This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure ...involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come.
Abundant crab burrows in carbon‐rich, muddy salt marsh soils act as preferential water flow conduits, potentially enhancing carbon transport across the soil–water interface. With increasing ...recognition of blue carbon systems (salt marshes, mangroves, and seagrass) as hotspots of soil carbon sequestration, it is important to understand drivers of soil carbon cycling and fluxes. We conducted field observations and flow modeling to assess how crab burrows drive carbon exchange over time scales of minutes to weeks in an intertidal marsh in South Carolina. Results showed that continuous advective porewater exchange between the crab burrows and the surrounding soil matrix occurs because of tidally driven hydraulic gradients. The concentrations of dissolved inorganic (DIC) and organic (DOC) carbon in crab burrow porewater differ with that in the surrounding soil matrix, implying a diffusive C flux in the low‐permeability marsh soil. Gas‐phase concentrations of CO2 in ∼ 300 crab burrows were approximately six times greater than ambient air. The estimated total C export rate via porewater exchange (1.0 ± 0.7 g C m−2 d−1) was much greater than via passive diffusion transport (6.7 ± 2 mg C m−2 d−1) and gas‐phase CO2 release (0.93 mg C m−2 d−1). The burrow‐related carbon export was comparable to the regional salt marsh DIC export, groundwater‐derived DIC export, and the net primary production previously estimated using ecosystem‐scale approaches. These insights reveal how crab burrows modify blue carbon sequestration in salt marshes and contribute to coastal carbon budgets.
Wastewater treatment plants (WWTPs) are considered the main sources of chemicals of emerging concern (CECs) in aquatic environments, and can negatively impact aquatic ecosystems. In this study, WWTP ...influent, effluent, and sludge, and upstream and downstream waters from the WWTP recipient were investigated at 15 locations for a total of 164 CECs, including pharmaceuticals, personal care products, industrial chemicals, per- and polyfluoroalkyl substances (PFASs), and pesticides. In addition, zebrafish (Danio rerio) embryo toxicity tests (ZFET) were applied to WWTP influent and effluent, and upstream and downstream waters from WWTP recipients. A total of 119 CECs were detected in at least one sample, mean concentrations ranging from 0.11 ng/L (propylparaben) to 64,000 ng/L (caffeine), in wastewater samples and from 0.44 ng/L (ciprofloxacin) to 19,000 ng/L (metformin) in surface water samples. Large variations of CEC concentrations were found between the selected WWTPs, which can be explained by differences in CEC composition in influent water and WWTP treatment process. The sludge-water partitioning coefficient (Kd) of CECs showed a significant linear correlation to octanol/water partition coefficient (KOW) (p < 0.001), and thus could be used for predicting their fate in the aqueous and solid phase. The ΣCEC concentrations in WWTPs declined by on average 60%, based on comparisons of WWTP influent and effluent concentrations. The high concentrations of CECs in WWTP effluent resulted in, on average, 50% higher concentrations of CECs in water downstream of WWTPs compared with upstream. Some WWTP samples showed toxicity in ZFET compared with the respective control group, but no individual CECs or groups of CECs could explain this toxicity. These results could provide a theoretical basis for optimization of existing treatment systems of different designs, and could significantly contribute to protecting recipient waters.
Display omitted
•164 CECs s were studied in 15 WWTPs and its recipients•The zebrafish embryo toxicity tests were applied to WWTP and recipient samples•Most target CECs were efficiently removed in the WWTPs•The zebrafish embryos were not affected by the effluent and recipient samples
Only a few dozens of the several thousand existing per- and polyfluoroalkyl substances (PFAS) are monitored usingconventional target analysis. This study employed suspectscreening to examine patterns ...of emerging and novel PFAS inGerman and Chinese river water affected by industrial pointsources. In total, 86 PFAS were (tentatively) identified andgrouped into 18 structure categories. Homologue patterns revealeddistinct differences betweenfluoropolymer production sites of thetwo countries. In the Chinese Xiaoqing River Basin, the C8homologue was the most prevalent compound of the emergingseries of chlorinated perfluoroalkyl carboxylic acids (Cl-PFCAs)and perfluoroalkylether carboxylic acids (PFECAs). In contrast, C6and shorter homologues were dominant in the German Alz River.This indicates that the phaseout of long-chain compounds in Europe and their ongoing production in Asian countries also apply tounregulated emerging PFAS classes. Additional characteristics to differentiate the point sources were the peak area ratio ofperfluorobutane sulfonic acid (PFBS) versus the emerging compound hydro-substituted PFBS (H-PFBS) as well as the occurrenceof byproducts of the sulfonated tetrafluoroethylene-based polymer Nafion. The large number of identified unregulated PFASunderlines the importance of a grouping approach on a regulatory level, whereas the revealed contamination patterns can be used toestimate, prioritize, and minimize contributions of specific sources
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.
Display omitted
•Odum's outwelling hypothesis is framed in a marine carbon sequestration context.•Mangrove, saltmarsh, seagrass and macroalgae ecosystems effectively sequester carbon.•Outwelling may exceed sediment burial as a long-term carbon sequestration mechanism.•Carbon outwelling estimates remain spatially and temporally limited.•It is time to focus not only on local carbon burial, but also exports to the sea.
Free-flowing rivers (FFRs) support diverse, complex and dynamic ecosystems globally, providing important societal and economic services. Infrastructure development threatens the ecosystem processes, ...biodiversity and services that these rivers support. Here we assess the connectivity status of 12 million kilometres of rivers globally and identify those that remain free-flowing in their entire length. Only 37 per cent of rivers longer than 1,000 kilometres remain free-flowing over their entire length and 23 per cent flow uninterrupted to the ocean. Very long FFRs are largely restricted to remote regions of the Arctic and of the Amazon and Congo basins. In densely populated areas only few very long rivers remain free-flowing, such as the Irrawaddy and Salween. Dams and reservoirs and their up- and downstream propagation of fragmentation and flow regulation are the leading contributors to the loss of river connectivity. By applying a new method to quantify riverine connectivity and map FFRs, we provide a foundation for concerted global and national strategies to maintain or restore them.
PDF
