Disturbance and Coral Community Structure in the Intertidal Coral Reefs of the Southern Gulf of Kutch

Abstract

I quantified the coral community structure using quadrat sampling in two intertidal coral reefs off the island of Narrara and Pirotan in the Southern Gulf of Kutch. These areas are subject to heavy disturbance from industries, and anthropogenic pressures on the reef are considerable. Species richness and indices of diversity indicate the reefs are of low diversity. Species diversity values indicate that Narrara has a lower species diversity (H'= 2.822, SE 0.139) than Pirotan (H'= 3.95, SE 0.126) (t-test d.f. = 236, P < 0.00 I). However, environmental data collected from both areas indicate that the levels of disturbance, measured as the mean percentages of dead coral, sediment load on live coral, algae cover and bleaching, was generally higher in Pirotan than in Nanara. Multivariate techniques of analysis were used to elucidate these trends. Community clustering using the statistical package TWINSPAN and direct ordination using Canonical Correspondence (statistical package CANOCO) were carried out. Axis I correlated well with the percentage of boulders in the reef and the density of algae, and Axis 11 correlated well with sediment load on the coral. Species showed clear patterns of distribution along these axes. I set up experiments to test the effect of crude oil and bittern, a salt manufacturing by-product, on two common reef Scleractinidia, Porites compressa and Favia favus. Significant effects were seen with both pollutants. Most of the replicates showed some level of recovery though this recovery did not result in any of the coral regaining total health. Indices of coral sensitivity and coral recovery was developed and this proved a valuable means of comparing the response of different species. Bittern and crude oil stressed the corals and resulted in sediment deposition. The immediate cause of mortality was caused by sediment deposits on the coral boulders. In highly turbid reefs, coral species might be affected more by high sedimentation levels than by other environmental stressors