Groundwater discharge delivers elevated dissolved carbon into nearshore coral reefs

While submarine groundwater discharge （SGD） can export substantial carbon into coastal waters， there have been limited studies to explore the impacts of SGD in delivering carbon from mangrove beach to coral reefs. Here， we used radioactive isotopes to quantify SGD derived dissolved inorganic and organic carbon （DIC and DOC） exports along a typical coastal landscape gradient of mangrove-sandy beach-coral reef， and then placed those observations into a global context. Temporarily， average SGD in coral reefs was 11 ±5 cm d^-1， leading to a groundwater borne DIC and DOC of 98 ±48 and 19 ±9 mmol m^-2 d^-1， respectively. Spatially， SGD ranged from 7.1 ±1.3 to 6.6 ±0.9 cm d^-1 in mangrove mudflats， from 7.0 ±1.3 to 17.1 ±1.9 cm d^-1 in sandy aquifers， corresponding DIC export ranging from 90 ±13 to 153 ±44 and from 36 ±7 to 161 ±20 mmol m^-2 d^-1， respectively. For DOC， mangrove mudflats presented an efflux of 61 ±12 mmol m^-2 d^-1， while intertidal sandy beach recorded a consumption of 1.7 ±0.3 mmol m^-2 d^-1. The study underscored the importance of both mangrove mudflat and sand aquifers in exporting carbon components to coral reefs. Further global investigations revealed that SGD into nearshore coral reefs was 26 ±41 cm d^-1. This substantial SGD derived DIC into coral reefs may be considered to have a profoundly influence on the growth and degradation of coral reefs.