecosystems (Muscatine and Porter, 1977). Mass bleaching events in the last decade have prompted increased research into zooxanthellae population dynamics, and zooxanthellae density counts are useful in quantifying bleaching responses (Fagoonee et al, 1999). The health of reef corals has been an issue worldwide for several years, and made more prominent by incidences of mass bleaching. The prevalent concept of mass bleaching is that something abnormal is occurring on corals during a time period of so called “bleaching years” that is not happening in other years. It has been postulated that bleaching is the result of a seasonal cycle of zooxanthellae densities in response to seasonal environmental conditions, and that mass bleaching events are the visible result of the extreme conditions of an abnormal year (Fagoonee et al, 1999). Research projects on bleaching events are recording such unusual events from field observation of discoloration of corals (Fitt et al, 2000)

Methods

Results

Conclusion