*1.2. Water Quality and Coral Reefs along the North Coast of Timor-Leste*

Pollution arising from disturbed coastal regions and watersheds poses a serious threat to coral reefs globally. This type of pollution includes a wide range of compounds such as agrochemicals (pesticides), inorganic nutrients (nitrate, ammonia, and phosphate), soils and sediments, and fossil fuel residues that flow from disturbed landscapes. Many of these compounds negatively affect coral physiology by reducing calcification rates, fecundity, fertilization success, and larval development [33]. This can degrade reef communities reducing coral cover, community composition, diversity, and structural complexity [34,35]. High levels of marine pollution can increase the prevalence and severity of disease and susceptibility to bleaching [36–40]. Dissolved inorganic nitrogen (DIN = ammonium + nitrate + nitrite) measurements on reefs are generally <1.5 μM (individual species ammonium, nitrate, nitrite < 1 μM) with even lower phosphate concentrations (<0.3 μM; Table S2) [37,41–47]. A greater prevalence of disease has been associated with elevated concentrations of DIN from anthropogenic sources (fertilizer, sewage pollution, etc.) and phosphate ranging from 3.6 μM to 25.6 μM and 0.3 μM to 0.4 μM, respectively [41,42,45–47].

The isotopic signature of nutrients such as nitrogen can often serve as a tracer for different sources of coastal pollution, with different forms of input having different impacts

(sewage can increase pathogen concentrations) and solutions [32,48–57]. Stable isotope analyses of nitrogen stored in macroalgae can provide a nutrient signal integrated over time versus water sampling, which is highly variable over space and time [58]. Generally, δ15N signatures in algae associated with urban wastewater are >10‰ [59–62]. Natural and synthetic fertilizers display a large range from −4‰ to +4‰ of <sup>δ</sup>15N values while nitrogen fixation typically has a negative <sup>δ</sup>15N signature between −2 and 0‰ [63]. Upwelling can have variable δ15N values ranging from 5 to12‰ [50,64–70]. Both fertilizer use and waste infrastructure are expected to be developed as described in the national strategic development plan [19].
