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  1. Free, publicly-accessible full text available June 1, 2023
  2. Free, publicly-accessible full text available June 28, 2023
  3. Abstract Lake ecosystems, as integrators of watershed and climate stressors, are sentinels of change. However, there is an inherent time-lag between stressors and whole-lake response. Aquatic metabolism, including gross primary production (GPP) and respiration (R), of stream–lake transitional zones may bridge the time-lag of lake response to allochthonous inputs. In this study, we used high-frequency dissolved oxygen data and inverse modeling to estimate daily rates of summer epilimnetic GPP and R in a nutrient-limited oligotrophic lake at two littoral sites located near different major inflows and at a pelagic site. We examined the relative importance of stream variables in comparison to meteorological and in-lake predictors of GPP and R. One of the inflow streams was substantially warmer than the other and primarily entered the lake’s epilimnion, whereas the colder stream primarily mixed into the metalimnion or hypolimnion. Maximum GPP and R rates were 0.2–2.5 mg O 2 L −1  day −1 (9–670%) higher at littoral sites than the pelagic site. Ensemble machine learning analyses revealed that > 30% of variability in daily littoral zone GPP and R was attributable to stream depth and stream–lake transitional zone mixing metrics. The warm-stream inflow likely stimulated littoral GPP and R, while the cold-stream inflow only stimulatedmore »littoral zone GPP and R when mixing with the epilimnion. The higher GPP and R observed near inflows in our study may provide a sentinel-of-the-sentinel signal, bridging the time-lag between stream inputs and in-lake processing, enabling an earlier indication of whole-lake response to upstream stressors.« less
    Free, publicly-accessible full text available July 1, 2023
  4. Abstract

    Lakes and reservoirs, as most humans experience and use them, are dynamic bodies of water, with surface extents that increase and decrease with seasonal precipitation patterns, long-term changes in climate, and human management decisions. This paper presents a new global dataset that contains the location and surface area variations of 681,137 lakes and reservoirs larger than 0.1 square kilometers (and south of 50 degree N) from 1984 to 2015, to enable the study of the impact of human actions and climate change on freshwater availability. Within its scope for size and region covered, this dataset is far more comprehensive than existing datasets such as HydroLakes. While HydroLAKES only provides a static shape, the proposed dataset also has a timeseries of surface area and a shapefile containing monthly shapes for each lake. The paper presents the development and evaluation of this dataset and highlights the utility of novel machine learning techniques in addressing the inherent challenges in transforming satellite imagery to dynamic global surface water maps.

  5. Abstract We examine the UV/X-ray properties of 1378 quasars in order to link empirical correlations to theoretical models of the physical mechanisms dominating quasars as a function of mass and accretion rate. The clarity of these correlations is improved when (1) using C iv broad emission line equivalent width (EQW) and blueshift (relative to systemic) values calculated from high signal-to-noise ratio reconstructions of optical/UV spectra and (2) removing quasars expected to be absorbed based on their UV/X-ray spectral slopes. In addition to using the traditional C iv parameter space measures of C iv EQW and blueshift, we define a “C iv ∥ distance” along a best-fit polynomial curve that incorporates information from both C iv parameters. We find that the C iv ∥ distance is linearly correlated with both the optical-to-X-ray slope, α ox , and broad-line He ii EQW, which are known spectral energy distribution indicators, but does not require X-ray or high spectral resolution UV observations to compute. The C iv ∥ distance may be a better indicator of the mass-weighted accretion rate, parameterized by L / L Edd , than the C iv EQW or blueshift alone, as those relationships are known to break down at themore »extrema. Conversely, there is only a weak correlation with the X-ray energy index (Γ), an alternate L / L Edd indicator. We find no X-ray or optical trends in the direction perpendicular to the C iv distance that could be used to reveal differences in accretion disk, wind, or corona structure that could be widening the C iv EQW–blueshift distribution. A different parameter (such as metallicity) not traced by these data must come into play.« less
    Free, publicly-accessible full text available June 1, 2023
  6. Free, publicly-accessible full text available April 25, 2023
  7. Free, publicly-accessible full text available March 1, 2023
  8. A bistable [2]pseudorotaxane 1⊂CBPQT·4PF 6 and a bistable [2]rotaxane 2·4PF 6 have been synthesised to measure the height of an electrostatic barrier produced by double molecular oxidation (0 to +2). Both systems have monopyrrolotetrathiafulvalene (MPTTF) and oxyphenylene (OP) as stations for cyclobis(paraquat- p -phenylene) (CBPQT 4+ ). They have a large stopper at one end while the second stopper in 2 4+ is composed of a thioethyl (SEt) group and a thiodiethyleneglycol (TDEG) substituent, whereas in 1⊂CBPQT 4+ , the SEt group has been replaced with a less bulky thiomethyl (SMe) group. This seemingly small difference in the substituents on the MPTTF unit leads to profound changes when comparing the physical properties of the two systems allowing for the first measurement of the deslipping of the CBPQT 4+ ring over an MPTTF 2+ unit in the [2]pseudorotaxane. Cyclic voltammetry and 1 H NMR spectroscopy were used to investigate the switching mechanism for 1⊂CBPQT·MPTTF 4+ and 2·MPTTF 4+ , and it was found that CBPQT 4+ moves first to the OP station producing 1⊂CBPQT·OP 6+ and 2·OP 6+ , respectively, upon oxidation of the MPTTF unit. The kinetics of the complexation/decomplexation process occurring in 1⊂CBPQT·MPTTF 4+ and in 1⊂CBPQT·OP 6+ weremore »studied, allowing the free energy of the transition state when CBPQT 4+ moves across a neutral MPTTF unit (17.0 kcal mol −1 ) or a di-oxidised MPTTF 2+ unit (24.0 kcal mol −1 ) to be determined. These results demonstrate that oxidation of the MPTTF unit to MPTTF 2+ increases the energy barrier that the CBPQT 4+ ring must overcome for decomplexation to occur by 7.0 kcal mol −1 .« less
    Free, publicly-accessible full text available March 16, 2023