Effects of tourism-derived sewage on coral reefs: Isotopic assessments identify effective bioindicators.

Pulau Redang and Pulau Tioman have experienced huge tourism growth over the last two decades, but minimal sewage treatment may threaten the resilience of their coral reefs. This study uses stable isotope techniques to identify suitable bioindicators of sewage nutrients (δ¹⁵N) at these islands by measuring macroalgae (Lobophora spp.), gastropods (Drupella spp.), scleractinian coral (Acropora spp.), and leather coral (Sinularia spp.). At tourist hubs using seepage septic tank systems, enrichment of Acropora δ¹⁵N (Redang, +0.7‰) and Sinularia δ¹⁵N (Tioman, +0.4‰) compared to pristine background levels indicate enhanced sewage nutrient discharge. Carbon isotopes and survey data suggest that sedimentation did not confound these δ¹⁵N trends. Potential damaging effects of sewage discharge on the coral reef communities at both islands are highlighted by strong correlations between Acropora δ¹⁵N and regional variation in coral reef community structure, and exclusive occurrence of degraded reefs at regions of high sewage influence. • Stable isotope baseline for Acropora , Drupella and Sinularia in Redang and Tioman • Elevated δ¹⁵N at tourist hubs of both islands indicate sewage nutrient enrichment • Co-occurrence of elevated δ¹⁵N alongside degraded coral reefs • Significant correlation between δ¹⁵N and regional variance in community structure • Drupella spp. are likely to have non-acroporid feeding strategies [ABSTRACT FROM AUTHOR]