Radium-derived porewater exchange and dissolved N and P fluxes in mangroves

Mangroves are increasingly being recognized as a major player in coastal hydrological and biogeochemical cycles with their complex belowground structure (i.e., crab burrows) facilitating porewater exchange and submarine groundwater discharge. Here, we quantify porewater exchange rates and associated fluxes of nutrients at six mangrove dominated sites covering a broad latitudinal gradient (∼12.4°S to ∼38.3°S) in Australia. Porewater exchange rates were calculated using the natural tracer radium (²²³Ra and ²²⁴Ra) and ranged from 1.5 cm day⁻¹ in the temperate region to 30.9 cm day⁻¹ in the tropics. When porewater exchange rates were multiplied by the global weighted area of mangroves in each of their respective climate zones, this would equate to global porewater exchange rates of 6.4 × 10¹² m³ yr⁻¹ (²²³Ra) and 7.2 × 10¹² m³ yr⁻¹ (²²⁴Ra) which is equivalent to 17–19% of global riverine freshwater flows. This porewater exchange rate could recirculate a volume of water equal to continental shelf waters adjacent to mangroves every 12.4 (²²³Ra) and 10.1 (²²⁴Ra) years. The radium-derived estimates are within 40% of previous values based on ²²²Rn (1.3 × 10¹³ m³ yr⁻¹). The mangrove sites studied were seen to be both nitrogen (N) and phosphorous (P) limited which was driven by a combination of porewater exchange, low incoming surface water nutrient fluxes, and the high contribution of dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP). The average porewater total dissolved nitrogen (TDN) and total dissolved phosphorous (TDP) concentrations were ∼3.4 and 2.7 times greater respectively than in surface waters. The average surface water export of TDN was 2.39 ± 1.30 mmol N m⁻² day⁻¹. If upscaled to the global mangrove area, these TDN exports (1.69 Tg N yr⁻¹) would be equal to 6 ± 3% of global river exports. In contrast, there was an overall import of total dissolved phosphorus (TDP) from coastal waters (0.95 ± 0.40 mmol P m⁻² day⁻¹) into the mangrove creeks. Globally, this import would be equal to 1.48 Tg P yr⁻¹ or 5 ± 2% of global riverine fluxes to the coastal ocean. Therefore, while mangroves may increase N:P ratios of the coastal ocean, they seem to be minor global contributors to coastal N and P loads.