The influence of a pollution gradient from a large urbanised area on coral reef fish population structure and coral reef resilience at Jakarta, Indonesia

Abstract

Coral reefs are considered as the most complex and diverse marine ecosystems (Knowlton, 2001; Jackson et al., 2001). Coral reefs have a high degree of natural variability caused by large scale natural disturbances (Green et al., 1999). However, anthropogenic disturbances now threaten coral reefs as well. Most coral reef ecosystems are present near coasts, and are therefore also subjected to stressors associated with urbanization, including land-based pollution, agricultural runoff, deforestation, oil spills, coral mining, overfishing, untreated sewage, siltation, marine litter, nutrient overloads and resource extraction (Jackson et al., 2001; Green & Bellwood, 2009; Cleary et al., 2008). Because of these human activities, the composition and functional characteristics of coral reefs change. Herbivores reef fishes are known to be one of the determining factors whether coral reefs are likely to recover after disturbances. Herbivorous fishes play a crucial role in coral reef resilience by limiting the establishment and growth of algal communities that disturb coral recruitment (Hughes et al., 2007; Green & Bellwood, 2009). Consequently, functional groups of herbivores prevent the occurrence of coral-algae phase shifts (Hughes et al., 2007). The total fish biomass and environmental parameters within reefs of the Pulau Seribu island complex, located near the coast of Jakarta, Indonesia, were measured in order to address the question ‘How does the pollution gradient from a large urbanised area affect coral reef fish populations as an indicator for coral reef resilience against regime shifts from a coral dominated reef structure towards macroalgal communties in the Indonesia’s Jakarta Bay – Pulau Seribu reef complex?’ Total biomass of all fish species, total biomass of herbivores and total biomass of the functional herbivorous group Grazers/Detrivores were all negatively influenced by urbanisation. Also, fish diversity was negatively affected by urbanisation. Only two significant relations were found, focusing on the influence of herbivorous biomass on coral reef structure; between biomass browsers and sponges cover (p=0,001) and between biomass of scrapers/small excavators and algae cover (P=0,032); indicating a small influence of herbivorous biomass variation on the coral reef structure. In terms of coral reef resilience, results suggest that the locations near the urbanised area of Jakarta experience low resilience, indicated by the low amount of herbivorous biomass, therefore increasing the chance of going from a coral dominated to an algae dominated system with increasing the human pressure. The influence of herbivorous reef fishes on coral reef resilience is limited. Still there seems to a relation between coral reef fishes and coral reef resilience, as evidenced by the found relations between reef fish diversity and cover of coral, sponges, rubble and sand.