Abundance and physiology of dominant soft corals linked to water quality in Jakarta Bay, Indonesia

Abstract

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.

Keywords: Coral reefs; Electron transport system (ETS) activity; Eutrophication; Megacity; Nephthea spp.; Phase shifts; Photosynthetic yield; Pollution; Sarcophyton spp..

Figure 1. Study area.

The map includes boundaries of the Thousand Islands Marine National Park and study sites from nearshore reefs (within Jakarta Bay), as well as from the outer Thousand Islands (mid- and offshore): AB, Ayer Besar; UJ, Untung Jawa; R, Rambut; PS, Pari South; PN, Pari North; P, Panggang; C, Congkak; B, Bira.

Figure 2. Live benthic cover.

Mean cover (± SD) for hard and soft corals (A), the two soft coral genera Sarcopyhton spp. and Nephthea spp. (B) as well as macroalgae (C) for sites along the Thousand Islands (x-axis refers to distance to Jakarta). p-values (p > 0.05; one-way ANOVA) and significant post hoc results (p > 0.05; Student-Newman-Keuls) for differences among sites are given for each group. Consider different scales on y-axis. Study sites: AB, Ayer Besar; UJ, Untung Jawa; R, Rambut; PS, Pari South; PN, Pari North; P, Panggang; C, Congkak; B, Bira.

Figure 3. Mean electron transport system (ETS) activity Nephthea spp. and Sarcophyton spp. activity Nephthea spp. (A) and Sarcophyton spp. (B) for sites along the Thousand Islands (x-axis refers to distance to Jakarta).

p-values (p > 0.05; one-way ANOVA) and significant post hoc results (p > 0.05; Student-Newman-Keuls) for differences among sites are given for each group. AB, Ayer Besar; UJ, Untung Jawa; R, Rambut; PS, Pari South; PN, Pari North; P, Panggang; C, Congkak; B, Bira.

Figure 4. Mean electron transport system (ETS) activity Nephthea spp. (A) and Sarcophyton spp. (B) for sites along the Thousand Islands (x-axis refers to distance to Jakarta).

p-values (p > 0.05; one-way ANOVA) and significant post hoc results (p > 0.05; Student-Newman-Keuls) for differences among sites are given for each group. AB, Ayer Besar; UJ, Untung Jawa; R, Rambut; PS, Pari South; PN, Pari North; P, Panggang; C, Congkak; B, Bira.

Figure 5. Visualization of the metabolic condition of Sarcophyton spp. and Nephthea spp.

Visualization of the metabolic condition indicated by photosynthetic yield (F_v/F_m) and electron transport system (ETS) activity of the two soft coral genera Sarcophyton spp. and Nephthea spp. based on distance-based redundancy analysis (dbRDA). Water quality factors (A) Sarcophyton spp. and (C) Nephthea spp.) and benthic factors (B) Sarcophyton spp. and (D) Nephthea spp.) are overlain for both genera. Study sites: AB, Ayer Besar; UJ, Untung Jawa; R, Rambut; PS, Pari South; PN, Pari North; P, Panggang; C, Congkak; B, Bira.