Coastal ecosystems play a major role in marine carbon budgets, but substantial uncertainties remain in the sources and fluxes of coastal carbon dioxide (CO 2 ). Here, we assess when, where, and how submarine groundwater discharge (SGD) releases CO 2 to shallow coastal ecosystems. Time-series observations of dissolved CO 2 and radon ( 222 Rn, a natural groundwater tracer) across 40 coastal systems from 14 countries revealed large SGD-derived CO 2 fluxes. The mean groundwater partial pressure of CO 2 was 35 times higher than surface seawater. The mean SGD-derived CO 2 flux was 148 ± 226 millimoles per square meter per day (mmol m −2 day −1 ), resulting in a mean water-air CO 2 flux of 80 ± 133 mmol m −2 day −1 . Tidal rather than diel cycles drove CO 2 enrichment in most ecosystems. Tidally driven SGD was the primary CO 2 source in mangroves, salt marshes, tidal flats, estuaries, and canals. Overall, we expand current knowledge of marine carbon cycles by demonstrating SGD as an important source of CO 2 that requires inclusion in coastal carbon budgets.

Mangroves have high CO2 sequestration capacity, storing large amounts of carbon on their biomass and sediments/soil. Mangrove carbon is also transported to the ocean, i.e. outwelling or lateral fluxes, where it can be stored for long time scales. Here, we used radium isotopes (224Ra and 223Ra) to resolve carbon and alkalinity outwelling to the ocean from two mangrove seascapes in Brazil. We sampled porewaters to define the source composition, mangrove creek waters to resolve tidal cycles, and performed transects away from the mangrove into continental shelf to trace mangrove carbon across the seascape. High-resolution observations of radium isotopes in the creek indicated that tidal pumping is the main driver of carbon exchange. Low pH (6.8 – 7.0) and high 224Ra activities (165 – 290 dpm 100L-1) were found during low tides, indicating mangrove porewater exchange. Radium mass balance models revealed porewater exchange at 20.0 ± 25.4 cm d-1 in the tropical mangrove and 3.0 ± 2.0 cm d-1 at the sub-tropical mangrove. Radium-derived transport rates of mangrove porewater to the continental shelf were higher in the mesotidal tropical (667 ± 313 m d-1) than the microtidal subtropical (371 ± 168 m d-1) seascape. Radium isotopes were positively correlated (p < 0.05) with dissolved inorganic (DIC), organic (DOC) and particulate organic (POC) carbon across the entire seascape. DIC as bicarbonate (HCO3-) was the main form of carbon on all scales in both mangrove seascapes, representing 57 – 82% of the total carbon pool. DOC and POC accounted for 5 – 12% and 1 – 7% of total carbon, respectively. Although mangrove waters emitted CO2 to the atmosphere (38.4 – 142.9 mmol m-2 d-1), both bays and continental shelves were a CO2 sink (-1.9 – -0.6 mmol m-2 d-1). Porewater-derived carbon outwelling exceeded carbon fluxes at the mangrove-bay and bay-shelf interfaces, indicating carbon transformations across the seascape continuum. Total carbon outwelled from mangroves were 3 – 4 times higher than soil carbon burial at both mangrove sites. Bicarbonate outwelling (31.0 – 71.6 mmol m-2 d-1) reaching the continental shelves increased mangrove soil carbon sequestration capacity by 234% in these mangrove systems. Hence, overlooking outwelling as a blue carbon sink mechanism would underestimate the role of mangroves in sequestering CO2 and mitigating climate change.

Mangroves store significant amounts of carbon in both sediment and water. Methane (CH4) is often produced in anoxic, organic-rich sediments during carbon degradation and released to overlying waters via porewater exchange. Yet, a portion of CH4 can be oxidized to CO2 before emission. Here, we investigate whether CH4 oxidation impacts its emissions using high-temporal resolution CH4 concentration and stable isotope (delta C-13-CH4) observations collected over 14 tidal cycles in 2 Brazilian mangrove creeks with no river inputs. We found higher CH4 concentrations (similar to 150 nM) more depleted in C-13 (-75 parts per thousand) during low tide than high tide at both creeks. Similar delta C-13-CH4 values between low tide surface waters and porewaters further suggest tidally driven porewater exchange as the main source of CH4. More C-13-enriched CH4 in surface waters and surface sediments than deep sediments indicate partial CH4 oxidation prior to exchange with the atmosphere. A stable isotope mass balance revealed that 17-58% of CH4 was oxidized at rates of 3-25 mu mol m(-2) d(-1) in the water column of tidal creeks. A larger portion of deep porewater CH4 (45-61%) was oxidized in sediments prior to porewater exchange with surface creek waters. The two mangrove creeks had average water-air CH4 fluxes of 51-109 mu mol m(-2) d(-1) over spring-neap tidal cycles. These aquatic CH4 emissions offset only < 3% of the mangroves' soil carbon sequestration. Overall, CH4 oxidation in both surface water and sediment attenuated CH4 emissions to the atmosphere.

Seagrass meadows are effective sinks of atmospheric carbon dioxide (CO2). However, there is little insight on how methane (CH4) emissions may potentially offset carbon sequestration in seagrass meadows. Here, we resolve diel and seasonal dynamics of CH4 and CO2 water-air fluxes over a cold-temperate Zostera marina seagrass meadow using high-resolution timeseries observations in seawater. CH4 was emitted from the seagrass-dominated coastal bay year-round to atmosphere with CH4 fluxes ranging from 0.2 to 2.6 mu mol m-2 d-1. These fluxes are at the lower end of earlier estimates based mostly on short-term (i.e., 1 day) observations. The 13-fold seasonal fluctuations in CH4 emissions were greater than the 6-fold diel fluctuation. Radon observations imply that dissolved CH4 was primarily originated from sediment porewater. The main fate of CH4 in the water was outgassing to the atmosphere via wind forcing. Oxygen and temperature partially controlled dissolved CH4 seasonal dynamics. There was an annual average uptake of CO2 from the atmosphere (-0.9 +/- 1.5 mmol m-2 d-1) driven by enhanced photosynthesis in the spring and summer. The CO2-equivalent CH4 outgassing (0.5 +/- 0.6 g CO2 eq m-2 yr-1) offsets only 0.8% of the sediment carbon accumulation in this cold-temperate Z. marina meadows over a 20-year time horizon. The CO2-equivalent CH4 flux was 6% of the average annual CO2 uptake. Hence, CH4 emissions from this cold-temperate seagrass meadow acted as a minor offset to carbon sequestration.

This dataset is a global compilation of 40 coastal ecosystems across 14 countries. It consists of continuous time series for carbon dioxide and radon. Other parameters included are latitude, longitude, depth, temperature, salinity, dissolved oxygen, and water-air flux for carbon dioxide. Timestamps of each measurement correspond to the local time zone. Carbon dioxide and radon were measured using Licor and RAD7, respectively. Depth, temperature, salinity, and dissolved oxygen were measured using probes. This dataset was used to interpret the impact of SGD on CO2 dynamics in globally distributed coastal ecosystems. Data sources are mentioned in the sheet References. Column names in the "Master File" sheet: Location: Name of the location of measurement Location ID: ID of the location Country: Country of location Ecosystem Type: categorical ecosystem type Latitude: Latitudinal coordinate of the location (DD) Longitude: Longitudinal coordinate of the location (DD) Date (DD/MM/YYYY): Date when the measurement was taken (DD/MM/YYYY) Time (hh:mm): Local time of when the measurement was taken Time since start (Days): Duration of the continuous measurement since the first measurement was started (days) Wind Speed (m/s): Wind speed at the location at the time of measurement Depth (m): Depth at which the measurement was taken (m) Temperature (°C): Temperature at the point of radon/carbon dioxide measurement Salinity: Salinity of water at the point of radon/carbon dioxide measurement DO (%): Dissolved oxygen in the water at the point where radon measurement was taken (%) Radon (dpm/L): Radon measurement in water (dpm/L). Measured using Durridge RAD7 CO2 (µatm) at sst: measurement of Carbon Dioxide dissolved in water at sea surface temperature (µatm). Measured using LiCor. Water-air CO2 flux (mmol m-2 day-1): Calculated water-air flux of carbon dioxide.

Tidewater glaciers are highly vulnerable to climate change due to warming from both atmospheric and seawater sources. Most tidewater glaciers are rapidly retreating, but little is known about how glacial melting modifies coastal biogeochemical cycles. Here, we investigate carbonate and nutrient dynamics and fluxes in an expanding proglacial tidal lagoon connected to Europe's largest glacier in Iceland (Vatnaj & ouml;kull). The lagoon N:P:Si ratios (2:1:30) imply a system deficient in nitrogen. The large variations in the freshwater endmembers highlighted the complexity of resolving sources and transformations. The lagoon acted as a sink of dissolved inorganic carbon (DIC). Floating chamber incubations revealed a CO2 uptake of 26 +/- 15 mmol m-2 d-1. Lagoon waters near the glacier had a 170% higher CO2 uptake than near the lagoon mouth, likely driven by primary production stimulated by nitrogen-rich bottom water upwelling. The lateral DIC and total alkalinity (TA) flux rates (outwelling) from the lagoon to the ocean were -1.5 +/- 0.1 (export to ocean) and 23 +/- 5 mmol m-2 d-1 (import into the lagoon) respectively. All samples were undersaturated with respect to aragonite due to glacial meltwater dilution of TA and CO2 uptake. This implies dilution of oceanic alkalinity, lowering the nearshore buffering capacity against ocean acidification. Marine terminating glaciers are rapidly retreating and releasing freshwater, sediments, carbon, and nutrients to the ocean. We investigated glacier-ocean exchange in a climate change hotspot in Iceland. The glacier-fed lagoon was a sink of atmospheric carbon dioxide with greater uptake close to the glacier where primary production is enhanced. Nutrients, dissolved inorganic and organic carbon were exported from the glacier-fed lagoon to the ocean. Substantial amounts of dissolved inorganic carbon were consumed within the estuarine lagoon prior to exchange with the ocean. Glacier meltwater diluted alkalinity and acidified the lagoon, contributing to local ocean acidification. We emphasize the need to resolve carbon transformations and transport at the land-ocean interface in areas impacted by glaciers. The tidewater glacier enhanced CO2 uptake within the lagoon Estuarine transformations within the lagoon modify carbon and nutrient transport to the coastal ocean Glacial meltwater diluted lagoon alkalinity and enhanced local ocean acidification

Mangrove soils are highly enriched in organic carbon. Tidal pumping drives seawater and oxygen into mangrove sediments during flood tide and releases carbon-rich porewater during ebb tides. Here, we resolve semi-diurnal (flood/ebb tides), diel (day/night) and weekly (neap/spring tides) drivers of porewater-derived CO2 fluxes in two mangroves and update global estimates of CO2 emissions. Tidal pumping controlled pCO2 variability within the mangrove creeks. The highest values of pCO2 (2,585-6,856 µatm) and 222Rn (2,315-6,159 dpm m-3) and lowest values of pH (6.8-7.1) and dissolved oxygen (1.7-3.7 mg L-1) at low tides were due to enhanced porewater export. 222Rn and pCO2 in mangrove porewater were respectively 4-15 and 38-41 times greater than surface waters. pCO2 increased by 50±30% from high to low tide, 9±22% from day to night and 57±5% from neap to spring tide with clear changes on hourly, diel, and weekly time scales. Both porewater-derived CO2 and water-air outgassing increased with tidal amplitudes (r2 = 0.34, p < 0.05). Combining our new estimates with literature data, global porewater-derived (16 sites) and water-atmosphere (52 sites) CO2 fluxes in mangroves would be upscale to 45±12 and 41±10 Tg C y-1, respectively. These fluxes account for 25% of net primary production and 238% of sediment carbon burial rates in global mangroves. Overall, our local observations and global compilation suggest that porewater-derived CO2 exchange is a major but often unaccounted source of CO2 in mangroves – which can be emitted to the atmosphere or laterally exported to the ocean – and should be included in carbon budgets to solve global imbalances.

Mangroves and saltmarshes are biogeochemical hotspots storing carbon in sediments and in the ocean following lateral carbon export (outwelling). Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and natural biogeochemical processes, e.g., wetland inputs. Here, we investigate how mangroves and saltmarshes influence coastal carbonate chemistry and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to blue carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of intertidal wetlands export more DIC than alkalinity, potentially decreasing the pH of coastal waters. Porewater-derived DIC outwelling (81 ± 47 mmol m −2 d −1 in mangroves and 57 ± 104 mmol m −2 d −1 in saltmarshes) was the major term in blue carbon budgets. However, substantial amounts of fixed carbon remain unaccounted for. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant but often overlooked carbon sequestration mechanism.

Submarine groundwater discharge (SGD) is an important pathway for carbon and nutrients to the coastal ocean, sometimes exceeding river inputs. SGD fluxes can have implications for long-term carbon storage, ocean acidification and nutrient dynamics. Here, we used radium (Ra-223 and Ra-226) isotopes to quantify SGD-derived fluxes of dissolved inorganic (DIC) and organic (DOC) carbon, nitrate (NO3-), nitrite (NO2-), ammonium (NH4+) and phosphate (PO43-) in a spring-fed coastal bay in the Japan Sea. The average coastal water residence times using Ra-223/Ra-226 ratios was 32.5 +/- 17.9 days. Fresh and saline SGD were estimated using a radium mixing model with short- and long-lived isotopes. The volume of fresh SGD entering the bay (4.6 +/- 4.6 cm day(-1)) was more than twice that of the volume of saline SGD (1.9 +/- 2.1 cm day(-1)). Fresh SGD (mmol m(2) day(-1)) was the main source of DOC (2.7 +/- 2.6), DIC (13.9 +/- 13.7), PO43- (0.3 +/- 0.3) and NO3- (6.6 +/- 6.5) to the coastal ocean, whereas saline SGD was the main source of NH4+ (0.2 +/- 0.2). Total SGD-derived carbon and nutrient fluxes were 4 - 7 and 2-16 times greater than local river inputs. Positive correlations between chlorophyll-a, Ra-226 and delta C-13-DIC indicate that SGD significantly (p < 0.05) enhances primary productivity nearshore. Overall, fresh SGD of nitrogen and carbon to seawater drove chlorophyll-a, decreased DIC/Alkalinity ratios, and modified the carbonate biogeochemistry of the coastal ocean.

Abstract Lateral fluxes (i.e., outwelling) of dissolved organic (DOC) and inorganic (DIC) carbon and total alkalinity were estimated using radium isotopes at the groundwater, mangrove creek, and continental shelf scales in the Amazon region. Observations of salinity and radium isotopes in the creek indicated tidally driven groundwater exchange as the main source of carbon. Radium‐derived transport rates indicate that mangrove carbon is exported out of the continental shelf on timescales of 22 ± 7 d. Bicarbonate was the main form (82% ± 11%) of total dissolved carbon in all samples, followed by DOC (13% ± 12%) and CO2 (5% ± 4%). DIC (18.7 ± 15.7 mmol m−2 d−1) exceeded DOC (3.0 ± 4.1 mmol m−2 d−1) outwelling at all spatial scales. The interpretation of outwelling across the mangrove‐ocean continuum is related to the spatial and temporal scales investigated. At all scales, outwelling represented a major coastal carbon pathway driving bicarbonate storage in the ocean.

The term ‘Blue Carbon’ was coined about a decade ago to highlight the important carbon sequestration capacity of coastal vegetated ecosystems. The term has paved the way for the development of programs and policies that preserve and restore these threatened coastal ecosystems for climate change mitigation. Blue carbon research has focused on quantifying carbon stocks and burial rates in sediments or accumulating as biomass. This focus on habitat-bound carbon led us to losing sight of the mobile blue carbon fraction. Oceans, the largest active reservoir of carbon, have become somewhat of a blind spot. Multiple recent investigations have revealed high outwelling (i.e., lateral fluxes or horizontal exports) of dissolved inorganic (DIC) and organic (DOC) carbon, as well as particulate organic carbon (POC) from blue carbon habitats. In this paper, we conceptualize outwelling in mangrove, saltmarsh, seagrass and macroalgae ecosystems, diagnose key challenges preventing robust quantification, and pave the way for future work integrating mobile carbon in the blue carbon framework. Outwelling in mangroves and saltmarshes is usually dominated by DIC (mostly as bicarbonate), while POC seems to be the major carbon species exported from seagrass meadows and macroalgae forests. Carbon outwelling science is still in its infancy, and estimates remain limited spatially and temporally. Nevertheless, the existing datasets imply that carbon outwelling followed by ocean storage is relevant and may exceed local sediment burial as a long-term (>centuries) blue carbon sequestration mechanism. If this proves correct as more data emerge, ignoring carbon outwelling may underestimate the perceived sequestration capacity of blue carbon ecosystems.

Natural and anthropogenic pressures drive coastal eutrophication worldwide, depending on the system's physical and biogeochemical dynamics in multiple spatial and temporal scales. Understanding the complexity of this process is essential to support management efforts and sustainability. Nutrients load to the Bay of Santa Catarina Island (BSCI), an important area for mollusc aquaculture, fisheries and tourism in Brazil, were assessed to identify the pressures of the eutrophication process. An updated Driver-Pressure-State-Impact-Response framework was used to facilitate the understanding of the relationship between human activities and impacts on human welfare. Pressures from runoff and effluents from combined sources resulted in inputs of 1998 t N.year(-1) and 155 t P.year(-1) to the system. The watersheds were characterized as meso-active to eury-active for both N and P yields. In addition to the local anthropogenic pressures, meso-scale events, such as the seasonal influence of the Plata Plume River, act as an external source of nutrients, sometimes associated with harmful algae bloom events. The results show that eutrophication and its symptoms could impact 85% of the ecosystem services of the region. Management of eutrophication at BSCI requires integrated actions between the nine municipalities of the watershed, but there are obstacles in environmental legislation and political interest to promote it. This study provides the scientific basis for stakeholders and decision-makers to establish priorities and actions in coastal municipalities to minimize eutrophication.

Marine heatwaves (MHWs) are a major concern worldwide due to their increasing impacts in recent years, and these extreme events may trigger deoxygenation of coastal waters affected by sewage and eutrophication. Here we investigate the combined effects of MHWs and nutrient enrichment on the water quality and biodiversity of the Bay of Santa Catarina Island (Brazil). We used historical (1994–2020) sea surface temperature data from satellites and in situ physical, chemical and biological parameters to assess temporal trends. Oxygen levels have been decreasing whilst phosphorus levels have been increasing in the bay. During the austral summer of 2020 a regional sea surface heatwave was detected by satellite, lasting for 9 days and coinciding with our research cruise. During this period, seawater temperatures reached 29.8°C and anoxia was detected for the first time in the bay. A decrease in macrobenthic and phytoplankton community richness correlated with decreases in oxygen both through time and towards more urbanized areas. Overall, poor wastewater treatment is a key stressor that combined with MHWs to degrade coastal waters. Mitigation strategies are needed to minimize the impact of MHWs, including improved sewage treatment, restoration and conservation of wetlands and the use of nature-based technologies to promote coastal ecosystem recovery.

Understanding the different scales of temporal variability is crucial to improve the knowledge of the biogeochemical processes in the land-ocean interface. In this study, we evaluated the role of continental runoff and intrusion of oceanic water masses in the trophic state of the Bay of Santa Catarina Island (BSCI) over the last three decades (1993–2019) by using multiple biogeochemical and eutrophication assessment tools. The sub-watersheds of BSCI showed high concentrations of nutrients, fecal coliform and chlorophyll-a, directly correlated to the number of inhabitants. Worst-case scenarios were found in summer and fall seasons due to sewage inputs caused by mass tourism and the inefficiency or even absence of treatment systems, boosted by strong rainfall. The intrusion of the South Atlantic Central Water and the Plata Plume Water into the BSCI favored autotrophy in the summer and heterotrophy in the winter, coupled with low and high residence time, respectively. El Niño events enhanced rainfall and continental runoff, exporting elevated nutrients and phytoplankton biomass loads from the eutrophic rivers to the continental shelf. The pattern reverses during La Niña, when chlorophyll and nutrient peaks were detected inside the bay. Eutrophication evaluation indicated that the trophic state oscillated from moderate to high and that these conditions tend to remain the same in future scenarios due to the moderate residence time of the water, anthropogenic pressures, periodic algal blooms and the intrusion of nutrient-rich oceanic water masses. Management actions, such as the improvement of the wastewater treatment system and wetlands restoration, are needed in order to mitigate eutrophication and the loss of ecosystem services and functions.

Sewage treatment is a way of controlling coastal eutrophication worldwide. Yet is it enough to ensure water quality and avoid the effects of eutrophication? In this study, we hypothesize that sewered and non-sewered regions of Conceição lagoon (Southern Brazil) have similar water quality, due to the poorly designed sewerage system and the lack of effective public policies. The LOICZ (Land–Ocean Interactions in the Coastal Zone) biogeochemical model was used to understand how the lagoon responds to nutrient inputs from the watershed. There was no significant difference in the water quality between the urbanized rivers. Both regions (sewered and non-sewered) showed high concentrations of phosphate, ammonium and fecal coliform. The percentage of urbanization was directly correlated with deoxygenation, nutrients and coliform. Lower concentrations of nutrients and oxic waters were found in the non-urbanized rivers and in the lagoon. As such, biogeochemical and physical processes counterbalance the nutrient inputs. However, the sewage runoff might be fueling the dead zone of the lagoon, since the water residence time and nutrient retention are high. Our results indicate that the sewage treatments are inefficient in preventing the input of organic matter and nutrients to the system. Additionally, new management strategies are required to revert eutrophication, such as the expansion of the serviced area by the sewage system and the improvement of its technology.

This study investigated how the biogeochemical processes and water quality of Madre's Estuarine System (MES) are influenced by different environmental conditions and nutrient runoff from a poor-regulated watershed. We applied the Land-Ocean Interactions in the Coastal Zone (LOICZ) model to estimated water and nutrient fluxes and the net ecosystem metabolism. The closest region to agriculture, the inner estuary, showed hypoxic and anoxic conditions, nutrients maxima in the bottom waters and organic enrichment in the sediment. In contrast, lower nutrients concentrations and oxic waters were found in the middle estuary, where autotrophic conditions predominated. The system's trophic state was directly correlated with the volume of precipitation, wind intensity and depth. The water column was oligotrophic and predominantly composed by freshwater during rainfall events. Southerly winds forced the intrusion of coastal waters into the estuary, increasing water column stratification, both physical and chemical, and the trophic state. This study demonstrated that nutrient runoff from the irrigate rice culture and untreated sewage are boosting symptoms of eutrophication. Also, the high turbidity levels might be replacing phytoplankton species by an opportunistic macrophyte. The low water residence time was an important mechanism to improve the water quality, exporting elevated nutrients loads to the continental shelf.

ABSTRACT The Conceição Lagoon, located in south Brazil, is a semi-enclosed coastal ecosystem that has seasonal hypoxic and anoxic conditions in its vertically stratified central region, characterized as a site of retention and mineralization of organic matter. This study investigates water column dynamics in the central region of the Conceição Lagoon (CCL) and its relation to physical and chemical variables, in order to understand the hypoxic and anoxic events. Surface, halocline and bottom waters were evaluated at three sampling sites along the CCL. The samples were collected in triplicate during the summer, fall and winter of 2014. Hypoxic and/or anoxic events occurred in the summer (1/21) at the halocline (3 m) and bottom (4 m) waters, and in the fall (2/5) in the bottom water (4.5 m). Positive values of apparent oxygen utilization showed mineralization processes in the halocline and bottom waters. The lowest vertical stratification index was recorded in August (southern winter), which was associated with wind speed (14.7 m.s-1) and direction (southern quadrant). Nutrient concentrations were higher in winter, related to increasing of water mixing. This was the first study to evaluate the dynamics of hypoxic and anoxic events in the CCL and how nutrients respond to the physical structure of the water column.