Carbon In Coastal Waters Research Articles

A Shift from Snow to Rain in Midlatitude Japan Increases Fresh Submarine Groundwater Discharge and Doubled Inorganic Carbon Flux over 20 Years.

Fresh submarine groundwater discharge (FSGD), a well-established land-to-sea pathway for water and materials, has increased (∼30%) along the midlatitude coast of Japan with the shift from snowfall to rainfall since the 1980s. The terrestrial-derived FSGD in this area has a residence time of 10-20 years, and its material flux may respond quickly to the effects of recent warming. In this study, FSGD collected directly from the seafloor (∼10 m water depth) during two different periods (2017-2021 and 2001-2003) was observed using the same consistent method to evaluate the warming-related effects on FSGD material fluxes. In the FSGD system, dissolved constituents and the isotopic composition decreased by 10-30% and residence time by one-third compared to the previous data, even though homogeneous water quality was maintained among the FSGD-connected aquifers for each period. Most concerning is that these changes resulted in a drop in pH and an increase in FSGD-derived carbon flux. Total nutrient fluxes from land have roughly halved, resulting in lower primary productivity and an estimated doubling of excess dissolved inorganic carbon in coastal waters. Our findings present direct evidence of the sensitivity of carbon flux to climate change and the urgency for carbon-related FSGD research worldwide.

A Semi-Analytical Optical Remote Sensing Model to Estimate Suspended Sediment and Dissolved Organic Carbon in Tropical Coastal Waters Influenced by Peatland-Draining River Discharges off Sarawak, Borneo

Coastal water quality degradation is a global challenge. Marine pollution due to suspended sediments and dissolved matter impacts water colour, biogeochemistry, benthic habitats and eventually human populations that depend on marine resources. In Sarawak (Malaysian Borneo), peatland-draining river discharges containing suspended sediments and dissolved organic carbon influence coastal water quality at multiple locations along the coast. Optical remote sensing is an effective tool to monitor coastal waters over large areas and across remote geographic locations. However, the lack of regional optical measurements and inversion models limits the use of remote sensing observations for water quality monitoring in Sarawak. To overcome this limitation, we have (1) compiled a regional spectral optical library for Sarawak coastal waters, (2) developed a new semi-analytical remote sensing model to estimate suspended sediment and dissolved organic carbon in coastal waters, and (3) demonstrated the application of our remote sensing inversion model on satellite data over Sarawak. Bio-optical data analysis revealed that there is a clear spatial variability in the inherent optical properties of particulate and dissolved matter in Sarawak. Our optical inversion model coupled with the Sarawak spectral optical library performed well in retrieving suspended sediment (bias = 3% and MAE = 5%) and dissolved organic carbon (bias = 3% and MAE = 8%) concentrations. Demonstration products using MODIS Aqua data clearly showed the influence of large rivers such as the Rajang and Lupar in discharging suspended sediments and dissolved organic carbon into coastal waters. The bio-optical parameterisation, optical model, and remote sensing inversion approach detailed here can now help improve monitoring and management of coastal water quality in Sarawak.

Remote sensing approach for the estimation of particulate organic carbon in coastal waters based on suspended particulate concentration and particle median size

The particulate organic carbon (POC) content retrieved by remote sensors is influenced by the suspended particulate concentration (SPC) and the particle size distribution (PSD). The objective of this study was to provide study case of remote sensing monitoring of non-optical activity substance POC in Hangzhou bay, China. A modified empirical remote sensing algorithm was established based on SPC and median particle size (D50) to describe the influence of PSD variation on remote sensing reflectance (Rrs). The algorithm was applied to MODIS data to reveal POC spatial and temporal variations. The results show that the accuracy of the remote sensing estimation algorithm, established on the basis of Mie theory, is relatively higher than the empirical model simply based on the statistical correlation between Rrs and POC. The POC in Hangzhou bay caused by spring and neap tides vary significantly.

Deriving Particulate Organic Carbon in Coastal Waters from Remote Sensing: Inter-Comparison Exercise and Development of a Maximum Band-Ratio Approach

Recently, different algorithms have been developed to assess near-surface particulate organic matter (POC) concentration over coastal waters. In this study, we gathered an extensive in situ dataset representing various contrasted bio-optical coastal environments at low, medium, and high latitudes, with various bulk particulate matter chemical compositions (mineral-dominated, 50% of the data set, mixed, 40%, or organic-dominated, 10%). The dataset includes 606 coincident measurements of POC concentration and remote-sensing reflectance, Rrs, with POC concentrations covering three orders of magnitude. Twelve existing algorithms have then been tested on this data set, and a new one was proposed. The results show that the performance of historical algorithms depends on the type of water, with an overall low performance observed for mineral-dominated waters. Furthermore, none of the tested algorithms provided satisfactory results over the whole POC range. A novel approach was thus developed based on a maximum band ratio of Rrs (red/blue, red/yellow or red/green ratio). Based on the standard statistical metric for the evaluation of inverse models, the new algorithm presents the best performance. The root-mean square deviation for log-transformed data (RMSDlog) is 0.25. The mean absolute percentage difference (MAPD) is 37.48%. The mean bias (MB) and median ratio (MR) values are 0.54 μg L−1 and 1.02, respectively. This algorithm replicates quite well the distribution of in situ data. The new algorithm was also tested on a matchup dataset gathering 154 coincident MERIS (MEdium Resolution Imaging Spectrometer) Rrs and in situ POC concentration sampled along the French coast. The matchup analysis showed that the performance of the new algorithm is satisfactory (RMSDlog = 0.24, MAPD = 34.16%, MR = 0.92). A regional illustration of the model performance for the Louisiana continental shelf shows that monthly mean POC concentrations derived from MERIS with the new algorithm are consistent with those derived from the 2016 algorithm of Le et al. which was specifically developed for this region.

Tracing Different Freshwater Sources for Nutrients and Dissolved Organic Matter in Coastal Waters off Jeju Island Using Radon

In order to determine the origins of dissolved inorganic nutrients and dissolved organic matter in coastal waters off Jeju Island, Korea, we measured the concentrations of 222Rn, nutrients, fluorescent dissolved organic matter (FDOM), and dissolved organic carbon in coastal waters during September 2016 and October 2017. The lower salinity waters occurring in the southern coastal waters off Jeju (SCW) were characterized by higher nutrients and humic-like FDOM, while those in the western coastal waters off Jeju (WCW) showed no trend. A significant negative correlation between salinity and 222Rn in the SCW (r2 = 0.62, n = 7 for 2016; r2 = 0.56, n = 14 for 2017) indicates that freshwaters occurring in the SCW originate from fresh groundwater. However, depleted 222Rn activities in the fresh surface waters of the WCW indicate that those in the WCW originate mainly from the remote Changjiang diluted water. In general, the coastal ecosystem in the SCW is characterized by the dominance of brown seaweeds, and the WCW is by scleractinian corals. Our results imply that high nutrient concentrations in the SCW provide favorable conditions for the growth of seaweeds, while relatively low nutrient concentrations and high FDOM concentrations in the WCW provide favorable conditions for survival of corals.

Microbial metabolism of methanol and methylamine in the Gulf of Mexico: insight into marine carbon and nitrogen cycling.

One carbon (C1) metabolism plays an important role in marine carbon cycling but the dynamics and modes of C1 transformations are not fully understood. We made contemporaneous measurements of methylamine and methanol metabolism to elucidate the role of C1 compounds as sources of carbon, energy and nitrogen. Methanol and methylamine were predominantly used as an energy source in offshore waters (oxidation rate constant: kmethanol : 0.02-0.10 day-1 ; kmethylamine : 0.01-0.18 day-1 ), but were also important sources of biomass carbon in coastal waters (assimilation rate constant: kmethanol : 0.04-0.10 day-1 ; kmethylamine : 0.01-0.05 day-1 ). The relative extent of assimilation versus oxidation for these substrates correlated positively with chlorophyll, nutrients and heterotrophic bacterial production. Methanol oxidation and assimilation were stimulated significantly by nutrient addition. In contrast, methylamine metabolism was inhibited by ammonium or nitrate, suggesting that methylamine served as a nitrogen source. A preliminary metagenomic survey revealed a diverse population of putative C1-utilizing microorganisms. These results show that the remineralization of methylamine could provide both C and N sources for microbes. Both methanol and methylamine contribute to microbial energetic and carbon substrate demands with a distinctly different signature in nearshore versus offshore environments.

Long-term variations in oxygen in sub-tropical coastal waters: Influence of sewage effluent

The increase in population in the Pearl River delta region has increased domestic sewage and industrial discharges, which influences biogeochemical (e.g. carbon dioxide release) and environmental (e.g. oxygen depletion) conditions. Dissolved oxygen and dissolved inorganic carbon concentrations along with salinity, temperature, pH, and chlorophyll a at the surface were collected during seven cruises at eight stations in different months in 2005–2006 to document the seasonal influence of anthropogenic inputs on dissolved oxygen in different areas of Hong Kong waters. Long-term data during 1997–2006 were used to assess air-sea exchange of dissolved oxygen. Near the Pearl River estuary, dissolved oxygen was undersaturated, while partial pressure carbon dioxide was oversaturated, which indicated that Pearl River estuary influenced waters were heterotrophic and represented net sources of carbon dioxide to the atmosphere. In Victoria Harbour, where there was greater sewage effluent influence, however, the degree of dissolved oxygen undersaturation was even more than the Pearl River estuary influenced waters throughout the year. In contrast, the eastern waters, where there was less influence of anthropogenic inputs, showed seasonal variability: undersaturation of dissolved oxygen (∼90%) in the dry season shifting to slight oversaturation (∼105%) in the wet season. The monthly average air-sea influx of oxygen decreased by ∼50 to 200 mmol m−2 d−1 in the wet season relative to that in the dry season in Victoria Harbour, which was coupled with an increase in chlorophyll a from the dry to wet season. These findings are important in understanding why the eutrophication impact from nutrient enrichments in Hong Kong waters is not as severe as one would expect, and also how sewage effluent influences biogeochemical processes of dissolved oxygen and carbon in coastal waters.

Dissolved organic carbon: Spectral relationships in coastal waters1

Caution should be observed in using photometry to estimate concentrations of dissolved organic carbon in coastal waters, even within very limited geographical regions.