1. Habitat configuration is an important baseline to delineate protected area design, refine impact mitigation measures and define habitat protection plans for threatened species. For coastal delphinids, outlining their habitat configuration becomes a real challenge when faced with large distribution ranges that straddle international borders, leaving broad information gaps in uninvestigated areas.2. This study projected likely habitats of Indo-Pacific humpback dolphins, Sousa chinensis, in the Beibu Gulf (Gulf of Tonkin) based on occurrence data and remotely sensed oceanographic characteristics. Net primary productivity was derived to measure the ecosystem service of humpback dolphin habitats.3. Bathymetry and chlorophyll-a concentration are major variables contributing to humpback dolphin habitat configuration, which is characterized by shallow water depth and high primary productivity. Three major, likely habitats were identified in the northern Beibu Gulf from western Leizhou Peninsula to the China-Vietnam border, western Gulf of Tonkin from the Red River estuary to the central coast of Vietnam, and south-western Hainan Island. Less than 9% of likely habitats are currently protected by marine protected areas.4. Affinity to high primary productivity and shallow depths implies that prey abundance and foraging efficiency influence habitat selection by Indo-Pacific humpback dolphins. Anthropogenic activities potentially altering oceanographic characteristics may impact regional marine ecosystem functions, and hence habitat configuration. Habitat protection actions for Indo-Pacific humpback dolphins includeimplementing coordinated and systematic surveys in major habitats, associating core habitat protection with protected area networks and maritime function zoning, ensuring ecosystem function integrity within major habitats, and reducing both explicit lethal impacts and implicit anthropogenic impacts from activities that change oceanographic features. The habitat protection plan should not only consider marine habitats, but also adjacent coastal landscapes and river catchments.

Mangroves are blue carbon systems characterized by high soil carbon storage and sequestration.Soil carbon losses via groundwater or pore water pathways are potentially important yet poorly understood components of mangrove carbon budgets. Here we quantified submarine groundwater discharge (SGD) and associated dissolved inorganic carbon (DIC) and organic carbon (DOC) fluxes into a mangrove-dominated tropical bay (Maowei Sea) using a radon ( 222 Rn) mass balance model. The SGD fluxes in Maowei Sea were estimated to be 4.9 × 10 7 (0.36 ± 0.33 m/day) and 2.6 × 10 7 m 3 /day (0.20 ± 0.18 m/day) for the wet and dry seasons, respectively, implying that SGD may respond to precipitation. The SGD-derived DIC and DOC fluxes (mol·m À2 ·day À1 ) in the wet season (DIC: 0.70 ± 0.82; DOC: 0.31 ± 0.30) were higher than those in the dry season (DIC: 0.25 ± 0.24; DOC: 0.25 ± 0.23). These SGD-derived carbon fluxes exceed local river inputs and constituted >70% of the total DIC and DOC input into the bay. If scaled up to the global weighted mangrove area in combination with data from other 32 study sites, carbon fluxes via SGD into mangroves may be equivalent to 29-48% of the global riverine input into the ocean. Therefore, we suggest that SGD is a major component of coastal carbon budgets and that accounting for SGD helps to reduce uncertainties in mangrove blue carbon budgets.Plain Language Summary Mangrove soils sequester large amounts of carbon, but little is known on the loss of carbon via groundwater pathways. Here we quantified submarine groundwater discharge using radon and related dissolved inorganic and organic carbon fluxes in a mangrove-dominated tropical bay in China (Maowei Sea). Combining our results with literature data revealed that groundwater fluxes in mangroves are significant and should be considered in blue carbon assessments.

Submarine groundwater discharge (SGD) has been recognized as an important pathway for nutrients into estuaries, coasts, and the adjacent seas. In this study, 222Rn was used to estimate the SGD‐associated nutrient fluxes into an aquaculture area in a typical tropical bay (Maowei Sea, China). The SGD into the Maowei Sea during June 2016 was estimated to be 0.36 ± 0.33 m d−1 and was associated with SGD‐derived dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicon (DSi) fluxes (mol d−1) of (4.5 ± 5.5) × 106, (5.3 ± 9.1) × 104, and (9.4 ± 9.3) × 106, respectively. The SGD‐derived nutrients (i.e., DIN, DIP, and DSi) were more than 1.9, 0.9, and 3.6 times the amounts in the local river input and served as dominant sources in the nutrient budgets in the Maowei Sea. Moreover, the N/P ratios in the SGD around the Maowei Sea were high (mean: 64), and these ratios likely exceeded the environmental self‐purification capacity, thereby enhancing the biomass and changing the phytoplankton community structure. Therefore, SGD processes with derived nutrients may affect the biogeochemical cycles and marine ecological environment in the Maowei Sea. Furthermore, the N/P ratios (∼67) in oysters are very close to those in the SGD in the Maowei Sea; this coincidence suggests that the high N/P ratios in the SGD are likely to be one of the most important sources that support oyster aquaculture, which might weaken the burden of water eutrophication in the Maowei Sea.

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