Protecting water quality at catchment scales is complicated by the high spatiotemporal variability in water chemistry. Consequently, determining pollutant sources requires costly monitoring ...strategies to diagnose causes and guide management solutions. However, recent studies have shown that spatial patterns in water chemistry can be persistent at catchment scales, potentially allowing identification of pollution sources and sinks with just a few sampling campaigns. Here, we tested a new method to quantify spatial persistence (SP) of water chemistry patterns with data from synoptic samplings in 22 headwater subcatchments within a 375 km2 catchment in western France (March 2018 to July 2019). This new method to quantify SP reduces dependence on long‐term metrics such as flow‐weighted concentrations, which are usually uncertain or unavailable. We applied the method to 16 ecologically relevant water quality parameters, including soluble reactive phosphorus, nitrate, and dissolved organic carbon. The results showed an average SP of 0.68 among parameters during the study period. For most parameters, SP was higher during the high‐flow winter period but lower and more variable during the low‐flow summer period. We found that the SP ultimately depended on the ratio between the temporal and spatial coefficients of variation (variance explained: 70%) rather than the temporal synchrony among subcatchments (variance explained: 4%). These results demonstrate that in these temperate catchments, synoptic sampling during the high‐flow winter period allows efficient identification of source and sink subcatchments, while more frequent samplings are needed to characterize ecological conditions at low flow.
Key Points
We found high spatial persistence of water chemistry, despite high spatiotemporal variability in water chemistry
Spatial persistence of water chemistry is primarily determined by the ratio between spatial and temporal variability
A single synoptic sampling during the high‐flow season allows efficient identification of source and sink subcatchments
Litter decomposition is central to ecosystem functioning and depends, under constant abiotic conditions, on litter quality and decomposer activity. During the assembly of a plant community following ...disturbance, litter quality is expected to decrease, due to an increasing proportion of resource-conservative species, whereas decomposer activity is expected to increase, due to the establishment of decomposer populations and their response to local vegetation (“home-field advantage”, HFA). To date, the combined effect of these processes remains poorly understood. We studied 27 seminatural grasslands in western France, ranging from 1 to 25 years since last cultivation. We measured the functional composition of plant communities using litter traits (specific leaf area, leaf dry matter content, C/N ratio, phenolics), characterized the entire community of decomposers (macrofauna, mesofauna, microbes) and performed reciprocal litter transplants to quantify HFA. We found that, overall, decomposition was rapid, and HFA was not evident. While there was substantial among-grassland variation in decomposition and HFA, neither changed with grassland age. Litter quality and decomposer efficiency also remained, overall, unchanged. However, grassland age determined all measured litter traits and caused soil microbial C/N ratio to decline. Although these changes impacted decomposition individually, together they canceled out each other, resulting in constant decomposition across the chronosequence. Our results suggest that processes driving decomposition differ during grassland succession and suggest that HFA may be lower in communities with high litter quality. Moreover, simultaneous assembly processes have opposing and therefore stabilizing effects on decomposition, possibly explaining the outstanding resilience of primary production in temperate grassland ecosystems.
Magnesium alloys normally contain minor amounts (~0.3%) of Mn to achieve improved corrosion resistance by controlling the level of Fe during melting. It has been reported recently that minor Mn ...additions can significantly enhance the age hardenability of die-cast Mg–Al–RE alloys. This paper reports that minor Mn additions also have a remarkable influence in improving the creep resistance of die-cast Mg–Al–RE alloys. The secondary creep rate of Mg–4Al–3La alloy at 175°C/75MPa is reduced by more than three orders of magnitude (from 5.9×10-7s-1 to 3.0×10-10s-1) by the addition of 0.32% Mn. The improvement in creep resistance is associated with the dynamic precipitation of nanoscale Al–Mn particles during creep. The findings in this work shed new light on creep resistance of Mg–Al based alloys.
Lake sediment oxygen isotope records (calcium carbonate-δ18O) in the western North American Cordillera developed during the past decade provide substantial evidence of Pacific ocean–atmosphere ...forcing of hydroclimatic variability during the Holocene. Here we present an overview of 18 lake sediment δ18O records along with a new compilation of lake water δ18O and δ2H that are used to characterize lake sediment sensitivity to precipitation-δ18O in contrast to fractionation by evaporation. Of the 18 records, 14 have substantial sensitivity to evaporation. Two records reflect precipitation-δ18O since the middle Holocene, Jellybean and Bison Lakes, and are geographically positioned in the northern and southern regions of the study area. Their comparative analysis indicates a sequence of time-varying north–south precipitation-δ18O patterns that is evidence for a highly non-stationary influence by Pacific ocean–atmosphere processes on the hydroclimate of western North America. These observations are discussed within the context of previous research on North Pacific precipitation-δ18O based on empirical and modeling methods. The Jellybean and Bison Lake records indicate that a prominent precipitation-δ18O dipole (enriched-north and depleted-south) was sustained between ~3.5 and 1.5ka, which contrasts with earlier Holocene patterns, and appears to indicate the onset of a dominant tropical control on North Pacific ocean–atmosphere dynamics. This remains the state of the system today. Higher frequency reversals of the north–south precipitation-δ18O dipole between ~2.5 and 1.5ka, and during the Medieval Climate Anomaly and the Little Ice Age, also suggest more varieties of Pacific ocean–atmosphere modes than a single Pacific Decadal Oscillation (PDO) type analogue. Results indicate that further investigation of precipitation-δ18O patterns on short (observational) and long (Holocene) time scales is needed to improve our understanding of the processes that drive regional precipitation-δ18O responses to Pacific ocean–atmosphere variability, which in turn, will lead to a better understanding of internal Pacific ocean–atmosphere variability and its response to external climate forcing mechanisms.
Petrography and stable isotopic (carbon and oxygen) studies of the Middle Eocene Garampani limestone from Jamunagar limestone quarry of the Umrangso area, North Cachar Assam, were carried out to ...determine the environment of deposition and the level of diagenesis to improve our understanding of the climate at the time of its formation. The major petrographic types documented here include wackestone, packstone, and grainstone which fall into the type I and II categories of the triangular classification scheme indicating formation in a short-lived, high-energy environment that was dissipating at the time of deposition. The samples have a distinct signature that includes extensive diagenesis with varying intensities of compaction, recrystallization, pressure solution, and neomorphism. In occurrences of stratigraphically significant foraminiferal assemblages and palynofossils, the Garampani Limestone is assigned Middle Eocene age. Most of the samples produced positive δ
13
C ‰ ranging from 1.64 to 0.76 ‰ (PDB) and correlated with a transgressive phase. The negative δ
18
O values range from −6.25 to −4.67 ‰ (PDB) suggesting that the Middle Eocene was generally characterized by a warmer climate.
In this review, we examine why blood pressure (BP) and sympathetic nerve activity (SNA) increase during a rise in central nervous system (CNS) P(CO(2)) (central chemoreceptor stimulation). CNS ...acidification modifies SNA by two classes of mechanisms. The first one depends on the activation of the central respiratory controller (CRG) and causes the much-emphasized respiratory modulation of the SNA. The CRG probably modulates SNA at several brain stem or spinal locations, but the most important site of interaction seems to be the caudal ventrolateral medulla (CVLM), where unidentified components of the CRG periodically gate the baroreflex. CNS P(CO(2)) also influences sympathetic tone in a CRG-independent manner, and we propose that this process operates differently according to the level of CNS P(CO(2)). In normocapnia and indeed even below the ventilatory recruitment threshold, CNS P(CO(2)) exerts a tonic concentration-dependent excitatory effect on SNA that is plausibly mediated by specialized brain stem chemoreceptors such as the retrotrapezoid nucleus. Abnormally high levels of P(CO(2)) cause an aversive interoceptive awareness in awake individuals and trigger arousal from sleep. These alerting responses presumably activate wake-promoting and/or stress-related pathways such as the orexinergic, noradrenergic, and serotonergic neurons. These neuronal groups, which may also be directly activated by brain acidification, have brainwide projections that contribute to the CO(2)-induced rise in breathing and SNA by facilitating neuronal activity at innumerable CNS locations. In the case of SNA, these sites include the nucleus of the solitary tract, the ventrolateral medulla, and the preganglionic neurons.
Three AE44 (Mg–4Al–4RE, wt%) alloys are prepared from individual RE elements, i.e., La, Ce, or Nd, by high‐pressure die casting and the creep resistance of these alloys is evaluated. It is shown that ...the choice of RE elements has a significant influence on the creep resistance of AE44, with the La‐containing alloy being the most creep‐resistant while the Nd‐containing alloy the least creep‐resistant. Microstructural examinations suggest that the observed difference in creep resistance cannot be accounted for by the thermal stability of the Al11RE3 phase, as reported previously, but can be related to the volume fraction of intermetallic phases and the content of manganese.
The choice of RE elements is shown to have a significant influence on the creep resistance of die‐cast magnesium alloy AE44, which cannot be accounted for by the thermal stability of the Al11RE3 phase as reported previously. The volume fraction of intermetallic phases and the Mn content are considered to be contributing factors for the observed differences in creep resistance.
Hot-tearing susceptibility is an important consideration for alloy design. Based on a review of previous research, an
a priori
indicator for the prediction of an alloy’s hot-tearing susceptibility is ...proposed in this article and is applied to a range of magnesium-rare earth (RE)-based alloys. The indicator involves taking the integral over the solid fraction/temperature curve between the temperature when feeding becomes restricted (coherency) and that when a three-dimension network of solid is formed (coalescence). The hot-tearing propensity of Mg-RE alloys is found to vary greatly depending on which RE is primarily used, due to the difference in the solidification range. Mg-Nd alloys are the most susceptible to hot tearing, followed by Mg-Ce-based alloys, while Mg-La alloys show almost no hot tearing. The proposed indicator can be well applied to hot-tearing propensity of the Mg-RE alloys. It is expected that the indicator could be used as an estimation of the relative hot-tearing propensity in other alloy systems as well.
Multiple paleoclimate proxies are required for robust assessment of past hydroclimatic conditions. Currently, estimates of drought variability over the past several thousand years are based largely ...on tree-ring records. We produced a 1,500-y record of winter precipitation in the Pacific Northwest using a physical model-based analysis of lake sediment oxygen isotope data. Our results indicate that during the Medieval Climate Anomaly (MCA) (900–1300 AD) the Pacific Northwest experienced exceptional wetness in winter and that during the Little Ice Age (LIA) (1450–1850 AD) conditions were drier, contrasting with hydroclimatic anomalies in the desert Southwest and consistent with climate dynamics related to the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). These findings are somewhat discordant with drought records from tree rings, suggesting that differences in seasonal sensitivity between the two proxies allow a more compete understanding of the climate system and likely explain disparities in inferred climate trends over centennial timescales.
Holocene variations of tropical moisture balance have been ascribed to orbitally forced changes in solar insolation. If this model is correct, millennial-scale climate evolution should be antiphased ...between the northern and southern hemispheres, producing humid intervals in one hemisphere matched to aridity in the other. Here we show that Holocene climate trends were largely synchronous and in the same direction in the northern and southern hemisphere outer-tropical Andes, providing little support for the dominant role of insolation forcing in these regions. Today, sea-surface temperatures in the equatorial Pacific Ocean modulate rainfall variability in the outer tropical Andes of both hemispheres, and we suggest that this mechanism was pervasive throughout the Holocene. Our findings imply that oceanic forcing plays a larger role in regional South American climate than previously suspected, and that Pacific sea-surface temperatures have the capacity to induce abrupt and sustained shifts in Andean climate.
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