Worldwide, coral reefs are challenged by multiple stressors due to growing urbanization, industrialization and coastal development. Coral reefs along the Thousand Islands off Jakarta, one of the largest megacities worldwide, have degraded dramatically over recent decades. The shift and decline in coral cover and composition has been extensively studied with a focus on large-scale gradients (i.e. regional drivers), however special focus on local drivers in shaping spatial community composition is still lacking. Here, the spatial impact of anthropogenic stressors on local and regional scales on coral reefs north of Jakarta was investigated. Results indicate that the direct impact of Jakarta is mainly restricted to inshore reefs, separating reefs in Jakarta Bay from reefs along the Thousand Islands further north. A spatial patchwork of differentially degraded reefs is present along the islands as a result of localized anthropogenic effects rather than regional gradients. Pollution is the main anthropogenic stressor, with over 80% of variation in benthic community composition driven by sedimentation rate, NO2, PO4 and Chlorophyll a. Thus, the spatial structure of reefs is directly related to intense anthropogenic pressure from local as well as regional sources. Therefore, improved spatial management that accounts for both local and regional stressors is needed for effective marine conservation.

Declining water quality is one of the main reasons of coral reef degradation in the Thousand Islands off the megacity Jakarta, Indonesia. Shifts in benthic community composition to higher soft coral abundances have been reported for many degraded reefs throughout the Indo-Pacific. However, it is not clear to what extent soft coral abundance and physiology are influenced by water quality. In this study, live benthic cover and water quality (i.e. dissolved inorganic nutrients (DIN), turbidity (NTU), and sedimentation) were assessed at three sites (< 20 km north of Jakarta) in Jakarta Bay (JB) and five sites along the outer Thousand Islands (20–60 km north of Jakarta). This was supplemented by measurements of photosynthetic yield and, for the first time, respiratory electron transport system (ETS) activity of two dominant soft coral genera, Sarcophyton spp. and Nephthea spp. Findings revealed highly eutrophic water conditions in JB compared to the outer Thousand Islands, with 44% higher DIN load (7.65 μM/L), 67% higher NTU (1.49 NTU) and 47% higher sedimentation rate (30.4 g m−2 d−1). Soft corals were the dominant type of coral cover within the bay (2.4% hard and 12.8% soft coral cover) compared to the outer Thousand Islands (28.3% hard and 6.9% soft coral cover). Soft coral abundances, photosynthetic yield, and ETS activity were highly correlated with key water quality parameters, particularly DIN and sedimentation rates. The findings suggest water quality controls the relative abundance and physiology of dominant soft corals in JB and may thus contribute to phase shifts from hard to soft coral dominance, highlighting the need to better manage water quality in order to prevent or reverse phase shifts.

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Coastal development, growing urbanization and industrialization are the most important stressors of coral reefs worldwide. Jakarta is one of the largest megacities worldwide. The coral reefs of the Thousand Islands north of Jakarta have degraded dramatically over the last 30-40 years. While large-scale gradients (i.e., regional drivers) have been extensively studied and shown shifts and declines in coral cover and composition, local drivers and their impact on spatial community composition have been neglected. The aim of our study is to investigate the spatial impact of anthropogenic stressors on local and regional scales on coral reefs north of Jakarta. Our results demonstrate that reefs in the north of the Thousand Islands are separated from the reefs in Jakarta Bay (JB), where a direct impact of Jakarta can be seen. Local anthropogenic effects rather than regional gradients have shaped a spatial patchwork of differentially degraded reefs along the nearshore islands. The main anthropogenic stressor is pollution and sedimentation rate, NO2, PO4 and chlorophyll-a explain over 80% of the variation. Surfactants and diesel-borne compounds from sewage and bilge water discharges are common pollutants. Responses to combinations of selected pollutant with elevated temperature (+3°C) were determined in the metabolic performance of the coral reef fish Siganus guttatus. During combined exposure, metabolic depression was observed. Effects of pollutants were not amplified by elevated temperature. In a study about two dominant soft coral genera, Sarcophyton spp. and Nephthea spp., on dissolved inorganic nutrients (DIN), turbidity (NTU), and sedimentation combined with measurements of photosynthetic yield and respiratory electron system (ETS) activity water quality seems to control the relative abundance and physiology of dominant soft corals in JB. In order to reverse or prevent phase shifts from hard to soft corals, there is a need to manage the water quality better. It is concluded that the intense anthropogenic pressure from local as well as regional sources is responsible for the spatial structure and health of reefs. Therefore, improved spatial management with a focus on both local and regional stressors is needed for effective marine conservation.

Knowledge on interactive effects of global (e.g. ocean warming) and local stressors (e.g. pollution) is needed to develop appropriate management strategies for coral reefs. Surfactants and diesel are common coastal pollutants, but knowledge of their effects on hard corals as key reef ecosystem engineers is scarce. This study thus investigated the physiological reaction of Pocillopora verrucosa from Lombok, Indonesia, to exposure with a) the water-soluble fraction of diesel (determined by total polycyclic aromatic hydrocarbons (PAH); 0.69 ± 0.14 mg L-1), b) the surfactant linear alkylbenzene sulfonate (LAS; 0.95 ± 0.02 mg L-1) and c) combinations of each pollutant with high temperature (+3°C). To determine effects on metabolism, respiration, photosynthetic efficiency and coral tissue health were measured. Findings revealed no significant effects of diesel, while LAS resulted in severe coral tissue losses (16–95% after 84 h). High temperature led to an increase in photosynthetic yield of corals after 48 h compared to the control treatment, but no difference was detected thereafter. In combination, diesel and high temperature significantly increased coral dark respiration, whereas LAS and high temperature caused higher tissue losses (81–100% after 84 h) and indicated a severe decline in maximum quantum yield. These results confirm the hypothesized combined effects of high temperature with either of the two investigated pollutants. Our study demonstrates the importance of reducing import of these pollutants in coastal areas in future adaptive reef management, particularly in the context of ocean warming.

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