Professor Isaac R Santos

Coastal waters may be a source or sink of carbon dioxide to the atmosphere. Little is known about short-term CO₂ dynamics within coastal lagoons often characterised by high anthropogenic nutrient loads and long water residence times. This study presents high-resolution timeseries observations of dissolved CO₂ and the groundwater tracer ²²²Rn every 10 min in Nissum Fjord in Denmark over four days. The hydrology of this coastal lagoon is anthropogenically controlled by tidal gate opening/closing cycles. Ocean inflows enhanced CO₂ by 50% to 423 ± 28 µatm compared to 271 ± 71 µatm when the eutrophic lagoon was draining to the ocean. Mixing between high CO₂ seawater and low CO₂ lagoon waters explained much of the CO₂ temporal trends. Negative correlations between p CO₂ and both dissolved oxygen and ²²²Rn imply an indirect effect of groundwater discharge on p CO₂ via primary production enhancement. The lower p CO₂ within the lagoon was likely related to enhanced nutrient inputs from rivers and groundwater sources sustaining high primary production that release oxygen while removing CO₂. Dissolved oxygen imply that primary production was a sink for CO₂ inside the lagoon. Low CO₂ suggest autotrophy, while persistent O₂ undersaturation imply net heterotrophy within the lagoon. On average, Nissum Fjord was a net CO₂ sink with an average flux of −41 mmol m^² d⁻¹. Variability in CO₂ gas water-air exchange is primarily governed by mixing during human-controlled flood gate operations and the direction of flow in and out of the lagoon.

Advective flows in coastal sediments occur on both small scales (centimeters) as porewater exchange (PEX), and large scales (>meters) as fresh groundwater discharge (FSGD) and seawater recirculation (RSGD). Constraining these benthic boundary exchange processes is essential to resolve biogeochemical budgets in global marginal seas. Here, we integrated field data and models to resolve porewater exchange and the different submarine groundwater discharge (SGD) components as nutrient sources in a highly dynamic macrotidal embayment. Small-scale PEX and large-scale RSGD are defined operationally based on the Ra–Rn mass balance model. Using Monte Carlo simulations, radium and radon mass balance models revealed that small-scale PEX 0.50 (0.32 ∼ 0.77) m³/m²/d exceeded large-scale fresh (<0.01 m³/m²/d) and recirculated 0.07 (0.04 ∼ 0.15) m³/m²/d SGD in macrotidal Hangzhou Bay. The seawater-based mass balance model results were corroborated by an independent sediment-based approach using ²²⁴Ra–²²⁸Th disequilibrium. The PEX- and SGD-associated nutrient fluxes in this bay, with extreme tidal ranges of 9 m, are an order of magnitude higher than local riverine loads. PEX accounted for over 90% of total nutrient inputs, highlighting the importance of recycled nutrients at the sediment–water interface. Tidal currents were the main driving forces of porewater exchange, pumping oxygen and organic matter into sediments while releasing nutrient-rich porewater. Resolving multi-scale advective benthic exchange provides deeper insight into complex nutrient sources driving eutrophication in coastal waters.

Nitrogen (N) availability regulates primary productivity and hence directly affects global oceanic carbon sequestration. Global fjords account for up to 11% of marine carbon burial. However, N loss via sediment burial remains largely unquantified. Here, we show that global fjords are hotspots of N burial, accounting for up to 18% of oceanic N burial despite only covering 0.1% of the ocean area. Burial is the dominant N loss mechanism, exceeding microbial N loss via denitrification and anammox, which are generally considered the major N loss mechanisms. Microbial N loss dominates in anoxic fjords and appears to be a function of temperature and nutrient availability. Overall, fjords efficiently sequester excess N in sediments over long time scales. Accelerated warming will promote both N burial from increased primary production and microbial N loss from warmer temperatures, affecting N budgets in fjords and in the ocean in general.

The fast changes in climate are driving global efforts to reduce greenhouse emissions and offset those that cannot be avoided. Interest in vegetated coastal ecosystems, known as Blue Carbon Ecosystems (BCEs), has grown rapidly due to their potential contribution to global carbon sequestration. Spain and Portugal host two of the main BCEs types; seagrass meadows and tidal marshes. To date, no comprehensive national assessment of BCEs carbon stocks has been conducted for Spain and Portugal. We have assessed the magnitude of the carbon sink associated with them across the entire Iberian Peninsula and insular Spanish territories and the potential CO₂ emission resulting from their degradation. The BCEs in the studied area are estimated to store 95 Tg CO₂-eq in the biomass and top meter of soil, equivalent to about 25% of the CO2 emissions of Spain and Portugal in 2022. The average rate of accumulation of organic carbon to the soil stock was estimated at 0.15 Tg CO₂-eq y⁻¹, equivalent to 0.04% of the annual anthropogenic CO_2eq emissions of these two countries (in 2022). Additionally, the loss of BCEs in Spain and Portugal over the last century may have released 11–27 Tg CO_2-eq, whereas we predicted that 1.3–5.6 Tg CO_2-eq will be released over the next 30 years. Which underscores the urge to increase conservation and restoration efforts. This study provides the first comprehensive Spanish and Portuguese national blue carbon inventory for its inclusion in NDCs, providing baseline data for the implementation of blue carbon offsetting projects.

Submarine groundwater discharge (SGD) derived nutrient inputs have been extensively documented. However, SGD-derived carbon fluxes remain largely unconstrained, representing a critical gap in most coastal carbon budgets. Here, we resolve SGD and dissolved carbon budgets in the Pearl River Estuary (PRE), the largest estuary in Southern China surrounded by the world's largest urban conglomerate. Broadly-defined SGD contributes 89%–96% of the dissolved inorganic carbon (DIC) pool (2–4 times riverine inputs) and 20%–70% of the dissolved organic carbon (DOC) fluxes of the PRE. SGD transports DIC exceeding total alkalinity (TAlk) by 2.7–7 times, potentially driving pH decline and acidification of nearshore waters. Groundwater pCO₂ values are 10–36 times higher than estuarine waters. SGD-derived DOC mineralization can decrease estuary water pH by 0.04–0.16 units and increase CO₂ by 6.0–90.0 μmol L⁻¹, affecting local coral populations and benthic organisms. SGD also reduces seawater dissolved oxygen (DO) by 12–150 μmol L⁻¹ and fuels the development of hypoxic zones. Overall, SGD regionally intensifies seawater hypoxia and acidification, creating challenging conditions for coral reef survival in an already stressed ecosystem. Our findings demonstrate that SGD should be integrated into carbon budgets and ecological assessments of the land-ocean continuum.

Mangroves host many marine species and support fisheries in developing (sub)tropical countries. The suitability of mangrove habitats depends strongly thier the water chemistry. Here, we show how global warming and rising atmospheric CO 2 will reduce dissolved oxygen and increase CO 2 in mangrove waters. Observations from 23 mangrove‐lined estuaries worldwide revealed that most sites already experience mild (34%–43% of the time) or severe (6%–32%) hypercapnic hypoxia, that is, high CO 2 and low oxygen conditions. Hypercapnic hypoxia mostly occurs during low tide, at low‐salinity sites, and in warm tropical regions. Climate change will decrease oxygen concentrations by 5%–35% and increase CO 2 concentrations by 8%–60% in mangrove waters by 2100. Overall, hypercapnic hypoxia events will occur more frequently, last longer, and become more severe. These shifts will reduce mangrove biodiversity and deteriorate habitat quality for commercially valuable fish. The strongest impact is expected in tropical developing countries. Plain Language Summary Mangrove forests are important coastal ecosystems that provide food, nursery grounds, and storm protection for millions of people, especially in tropical developing countries. Their value depends strongly on the chemistry of their surrounding waters. Using observations from 23 mangrove systems around the world, we show that mangrove waters already experience frequent periods of low oxygen and high carbon dioxide, which are stressful for many marine organisms. Climate change will worsen these problems by further reducing oxygen and raising carbon dioxide levels in mangrove waters. Stressful periods will occur more often, last longer, and become more severe. As a result, mangrove habitats will become less suitable for fish and other marine life, potentially reducing biodiversity and threatening coastal fisheries in developing countries.

Increased meltwater runoff from glaciers may drive localized ocean acidification and impact carbon dioxide (CO₂) uptake in the coastal ocean. However, the paucity of carbonate system observations from continental shelves receiving inputs from glaciers limits our understanding of cryosphere-ocean connectivity. Here, we contrast meltwater impacts on seawater carbonate chemistry and stable isotopes (δ¹³C-DIC) off marine- and land-terminating glacier outflows off Iceland. On the shelf outside a marine-terminating glacier, glacial meltwater reduced the seawater buffer capacity of receiving surface waters through dilution of total alkalinity, and increased CO₂ uptake through salinity-driven drawdown of pCO₂. Primary production acted as a counterbalance to the lowered [TA-DIC]. On the shelf area receiving meltwater from large glacial river deltas, CO₂ uptake was almost halved and the saturation state of aragonite was 0.2 units lower than on the marine-terminating glacier shelf. Reduced net autotrophy due to higher turbidity and upwelling of low-pH deep waters off the delta-dominated shelf likely explain those differences. The diverging carbonate dynamics on the two shelves build on previous observations that land-terminating glaciers can reduce the buffer capacity as well as CO₂ uptake potential of nearshore surface waters in comparison to marine-terminating glaciers. The future retreat of many marine-terminating glaciers onto land is likely to modify how meltwater will impact coastal seawater carbonate chemistry.

Organic‐rich mangrove soils release CH4, partially offsetting the climate benefits of high organic carbon sequestration. However, the local and global drivers and variability of mangrove CH4 emissions remain poorly understood. Here, we quantify water‐atmosphere CH4 emissions over hourly, daily, and weekly time scales in an Australian mangrove ecosystem. We then combine our new observations with earlier data sets to link temperature and mangrove CH4 emissions on the global scale. The water‐atmosphere CH4 emissions were partially controlled by temperature on both local and global scales. One degree warming increased mangrove water‐atmosphere CH₄ emissions by ∼23% locally and ∼13% globally. Globally scaled water‐atmosphere CH₄ emissions (0.07–0.10 Tg C yr⁻¹) currently offset 6%–8% of mangrove carbon burial. CH₄ emissions are predicted to increase by 10%–33% by 2100 under global warming scenarios and tropicalization. Therefore, mangrove CH₄ emissions should be considered in blue carbon assessments in the context of global warming.

Robust estimations of methane (CH₄) oxidation in marginal seas remain elusive, making CH₄ budgets particularly uncertain. Here, we investigate the CH₄ benthic source and bottom layer oxidation across the entire Baltic Sea using concentration and stable isotope (δ¹³C-CH₄) profiles along oxygen and salinity gradients. CH₄ concentrations were highest near the seafloor under low oxygen conditions. Comparison with previous local-scale studies implies increasing deep water CH₄ concentrations in the last decade. High CH₄ concentrations and δ¹³C-CH₄ values (−54.4 ± 17.7‰) in bottom waters indicate benthic sources. Oxygen, salinity, and ²²⁴ Ra (benthic tracer) explained the CH₄ distribution. Methane oxidation estimated from δ¹³C-CH₄ fractionation removed 49 ± 33% of benthic-produced CH₄ before reaching the surface, leading to small water–air fluxes (10.0 ± 9.2 μmol m⁻² d⁻¹). Overall, bottom layer CH₄ oxidation was highly effective attenuating CH₄ emissions to the atmosphere.

We report hydrogeochemical and isotopic observations across the Baltic Sea from two research expeditions: (1) a ~5000km cruise-track onboard the R/V Skagerak in 2023 and (2) a land-based sampling for terrestrial endmembers in 2024. The ship-based observations include continuous monitoring of hydrographic parameters, pH, and 222Rn in surface water. In addition, we collected 542 discrete samples from the water column, vertical profiles (n=69 stations), and meteorological data. Land observations include discrete samples from beach groundwater (n=77), nearshore surface water (n=47), and rivers close to the coastline (n=46). Discrete samples were analyzed for short-lived radium isotopes, nutrients, dissolved organic and inorganic carbon, total dissolved nitrogen, total alkalinity, methane, and stable isotopes (δ18OH2O, δ2HH2O, δ13CDIC, δ13CCO2, δ13CCH4). Data products include seven open-access files. This dataset forms the deposit for upcoming original research publications. This dataset will also
be valuable to researchers interested in the hydrogeochemistry of coastal seas, like the Baltic Sea, and more generally interested in submarine groundwater discharge and estuarine biogeochemistry.