Explore the vast range of titles available.

In Climate Change and Marine and Freshwater Toxins the editors have assembled contributions from a team of international experts to expand the framework for an appropriate assessment of climate change impacts on aquatic toxins.

Although halogen radicals are recognized to form as products of hydroxyl radical (•OH) scavenging by halides, their contribution to the phototransformation of marine organic compounds has received little attention. We demonstrate that, relative to freshwater conditions, seawater halides can increase photodegradation rates of domoic acid, a marine algal toxin, and dimethyl sulfide, a volatile precursor to cloud condensation nuclei, up to fivefold. Using synthetic seawater solutions, we show that the increased photodegradation is specific to dissolved organic matter (DOM) and halides, rather than other seawater salt constituents (e.g., carbonates) or photoactive species (e.g., iron and nitrate). Experiments in synthetic and natural coastal and estuarine water samples demonstrate that the halide-specific increase in photodegradation could be attributed to photochemically generated halogen radicals rather than other photoproduced reactive intermediates [e.g., excited-state triplet DOM (3DOM*), reactive oxygen species]. Computational kinetic modeling indicates that seawater halogen radical concentrations are two to three orders of magnitude greater than freshwater •OH concentrations and sufficient to account for the observed halide-specific increase in photodegradation. Dark •OH generation by gamma radiolysis demonstrates that halogen radical production via •OH scavenging by halides is insufficient to explain the observed effect. Using sensitizer models for DOM chromophores, we show that halogen radicals are formed predominantly by direct oxidation of Cl⁻ and Br⁻ by ³DOM*, an •OH-independent pathway. Our results indicate that halogen radicals significantly contribute to the phototransformation of algal products in coastal or estuarine surface waters.

Invisible to the naked eye lies a tremendous diversity of organic molecules and organisms that make major contributions to important biogeochemical cycles. However, how the diversity and composition of these two communities are interlinked remains poorly characterized in fresh waters, despite the potential for chemical and microbial diversity to promote one another. Here we exploited gradients in chemodiversity within a common microbial pool to test how chemical and biological diversity covary and characterized the implications for ecosystem functioning. We found that both chemodiversity and genes associated with organic matter decomposition increased as more plant litterfall accumulated in experimental lake sediments, consistent with scenarios of future environmental change. Chemical and microbial diversity were also positively correlated, with dissolved organic matter having stronger effects on microbes than vice versa. Under our experimental scenarios that increased sediment organic matter from 5 to 25% or darkened overlying waters by 2.5 times, the resulting increases in chemodiversity could increase greenhouse gas concentrations in lake sediments by an average of 1.5 to 2.7 times, when all of the other effects of litterfall and water color were considered. Our results open a major new avenue for research in aquatic ecosystems by exposing connections between chemical and microbial diversity and their implications for the global carbon cycle in greater detail than ever before.

Poly(butylene adipate-co-terephthalate) (PBAT) and poly(lactic acid) (PLA) are well-known biodegadable polyesters due to their outstanding performance. The biodegradation behavior of PLA/PBAT blends in freshwater with sediment is investigated in this study by analyzing the appearance, chemical structure and aggregation structure of their degraded residues via SEM, TG, DSC, gel permeation chromatography (GPC) and XPS. The effect of aggregation structure, hydrophilia and biodegradation mechanisms of PBAT and PLA on the biodegradability of PLA/PBAT blends is illuminated in this work. After biodegradation, the butylene terephthalate unit in the molecular structure of the components and the molecular weight of PLA/PBAT blends decreased, while the content of C-O bond in the composites increased, indicating that the samples indeed degraded. After 24 months of degradation, the increase in the relative peak area proportion of C-O to C=O in PLA/PBAT-25, PLA/PBAT-50 and PLA/PBAT-75 was 62%, 46% and 68%, respectively. The biodegradation rates of PBAT and PLA in the PLA/PBAT blend were lower than those for the respective single polymers.

Advances in water chemistry in the last decade have improved our knowledge about the genesis, composition, and structure of dissolved organic matter, and its effect on the environment. Improvements in analytical technology, for example Fourier-transform ion cyclotron (FT-ICR) mass spectrometry (MS), homo and hetero-correlated multidimensional nuclear magnetic resonance (NMR) spectroscopy, and excitation emission matrix fluorimetry (EEMF) with parallel factor (PARAFAC) analysis for UV–fluorescence spectroscopy have resulted in these advances. Improved purification methods, for example ultrafiltration and reverse osmosis, have enabled facile desalting and concentration of freshly collected DOM samples, thereby complementing the analytical process. Although its molecular weight (MW) remains undefined, DOM is described as a complex mixture of low-MW substances and larger-MW biomolecules, for example proteins, polysaccharides, and exocellular macromolecules. There is a general consensus that marine DOM originates from terrestrial and marine sources. A combination of diagenetic and microbial processes contributes to its origin, resulting in refractory organic matter which acts as carbon sink in the ocean. Ocean DOM is derived partially from humified products of plants decay dissolved in fresh water and transported to the ocean, and partially from proteinaceous and polysaccharide material from phytoplankton metabolism, which undergoes in-situ microbial processes, becoming refractory. Some of the DOM interacts with radiation and is, therefore, defined as chromophoric DOM (CDOM). CDOM is classified as terrestrial, marine, anthropogenic, or mixed, depending on its origin. Terrestrial CDOM reaches the oceans via estuaries, whereas autochthonous CDOM is formed in sea water by microbial activity; anthropogenic CDOM is a result of human activity. CDOM also affects the quality of water, by shielding it from solar radiation, and constitutes a carbon sink pool. Evidence in support of the hypothesis that part of marine DOM is of terrestrial origin, being the result of a long-term carbon sedimentation, has been obtained from several studies discussed herein.

This study investigated the trophic transfer, individual impact, and embryonic uptake of fluorescent nano-sized polystyrene plastics (nanoplastics) through direct exposure in a freshwater ecosystem, with a food chain containing four species. The alga Chlamydomonas reinhardtii , water flea Daphnia magna , secondary-consumer fish Oryzias sinensis , and end-consumer fish Zacco temminckii were used as test species. In the trophic transfer test, algae were exposed to 50 mg/L nanoplastics, defined as plastic particles <100 nm in diameter; higher trophic level organisms were exposed through their diet. In the direct exposure test, each species was directly exposed to nanoplastics. Microscopic analysis confirmed that the nanoplastics adhered to the surface of the primary producer and were present in the digestive organs of the higher trophic level species. Nanoplastics also negatively affected fish activity, as measured by distance traveled and area covered, and induced histopathological changes in the livers of fish that were directly exposed. Additionally, nanoplastics penetrated the embryo walls and were present in the yolk sac of hatched juveniles. These observations clearly show that nanoplastics are easily transferred through food chain, albeit because of high experimental dosages. Nevertheless, the results strongly point to the potential health risks of nanoplastic exposure.

An analysis of the relative effects of transpiration and evaporation, which can be distinguished by how they affect isotope ratios in water, shows that transpiration is by far the largest water flux from Earth’s continents, representing 80 to 90 per cent of terrestrial evapotranspiration and using half of all solar energy absorbed by land surfaces. Plants dominant in water-flux calculations Water fluxes from the land surface to the atmosphere are divided between evaporation, and transpiration from leaf stomata. Although a seemingly basic division between the physical and biological, there is still no consensus on the global partitioning between the two fluxes, resulting in uncertainties as to responses to future climate variations. Now, Scott Jasechko and colleagues use the isotopic signatures of transpiration and evaporation from a global data set of large lakes and reveal that enormous quantities of water — as much as 90% of total terrestrial evapotranspiration — are cycled through vegetation via transpiration. One conclusion to be drawn from this study is that the accuracy of biological — rather than physical — fluxes should be prioritized in work to improve climate models. Renewable fresh water over continents has input from precipitation and losses to the atmosphere through evaporation and transpiration. Global-scale estimates of transpiration from climate models are poorly constrained owing to large uncertainties in stomatal conductance and the lack of catchment-scale measurements required for model calibration, resulting in a range of predictions spanning 20 to 65 per cent of total terrestrial evapotranspiration (14,000 to 41,000 km 3 per year) (refs 1 , 2 , 3 , 4 , 5 ). Here we use the distinct isotope effects of transpiration and evaporation to show that transpiration is by far the largest water flux from Earth’s continents, representing 80 to 90 per cent of terrestrial evapotranspiration. On the basis of our analysis of a global data set of large lakes and rivers, we conclude that transpiration recycles 62,000 ± 8,000 km 3 of water per year to the atmosphere, using half of all solar energy absorbed by land surfaces in the process. We also calculate CO 2 uptake by terrestrial vegetation by connecting transpiration losses to carbon assimilation using water-use efficiency ratios of plants, and show the global gross primary productivity to be 129 ± 32 gigatonnes of carbon per year, which agrees, within the uncertainty, with previous estimates 6 . The dominance of transpiration water fluxes in continental evapotranspiration suggests that, from the point of view of water resource forecasting, climate model development should prioritize improvements in simulations of biological fluxes rather than physical (evaporation) fluxes.

Over the past few decades, groundwater wells installed in rural areas throughout the major river basins draining the Himalayas have become the main source of drinking water for tens of millions of people. Groundwater in this region is much less likely to contain microbial pathogens than surface water but often contains hazardous amounts of arsenic--a known carcinogen. Arsenic enters groundwater naturally from rocks and sediment by coupled biogeochemical and hydrologic processes, some of which are presently affected by human activity. Mitigation of the resulting health crisis in South and Southeast Asia requires an understanding of the transport of arsenic and key reactants such as organic carbon that could trigger release in zones with presently low groundwater arsenic levels.

Carbon dioxide (CO 2 ) transfer from inland waters to the atmosphere, known as CO 2 evasion, is a component of the global carbon cycle. Global estimates of CO 2 evasion have been hampered, however, by the lack of a framework for estimating the inland water surface area and gas transfer velocity and by the absence of a global CO 2 database. Here we report regional variations in global inland water surface area, dissolved CO 2 and gas transfer velocity. We obtain global CO 2 evasion rates of 1.8  petagrams of carbon (Pg C) per year from streams and rivers and 0.32  Pg C yr −1 from lakes and reservoirs, where the upper and lower limits are respectively the 5th and 95th confidence interval percentiles. The resulting global evasion rate of 2.1 Pg C yr −1 is higher than previous estimates owing to a larger stream and river evasion rate. Our analysis predicts global hotspots in stream and river evasion, with about 70 per cent of the flux occurring over just 20 per cent of the land surface. The source of inland water CO 2 is still not known with certainty and new studies are needed to research the mechanisms controlling CO 2 evasion globally. An analysis of regional variations in global inland water surface area, dissolved CO 2 and gas transfer velocity yields a global CO 2 evasion rate of 2.1 × 10 15 grams of carbon per year, which is higher than previous estimates owing to a larger contribution from streams and rivers. CO 2 transfer via inland waters There has been a growing recognition in recent decades that inland waters release considerable amounts of CO 2 into the atmosphere. Until now it has been difficult to obtain global estimates of how much carbon is transferred in this way. This study combines new data sets with a systematic approach to the problem and estimates the global emission rate at about two petagrams of carbon per year from streams, rivers, lakes and reservoirs, which is higher than previous figures because of the contributions from streams and rivers. This new analysis also identifies global hotspots in stream and river evasion, with about 70% of the CO 2 flux occurring over just 20% of the land surface.

Under global change, the ion concentration of aquatic ecosystems is changing worldwide. Many freshwater ecosystems are being salinized by anthropogenic salt inputs, whereas many naturally saline ones are being diluted by agricultural drainages. This occurs concomitantly with changes in other stressors, which can result in additive, antagonistic or synergistic effects on organisms. We reviewed experimental studies that manipulated salinity and other abiotic stressors, on inland and transitional aquatic habitats, to (i) synthesize their main effects on organisms' performance, (ii) quantify the frequency of joint effect types across studies and (iii) determine the overall individual and joint effects and their variation among salinity–stressor pairs and organism groups using meta-analyses. Additive effects were slightly more frequent (54%) than non-additive ones (46%) across all the studies ( n = 105 responses). However, antagonistic effects were dominant for the stressor pair salinity and toxicants (44%, n = 43), transitional habitats (48%, n = 31) and vertebrates (71%, n = 21). Meta-analyses showed detrimental additive joint effects of salinity and other stressors on organism performance and a greater individual impact of salinity than the other stressors. These results were consistent across stressor pairs and organism types. These findings suggest that strategies to mitigate multiple stressor impacts on aquatic ecosystems should prioritize restoring natural salinity concentrations. This article is part of the theme issue ‘Salt in freshwaters: causes, ecological consequences and future prospects’.