Increasing water nutrient reduces the availability of high-quality food resources for aquatic consumers and consequently simplifies river food webs.

While eutrophication has led to serious habitat degradation and biotic shifts in freshwater ecosystems, most current studies have focused on changes in community assemblages, with few considering the effect of eutrophication on food webs. We conducted a field study in subtropical headwater streams with a gradient of nutrient levels to examine the effect of increasing water nutrients on food webs by using the long-chain polyunsaturated fatty acid eicosapentaenoic acid (EPA) as a measure of the nutritional quality of food. Basal food resources (macrophytes, submerged leaf litter, and periphyton), and aquatic consumers (macroinvertebrates and fish) were collected, and their fatty acid (FA) profiles were analyzed. Our results showed that periphyton was the dominant source of EPA for macroinvertebrates and fish, and a high-quality resource for consumers. As water nutrient concentrations increased, nutritional quality of periphyton significantly decreased and, in turn, the correlation between FA profiles of periphyton and macroinvertebrates declined. However, periphyton FA profiles did not account for the variability of fish FA, which may be induced by the increasing proportions of omnivorous fish in eutrophic streams that derive EPA from other sources. Further, the reduced periphyton EPA was associated with decreased trophic links and simplified stream food webs. Our study highlights the importance of high-quality food resources for aquatic food webs as water nutrients increase in stream ecosystems and provides a nutritional perspective to understand the mechanisms how eutrophication affects aquatic ecosystems.

How do small dams alter river food webs? A food quality perspective along the aquatic food web continuum.

Damming of rivers poses a significant threat to freshwater ecosystems. Previous studies about the impact of damming on river ecosystems have mostly focused on large dams, with the impact of small dams largely unknown. Further, while the impacts of dams on aquatic communities have been widely studied, the effect on energy flow across river food webs remains unclear. In recent years, long-chain polyunsaturated fatty acid analysis (LC-PUFA) has emerged as a promising technique for assessing food quality and trophic interactions. In this study, LC-PUFA was applied to explore the nutritional effects of small dams on river food webs. A field investigation was conducted at upstream and downstream areas of three small dams in the headwaters of Dongjiang River, China, to evaluate the impact of small dams on the nutritional quality of basal food sources, and their consequent impacts on aquatic consumers and trophic links. Basal food sources (i.e., submerged leaves, macrophytes and periphyton) and aquatic consumers (i.e., macroinvertebrates and fish) were collected, and their fatty acid (FA) composition was measured. Our results showed that periphyton, rather than submerged leaves and macrophytes, was the primary high-quality food source for aquatic consumers, providing them with LC-PUFA, irrespective of whether sites were upstream or downstream. Damming the streams induced changes in aqueous nutrient concentrations (TP, PO4-P, DIN, and TN) from upstream to downstream of the dams, leading to significant variation in periphyton FA content. Compared with periphyton collected at downstream sites, periphyton at upstream sites contained higher LC-PUFA, but lower short-chain PUFA. Differences in periphyton LC-PUFA between the upstream and downstream areas of dams were reflected in the FA profiles of invertebrate grazers and filterers, and further transferred to fish. Furthermore, decreased periphyton nutritional quality at the downstream of the dams was one of the reasons for the simplification of stream food webs. Our results indicated that small dams negatively affected food webs, emphasizing the importance of high-quality food sources for stream ecosystems. We suggest that the trophic integrity of river food webs hinges on the dietary availability of periphyton supplying physiologically highly required nutrients for consumers and must thus not be compromised by damming of streams or other alterations.

Response of macrophyte litter decomposition in global blue carbon ecosystems to climate change.

Blue carbon ecosystems (BCEs) are important nature-based solutions for climate change-mitigation. However, current debates question the reliability and contribution of BCEs under future climatic-scenarios. The answer to this question depends on ecosystem processes driving carbon-sequestration and -storage, such as primary production and decomposition, and their future rates. We performed a global meta-analysis on litter decomposition rate constants (k) in BCEs and predicted changes in carbon release from 309 studies. The relationships between k and climatic factors were examined by extracting remote-sensing data on air temperature, sea-surface temperature, and precipitation aligning to the decomposition time of each experiment. We constructed global numerical models of litter decomposition to forecast k and carbon release under different scenarios. The current k averages at 27 ± 3 × 10-2 day-1 for macroalgae were higher than for seagrasses (1.7 ± 0.2 × 10-2 day-1 ), mangroves (1.6 ± 0.1 × 10-2 day-1 ) and tidal marshes (5.9 ± 0.5 × 10-3 day-1 ). Macrophyte k increased with both air temperature and precipitation in intertidal BCEs and with sea surface temperature for subtidal seagrasses. Above a temperature threshold for vascular plant litter at ~25°C and ~20°C for macroalgae, k drastically increased with increasing temperature. However, the direct effect of high temperatures on k are obscured by other factors in field experiments compared with laboratory experiments. We defined "fundamental" and "realized" temperature response to explain this effect. Based on relationships for realized temperature response, we predict that proportions of decomposed litter will increase by 0.9%-5% and 4.7%-28.8% by 2100 under low- (2°C) and high-warming conditions (4°C) compared to 2020, respectively. Net litter carbon sinks in BCEs will increase due to higher increase in litter C production than in decomposition by 2100 compared to 2020 under RCP 8.5. We highlight that BCEs will play an increasingly important role in future climate change-mitigation. Our findings can be leveraged for blue carbon accounting under future climate change scenarios.

The role of bio-geomorphic feedbacks in shaping microplastic burial in blue carbon habitats.

Coastal sediments are considered as hotspots of microplastics (MPs), with substantial MPs stocks found in blue carbon habitats such as mangroves and tidal marshes, where wave-damping vegetation reduces sediment erosion and enhances accretion. Here, we examined the effects of such bio-geomorphic feedbacks in shaping MPs burial, through a year-round field study in a mangrove habitat along the coast of South China. The results revealed that MPs abundance decreased significantly with the increase of cumulative sediment erosion as the strength of bio-geomorphic feedbacks declined. More shapes and colors of MPs were found at locations with weaker waves and less sediment erosion, where the average particle size was also higher. Our findings highlight the importance of bio-geomorphic feedbacks in affecting both the abundance and characteristics of the buried MPs. Such knowledge extends our understanding of MPs transport and burial from the perspective of bio-geomorphology, which is essential to assess and predict MPs accumulation patterns as well as its impacts on ecosystem functioning of the blue carbon habitats.

Fate and Effects of Macro- and Microplastics in Coastal Wetlands.

Coastal wetlands trap plastics from terrestrial and marine sources, but the stocks of plastics and their impacts on coastal wetlands are poorly known. We evaluated the stocks, fate, and biological and biogeochemical effects of plastics in coastal wetlands with plastic abundance data from 112 studies. The representative abundance of plastics that occurs in coastal wetland sediments and is ingested by marine animals reaches 156.7 and 98.3 items kg-1, respectively, 200 times higher than that (0.43 items kg-1) in the water column. Plastics are more abundant in mangrove forests and tidal marshes than in tidal flats and seagrass meadows. The variation in plastic abundance is related to climatic and geographic zones, seasons, and population density or plastic waste management. The abundance of plastics ingested by pelagic and demersal fish increases with fish length and dry weight. The dominant characteristics of plastics ingested by marine animals are correlated with those found in coastal wetland sediments. Microplastics exert negative effects on biota abundance and mangrove survival but positive effects on sediment nutrients, leaf drop, and carbon emission. We highlight that plastic pollution is widespread in coastal wetlands and actions are urged to include microplastics in ecosystem health and degradation assessment.

Effects of food and feeding regime on CO2 fluxes from mangrove consumers - Do marine benthos breathe what they eat?

Intertidal benthos link tertiary predators and primary producers in marine food webs as well as directly contribute to sediment CO2 emission. However, current methods for studying food sources of marine benthos are time-consuming and does not allow direct estimates on feeding regime-related (including different diets, active versus dormant) CO2 production. We examined the food sources of mangrove crabs and gastropods as well as their corresponding CO2 production using cavity-ring down spectroscopy to measure the δ13C-CO2 respiration for consumers, considering the effects of feeding regime, benthos taxa, and dominant feeding habit. Benthos taxa and feeding habit have significant impact on δ13C-CO2 respiration. Particularly, the δ13C-CO2 respiration for crabs (-23.9 ± 0.4‰) was significantly lower than that for gastropods (-17.5 ± 1.3‰). The δ13C-CO2 respiration for deposit-feeders was significantly higher than that for detritivores. There are significant differences in the amount of CO2 emitted and δ13C-CO2 respiration for crabs under different feeding regimes. The differences reflect diet-switching and fuel-switching by the crabs, i.e. 'you breathe what you eat'. Significant differences in CO2 production of crabs also exist between those feeding on microphytobenthos in the laboratory (0.13 ± 0.02 mmol g-1 day-1) and on field collection (i.e. just collected from the field) (0.31 ± 0.03 mmol g-1 day-1). CO2 production of crabs is strongly related to carapace width and length. The δ13C-CO2 respiration for mangrove crabs reflects their diet while crab-respired CO2 flux is related to crab size. These relationships enable partitioning the feeding habit and food sources of key benthos, and help incorporate their contribution into the overall sediment-atmosphere CO2 fluxes in mangrove forests.

The impact of super-typhoon Mangkhut on sediment nutrient density and fluxes in a mangrove forest in Hong Kong.

Cyclone disturbance results in mangrove foliage loss, tree mortality and other changes in ecosystem processes. However, the impact of cyclones on mangrove sediment nutrient density, sediment-air CO2 and CH4 fluxes and their isotopes remains largely unknown. Super-typhoon Mangkhut (maximum gust 256 km h-1) hit Hong Kong in September 2018. We investigated the influence of the cyclone by comparing pre- and post-cyclone sediment carbon cycling processes as well as nitrogen density during an 8-month period in a mangrove forest at Ting Kok, Hong Kong. Time and/or nitrogen density are the dominant drivers of the variation of dark sediment-air CO2 fluxes (Rd) and sediment nutrient density. Significant changes in Rd and their δ13CO2 values, sediment organic carbon density (SOC) and nitrogen density (SON) occurred after the cyclone. Rd were highest one month after the cyclone (0.05 ± 0.01 mmol m-2 min-1) and lowest before the cyclone (8.32 ± 2.84 µmol m-2 min-1). δ13CO2 of pre-cyclone Rd (-18.2 ± 1.7‰) was significantly higher than that of all post-cyclone fluxes (-22.9 ± 1.5‰ to -23.6 ± 1.8‰). Both SOC and SON were highest one month after the cyclone (23.05 ± 1.92 kg C m-3, 2.42 ± 0.11 kg N m-3, 20-40 cm). A significant positive relationship exists between Rd and SON. Sediment-air CH4 fluxes did not show significant changes over time but along the sea-land gradient (0.28 ± 0.21 to 0.61 ± 0.22 µmol m-2 min-1). Cyclone disturbance results in the pulse input of litter, which may explain the significant increase in post-cyclone Rd and lower δ13CO2 of Rd. With anticipated climate change-driven effects on cyclone occurrence and intensity, our data underscores the significance of incorporating the influence of cyclone disturbance in constraining the global nutrient budgets in mangroves.

Mangroves give cause for conservation optimism, for now.

Friess et al. discuss the results of conservation efforts for mangrove forests in recent years.

Improved estimates on global carbon stock and carbon pools in tidal wetlands.

Tidal wetlands are global hotspots of carbon storage but errors exist with current estimates on their carbon density due to the use of factors estimated from other habitats for converting loss-on-ignition (LOI) to organic carbon (OC); and the omission of certain significant carbon pools. Here we show that the widely used conversion factor (LOI/OC = 1.724) is significantly lower than our measurements for saltmarsh sediments (1.92 ± 0.01) and oversimplifies the polynomial relationship between sediment OC and LOI for mangrove forests. Global mangrove OC stock in the top-meter sediment reaches 1.93 Pg when corrected for this bias, and is 20% lower than the previous estimates. Ecosystem carbon stock (living and dead biomass, sediment OC and inorganic carbon) is estimated at 3.7-6.2 Pg. Mangrove deforestation leads to carbon emission rates at 23.5-38.7 Tg yr-1 after 2000. Mangrove sediment OC stock has previously been over-estimated while ecosystem carbon stock underestimated.

Feeding strategies for the acquisition of high-quality food sources in stream macroinvertebrates: Collecting, integrating, and mixed feeding.

Aquatic macroinvertebrates play an important functional role in energy transfer in food webs, linking basal food sources to upper trophic levels that include fish, birds, and humans. However, the trophic coupling of nutritional quality between macroinvertebrates and their food sources is still poorly understood. We conducted a field study in subalpine streams in Austria to investigate how the nutritional quality (measured by long-chain polyunsaturated fatty acids, LC-PUFAs) in macroinvertebrates changes relative to their basal food sources. Samples of macroinvertebrates, periphyton, and leaves were collected from 17 streams in July and October 2016 and their fatty acid (FA) composition was analyzed. Periphyton FA varied strongly with time and space, and their trophic effect on macroinvertebrate FA differed among functional feeding groups. The match between periphyton FA and macroinvertebrate FA decreased with increasing trophic levels, but LC-PUFA content increased with each trophic step from periphyton to grazers and finally predators. Macroinvertebrates fed selectively on, assimilated, and/or actively controlled their LC-PUFA, especially eicosapentaenoic acid (EPA, 20 : 5ω3) relative to their basal food sources in the face of spatial and temporal changes. Grazer FA profiles reflected periphyton FA with relatively good fidelity, and especially their EPA feeding strategy was primarily linked to periphyton FA variation across seasons. In contrast, shredders appeared to preferentially assimilate more EPA over other FA, which was determined by the availability of high-quality food over seasons. Predators may more actively control their LC-PUFA distribution with respect to different quality foods and showed less fidelity to the basal FA profiles in plants and prey. Overall, grazers and shredders showed relatively good fidelity to food FA profiles and performed as both "collectors" and "integrators" for LC-PUFA requirements across seasons, while predators at higher trophic levels were more "integrators" with added metabolic complexity leading to somewhat more divergent FA profiles. These results are potentially applicable for other aquatic consumers in freshwater and marine ecosystems.

Intuitionistic fuzzy analytical hierarchical processes for selecting the paradigms of mangroves in municipal wastewater treatment.

Municipal wastewater discharge is widespread and one of the sources of coastal eutrophication, and is especially uncontrolled in developing and undeveloped coastal regions. Mangrove forests are natural filters of pollutants in wastewater. There are three paradigms of mangroves for municipal wastewater treatment and the selection of the optimal one is a multi-criteria decision-making problem. Combining intuitionistic fuzzy theory, the Fuzzy Delphi Method and the fuzzy analytical hierarchical process (AHP), this study develops an intuitionistic fuzzy AHP (IFAHP) method. For the Fuzzy Delphi Method, the judgments of experts and representatives on criterion weights are made by linguistic variables and quantified by intuitionistic fuzzy theory, which is also used to weight the importance of experts and representatives. This process generates the entropy weights of criteria, which are combined with indices values and weights to rank the alternatives by the fuzzy AHP method. The IFAHP method was used to select the optimal paradigm of mangroves for treating municipal wastewater. The entropy weights were entrained by the valid evaluation of 64 experts and representatives via online survey. Natural mangroves were found to be the optimal paradigm for municipal wastewater treatment. By assigning different weights to the criteria, sensitivity analysis shows that natural mangroves remain to be the optimal paradigm under most scenarios. This study stresses the importance of mangroves for wastewater treatment. Decision-makers need to contemplate mangrove reforestation projects, especially where mangroves are highly deforested but wastewater discharge is uncontrolled. The IFAHP method is expected to be applied in other multi-criteria decision-making cases.

Spatially-explicit valuation of coastal wetlands for cyclone mitigation in Australia and China.

Coastal wetlands are increasingly recognised for their pivotal role in mitigating the growing threats from cyclones (including hurricanes) in a changing climate. There is, however, insufficient information about the economic value of coastal wetlands for cyclone mitigation, particularly at regional scales. Analysis of data from 1990-2012 shows that the variation of cyclone frequencies is related to EI Niño strength in the Pacific Ocean adjacent to Australia, but not China. Among the cyclones hitting the two countries, there are significant relationships between the ratio of total economic damage to gross domestic production (TD/GDP) and wetland area within cyclone swaths in Australia, and wetland area plus minimum cyclone pressure despite a weak relationship in China. The TD/GDP ratio is significantly higher in China than in Australia. Despite their extensive and growing occurrence, seawalls in China appear not to play a critical role in cyclone mitigation, and cannot replace coastal wetlands, which provide other efficient ecosystem services. The economic values of coastal wetlands in Australia and China are respectively estimated at US$52.88 billion and 198.67 billion yr-1 for cyclone mitigation, albeit with large within-country geographic variation. This study highlights the urgency to integrate this value into existing valuations of coastal wetlands.

Structural equation modelling reveals factors regulating surface sediment organic carbon content and CO2 efflux in a subtropical mangrove.

Mangroves are blue carbon ecosystems that sequester significant carbon but release CO2, and to a lesser extent CH4, from the sediment through oxidation of organic carbon or from overlying water when flooded. Previous studies, e.g. Leopold et al. (2015), have investigated sediment organic carbon (SOC) content and CO2 flux separately, but could not provide a holistic perspective for both components of blue carbon. Based on field data from a mangrove in southeast Queensland, Australia, we used a structural equation model to elucidate (1) the biotic and abiotic drivers of surface SOC (10cm) and sediment CO2 flux; (2) the effect of SOC on sediment CO2 flux; and (3) the covariation among the environmental drivers assessed. Sediment water content, the percentage of fine-grained sediment (<63µm), surface sediment chlorophyll and light condition collectively drive sediment CO2 flux, explaining 41% of their variation. Sediment water content, the percentage of fine sediment, season, landform setting, mangrove species, sediment salinity and chlorophyll collectively drive surface SOC, explaining 93% of its variance. Sediment water content and the percentage of fine sediment have a negative impact on sediment CO2 flux but a positive effect on surface SOC content, while sediment chlorophyll is a positive driver of both. Surface SOC was significantly higher in Avicennia marina (2994±186gm-2, mean±SD) than in Rhizophora stylosa (2383±209gm-2). SOC was significantly higher in winter (2771±192gm-2) than in summer (2599±211gm-2). SOC significantly increased from creek-side (865±89gm-2) through mid (3298±137gm-2) to landward (3933±138gm-2) locations. Sediment salinity was a positive driver of SOC. Sediment CO2 flux without the influence of biogenic structures (crab burrows, aerial roots) averaged 15.4mmolm-2d-1 in A. marina stands under dark conditions, lower than the global average dark flux (61mmolm-2d-1) for mangroves.

Paradigms of mangroves in treatment of anthropogenic wastewater pollution.

Mangroves have been increasingly recognized for treating wastewater from aquaculture, sewage and other sources with the overwhelming urbanization trend. This study clarified the three paradigms of mangroves in disposing wastewater contaminants: natural mangroves, constructed wetlands (including free water surface and subsurface flow) and mangrove-aquaculture coupling systems. Plant uptake is the common major mechanism for nutrient removal in all the paradigms as mangroves are generally nitrogen and phosphorus limited. Besides, sediments accrete and provide substrates for microbial activities, thereby removing organic matter and nutrients from wastewater in natural mangroves and constructed wetlands. Among the paradigms, the mangrove-aquaculture coupling system was determined to be the optimal alternative for aquaculture wastewater treatment by multi-criterion decision making. Sensitivity analysis shows variability of alternative ranking but underpins the coupling system as the most environment-friendly and cost-efficient option. Mangrove restoration is expected to be achievable if aquaculture ponds are planted with mangrove seedlings, creating the coupling system.

Effect of nitrate addition on reductive transformation of pentachlorophenol in paddy soil in relation to iron(III) reduction.

Reductive dechlorination is a crucial pathway for anaerobic biodegradation of highly chlorinated organic contaminants. Under an anoxic environment, reductive dechlorination of organic contaminants can be affected by many redox processes such as nitrate reduction and iron reduction. In the present study, batch incubation experiments were conducted to investigate the effect of nitrate addition on reductive dechlorination of PCP in paddy soil with consideration of iron transformation. Study results demonstrate that low concentrations (0, 0.5 and 1 mM) of nitrate addition can enhance the reductive dechlorination of PCP and Fe(III) reduction, while high concentrations (5, 10, 20 and 30 mM) of nitrate addition caused the contrary. Significant positive correlations between PCP degradation rates and the formation rates of dissolved Fe(II) (pearson correlation coefficients r = 0.965) and HCl-extractable Fe(II) (r = 0.921) suggested that Fe(III) reduction may enhance PCP dechlorination. Furthermore, consistent variation trends of PCP degradation and the abundances of the genus Comamonas, capable of Fe(III) reduction coupled to reductive dechlorination, and of the genus Dehalobacter indicated the occurrence of microbial community variation induced by nitrate addition as a response to PCP dechlorination.

Yap1 activation by H2O2 or thiol-reactive chemicals elicits distinct adaptive gene responses.

The yeast Saccharomyces cerevisiae transcription factor Yap1 mediates an adaptive response to oxidative stress by regulating protective genes. H(2)O(2) activates Yap1 through the Gpx3-mediated formation of a Yap1 Cys303-Cys598 intramolecular disulfide bond. Thiol-reactive electrophiles can activate Yap1 directly by adduction to cysteine residues in the C-terminal domain containing Cys598, Cys620, and Cys629. H(2)O(2) and N-ethylmaleimide (NEM) showed no cross-protection against each other, whereas another thiol-reactive chemical, acrolein, elicited Yap1-dependent cross-protection against NEM, but not H(2)O(2). Either Cys620 or Cys629 was sufficient for activation of Yap1 by NEM or acrolein; Cys598 was dispensable for this activation mechanism. To determine whether Yap1 activated by H(2)O(2) or thiol-reactive chemicals elicits distinct adaptive gene responses, microarray analysis was performed on the wild-type strain or its isogenic single-deletion strain Δyap1 treated with control buffer, H(2)O(2), NEM, or acrolein. Sixty-five unique H(2)O(2) and 327 NEM and acrolein Yap1-dependent responsive genes were identified. Functional analysis using single-gene-deletion yeast strains demonstrated that protection was conferred by CTA1 and CTT1 in the H(2)O(2)-responsive subset and YDR042C in the NEM- and acrolein-responsive subset. These findings demonstrate that the distinct mechanisms of Yap1 activation by H(2)O(2) or thiol-reactive chemicals result in selective expression of protective genes.