Molecular Traits of Dissolved Organic Matter in the Subterranean Estuary of a High-Energy Beach: Indications of Sources and Sinks

Advective flows of seawater and fresh groundwater through coastal aquifers form a unique ecohydrological interface, the subterranean estuary (STE). Here, freshly produced marine organic matter and oxygen mix with groundwater, which is low in oxygen and contains aged organic carbon (OC) from terrestrial sources. Along the groundwater flow paths, dissolved organic matter (DOM) is degraded and inorganic electron acceptors are successively used up. Because of the different DOM sources and ages, exact degradation pathways are often difficult to disentangle, especially in high-energy environments with dynamic changes in beach morphology, source composition, and hydraulic gradients. From a case study site on a barrier island in the German North Sea, we present detailed biogeochemical data from freshwater lens groundwater, seawater, and beach porewater samples collected over different seasons. The samples were analyzed for physico-chemistry (e.g., salinity, temperature, dissolved silicate), (reduced) electron acceptors (e.g., oxygen, nitrate, and iron), and dissolved organic carbon (DOC). DOM was isolated and molecularly characterized via soft-ionization ultra-high-resolution mass spectrometry, and molecular formulae were identified in each sample. We found that the islands’ freshwater lens harbors a surprisingly high DOM molecular diversity and heterogeneity, possibly due to patchy distributions of buried peat lenses. Furthermore, a comparison of DOM composition of the endmembers indicated that the Spiekeroog high-energy beach STE conveys chemically modified, terrestrial DOM from the inland freshwater lens to the coastal ocean. In the beach intertidal zone, porewater DOC concentrations, lability of DOM and oxygen concentrations, decreased while dissolved (reduced) iron and dissolved silicate concentrations increased. This observation is consistent with the assumption of a continuous degradation of labile DOM along a cross-shore gradient, even in this dynamic environment. Accordingly, molecular properties of DOM indicated enhanced degradation, and “humic-like” fluorescent DOM fraction increased along the flow paths, likely through accumulation of compounds less susceptible to microbial consumption. Our data indicate that the high-energy beach STE is likely a net sink of OC from the terrestrial and marine realm, and that barrier islands such as Spiekeroog may act as efficient “digestors” of organic matter.

Spatiotemporal Dissolved Silicate Variation, Sources, and Behavior in the Eutrophic Zhanjiang Bay, China

Dissolved silicate (DSi) is an important nutrient in coastal water, which is used by planktonic diatoms for cell division and growth. In this study, surface water samples were collected in the eutrophic Zhanjiang Bay (ZJB) in 2019, covering a seasonal variation of coastal water and land-based source water discharge. The spatiotemporal DSi distribution, land-based sources flux input and behaviors in ZJB were studied and discussed. The results show that the DSi concentration had significant differences in spatiotemporal scale. The average concentration of DSi in ZJB was 38.00 ± 9.48 μmol·L−1 in spring, 20.23 ± 11.27 μmol·L−1 in summer, 12.48 ± 1.42 μmol·L−1 in autumn and 11.96 ± 4.85 μmol·L−1 in winter. The spatiotemporal DSi distribution showed a decreasing gradient from the top to the mouth of ZJB, which was affected by land source inputs and hydrodynamics. The land-based sources’ input concentration of DSi in ZJB ranged from 0.021 to 0.46 mol·L−1, with an average of 0.14 mol·L−1, and the total annual flux of DSi was 1.06 × 109 mol, comprising up to 8.28%, 41.55% and 50.17% in dry, normal, and wet seasons, respectively. The Suixi River contributed the highest DSi flux proportion in all seasons. The DSi in land-based source water was mainly affected by water flow discharge, diatom uptake and impacts from anthropogenic activities. Compared with dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), the DSi was the limitation nutrient in ZJB. Additionally, the DSi concentration in the coastal water was negatively correlated with salinity. The seasonal DSi/DIN and DSi/DIP ratios in land-based sources discharge water was significantly higher than that in coastal water (p < 0.05). Land-based sources of DSi input played an important role in nutrients composition that sustained diatoms as the dominant species in ZJB.

Subduction-Zone Fluids

Fluids are essential to the physical and chemical processes in subduction zones. Two types of subduction-zone fluids can be distinguished. First, shallow fluids, which are relatively dilute and water rich and that have properties that vary between subduction zones depending on the local thermal regime. Second, deep fluids, which possess higher proportions of dissolved silicate, salts and non-polar gases relative to water content, and have properties that are broadly similar in most subduction systems, regardless of the local thermal structure. We review key physical and chemical properties of fluids in two key subduction-zone contexts—along the slab top and beneath the volcanic front—to illustrate the distinct properties of shallow and deep subduction-zone fluids.

Comparison of phytoplankton community structure in two tropical estuaries of East Coast of India

Phytoplankton community of two tropical river estuaries of the North-east coast of India was different due to dissolved nutrients concentration in those estuarine waters. The first study site was the Saptamukhi river estuary which is located in the Indian Sundarban (inside mangrove forest) and the second study site was Mahanadi estuary located in Orissa coast (adjacent to industrial and coastal fishing zone). The Saptamukhi estuary was mostly devoid of any anthropogenic influence and here the nutrient source was auto-generated. In Mahanadi estuarine water, the source of the major nutrients was industrial effluent and fishing waste. The Saptamukhi estuarine water had less dissolved phosphorus concentration but huge dissolved nitrogen available for phytoplankton uptake. A sufficient amount of dissolved silicate encouraged the diatom growth over other phytoplankton groups here. Due to the availability of the high amount of dissolved phosphorus and the low amount of dissolved nitrogen in the Mahanadi estuary the phytoplankton community was a mixed population of Bacillariophyceae, Cyanophyceae, Chlorophyceae and Dinophyceae and seasonal Chlorophycean bloom observed during the study period. In both the estuarine water primary productivity was high but the community respiration was higher and the estuaries were heterotrophic. Monsoonal runoff from land considerably changed the community in both estuarine water. Dissolved silicate concentration in both the estuarine water was sufficient for Diatom growth. Dissolve inorganic nitrogen and dissolved inorganic phosphate ratio played a major role for the community change of phytoplankton in two estuarine waters.

Molecular properties of dissolved organic matter (DOM) in the subterranean estuary of a high-energy beach: Finding proxies for reactive transport

&lt;p&gt;Advective flows of sea- and fresh groundwater through coastal aquifers form a unique ecohydrological interface, the subterranean estuary. Here, freshly produced marine organic matter and oxygen mix with groundwater, which is low in oxygen and contains aged organic carbon from terrestrial sources. Along the underground flow paths, dissolved organic matter (DOM) is degraded and inorganic electron acceptors are successively used up. Because of the different DOM sources and ages, exact degradation pathways are often difficult to delineate, especially in high-energy environments with dynamic changes in beach morphology, source composition, and hydraulic gradients. From a case study site on a barrier island in the German North Sea, we present detailed biogeochemical data from pore water samples collected in the shallow layer of the subterranean estuary. The samples were taken along the major flow paths of recirculating sea water and discharging fresh, meteoric groundwater, and analyzed for physico-chemistry, electron acceptors, and dissolved organic carbon (DOC). DOM was isolated and measured with soft-ionization ultra-high-resolution mass spectrometry, and chemical DOM characteristics were derived by assigning exact molecular formulae to the thousands of intact masses found in each sample. Using geographic and physico-(geo)chemical parameters (longitude, salinity, dissolved silicate, dissolved iron) as indicators of water origin and residence time, we evaluated the behavior of chemical DOM characteristics (H/C and O/C ratios, aromaticity) along the underground flow paths. Overall, DOC concentrations and an H/C-based molecular lability boundary index (MLB) decreased with decreasing oxygen concentrations and parallel increases of dissolved (reduced) iron and dissolved silicate concentrations, in line with the assumption that high H/C ratios are a trait of labile DOM which is continuously degraded. On the other hand, aromaticity indices and relative abundances of a &amp;#8220;humic-like&amp;#8221; fluorescent DOM fraction increased along the flow paths, likely through accumulation of compounds less susceptible to microbial attack. Our data indicates that even in a highly complex advective flow system like the subterranean estuary, molecular properties of DOM can be harnessed to identify key, perhaps even site- and season-specific biogeochemical processes.&lt;/p&gt;