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Abstract Fiddler crabs, as coastal ecosystem engineers, play a crucial role in enhancing biodiversity and accelerating the flow of material and energy. Here we show how widespread crab burrows modify the carbon sequestration capacity of different habitats across a large climatic gradient. The process of crab burrowing results in the reallocation of sediment organic carbon and humus. Crab burrows can increase more greenhouse gases emissions compared to the sediment matrix (CO2: by 17–30%; CH4: by 49–141%). Straightforward calculations indicate that these increased emissions could offset 35–134% of sediment carbon burial in these two ecosystems. This research highlights the complex interactions between crab burrows, habitat type, and climate which reveal a potential lower carbon sink function of blue carbon ecosystems than previously expected without considering crab burrows.

Abstract 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.

Loss of tidal wetlands is a world-wide phenomenon. Many factors may contribute to such loss, but among them are geochemical stressors such as exposure of the marsh plants to elevated levels on hydrogen sulfide in the pore water of the marsh peat. Here we report the results of a study of the geochemistry of iron and sulfide at different seasons in unrestored (JoCo) and partially restored (Big Egg) salt marshes in Jamaica Bay, a highly urbanized estuary in New York City where the loss of salt marsh area has accelerated in recent years. The spatial and temporal 2-dimensional distribution patterns of dissolved Fe2+ and H2S in salt marshes were in situ mapped with high resolution planar sensors for the first time. The vertical profiles of Fe2+ and hydrogen sulfide, as well as related solutes and redox potentials in marsh were also evaluated by sampling the pore water at discrete depths. Sediment cores were collected at various seasons and the solid phase Fe, S, N, C, and chromium reducible sulfide in marsh peat at discrete depths were further investigated in order to study Fe and S cycles, and their relationship to the organic matter cycling at different seasons. Our results revealed that the redox sensitive elements Fe2+ and S2– showed significantly heterogeneous and complex three dimensional distribution patterns in salt marsh, over mm to cm scales, directly associated with the plant roots due to the oxygen leakage from roots and redox diagenetic reactions. We hypothesize that the oxic layers with low/undetected H2S and Fe2+ formed around roots help marsh plants to survive in the high levels of H2S by reducing sulfide absorption. The overall concentrations of Fe2+ and H2S and distribution patterns also seasonally varied with temperature change. H2S level in JoCo sampling site could change from 5 mM in fall season, reflecting significantly seasonal variation in the rates of bacterial oxidation of organic matter at this marsh site. Solid phase Fe and S showed that very high fractions of the diagenetically reactive iron at JoCo and Big Egg were associated with pyrite that can persist for long periods in anoxic sediments. This implies that there is insufficient diagenetically reactive iron to buffer the pore water hydrogen sulfide through formation of iron sulfides at JoCo and Big Egg.

Rapid economic development in coastal areas has gradually increased the risk of coastal water quality deterioration. The assessment methods of coastal water quality are multifarious, but many depend on either subjective judgment or objective calculation. We proposed a weighted sum methodology by integrating the subjective analytic hierarchy process and objective entropy theory (AHP-entropy weight methodology) to obtain an overall evaluation of coastal water quality. The mathematical models to transform the biochemical and physical parameter values and soluble substance concentrations into index scores have been formulated in comparison to the national water quality classification scheme. The application of the AHP-entropy weight methodology was demonstrated in the nearshore area of Yangjiang city, China, based on 23 seawater sampling stations in autumn 2017 and spring 2018. Datasets including biochemical and physical parameters, nutrients, and heavy metals have been converted into water quality index scores based on the proposed mathematical model. Results revealed that the overall water quality fell into the "good" class in both sampling seasons. The spatial distribution of the water quality index scores demonstrated that the relatively worse water quality occurred in estuarine and nearshore areas, signifying the negative effect of coastal anthropogenic activities. The statistical analyses like the hierarchical cluster analysis interpreted that the river input acted as a main source of pollutants in the study area. The AHP-entropy weight methodology could be a preferred way to assist decision-makers in properly evaluating the current state of coastal water quality in an unbiased, objective manner., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Groundwater springs in karstified carbonate aquifers are known to transport carbon, nutrients and trace elements to the coastal ocean. The biogeochemical significance of submarine karstic springs and their impact on coastal primary production are often difficult to quantify. We investigated several karstic springs, including the first-order Port-Miou spring, in an urbanized watershed that is also severely impacted by sewage effluent (Calanques of Marseille-Cassis, France). Karstic springs were elevated in major nutrients and bioactive trace metals over Mediterranean seawater, with relatively low concentration ranges. Groundwater NO 3 - was likely derived from atmosphere-aquifer interactions, while DOC:DON ratios reveal that NO 2 - and NH 4 + was autochthonously produced during mixing between karst groundwater and seawater. Submarine groundwater discharge (SGD) during March 2018 (wet season, baseflow conditions) was 6.7 ± 2.0 m 3  s -1 for the entire investigated coastline, determined from simultaneous 224 Ra and 226 Ra mass balances. The contribution of groundwater PO 4 3- , the major limiting nutrient of the Mediterranean Sea, sustained only 1% of primary production adjacent to sewage outfall, but between 7 and 100% of the local primary production in areas that were not impacted by sewage. Groundwater and seawater Fe:DIN and Fe:DIP ratios suggest that Fe was not a limiting micro-nutrient during the period of study, where bioactive trace metal fluxes were dominated by sewage and atmospheric deposition, although excess Fe from groundwater may locally enhance N fixation. Groundwater solute fluxes may easily vary by a factor of two or more over time because karst aquifers are sensitive to precipitation, as is the case of the regional carbonate karstified aquifer of Port-Miou, highlighting the critical importance of properly characterizing nutrient and trace metal inputs in these coastal environments., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Inputs of submarine groundwater discharge (SGD) to the coastal ocean may alter local and regional-scale biology. Here, we report on nematode assemblages along the north shore of Long Island, NY. We test if nematode communities differed between sites impacted by mixed fresh-saline SGD and where SGD is exclusively saline. Diversity of nematodes was low at sites impacted by fresh SGD and communities were dominated by a few opportunistic genera. Moreover, a set of typical freshwater nematode genera restricted to impacted sites was observed. Their presence in the marine coastal zone is exceptional and underlines the structuring role that fresh SGD plays in the local ecosystem. Saline SGD structured nematode assemblages differently compared to sites impacted by fresh SGD. The number of nematode genera was markedly higher at saline SGD sites, with a different community structure. This study highlights the importance to which inputs of fresh SGD may have on local ecosystem diversity in marine coastal environments., (Copyright © 2018 Elsevier Ltd. All rights reserved.)