The influence of light on nitrogen cycling and the primary nitrite maximum in a seasonally stratified sea

In the seasonally stratified Gulf of Aqaba Red Sea, both release by phytoplankton and oxidation by nitrifying microbes contributed to the formation of a primary nitrite maximum (PNM) over different seasons and depths in the water column. In the winter and during the days immediately following spring stratification, formation was strongly correlated (R² = 0.99) with decreasing irradiance and chlorophyll, suggesting that incomplete reduction by light limited phytoplankton was a major source of . However, as stratification progressed, continued to be generated below the euphotic depth by microbial oxidation, likely due to differential photoinhibition of and oxidizing populations. Natural abundance stable nitrogen isotope analyses revealed a decoupling of the δ¹⁵N and δ¹⁸O in the combined and pool, suggesting that assimilation and nitrification were co-occurring in surface waters. As stratification progressed, the δ¹⁵N of particulate N below the euphotic depth increased from −5‰ to up to +20‰.N uptake rates were also influenced by light; based on ¹⁵N tracer experiments, assimilation of , , and urea was more rapid in the light (434 ± 24, 94 ± 17, and 1194 ± 48 nmol N L⁻¹ day⁻¹ respectively) than in the dark (58 ± 14, 29 ± 14, and 476 ± 31 nmol N L⁻¹ day⁻¹ respectively). Dark assimilation was 314 ± 31 nmol N L⁻¹ day⁻¹, while light assimilation was much faster, resulting in complete consumption of the ¹⁵N spike in less than 7 h from spike addition. The overall rate of coupled urea mineralization and oxidation (14.1 ± 7.6 nmol N L⁻¹ day⁻¹) was similar to that of oxidation alone (16.4 ± 8.1 nmol N L⁻¹ day⁻¹), suggesting that mineralization of labile dissolved organic N compounds like urea was not a rate limiting step for nitrification. Our results suggest that assimilation and nitrification compete for and that N transformation rates throughout the water column are influenced by light over diel and seasonal cycles, allowing phytoplankton and nitrifying microbes to contribute jointly to PNM formation. We identify important factors that influence the N cycle throughout the year, including light intensity, substrate availability, and microbial community structure. These processes could be relevant to other regions worldwide where seasonal variability in mixing depth and stratification influence the contributions of phytoplankton and non-photosynthetic microbes to the N cycle.     

Review: Hydraulic head measurements—new technologies, classic pitfalls

The hydraulic head is one of the most important metrics in hydrogeology as it underlies the interpretation of groundwater flow, the quantification of aquifer properties and the calibration of flow models. Heads are determined based on water-level measurements in wells and piezometers. Despite the importance of hydraulic head data, standard textbooks used in groundwater curricula provide relatively little discussion of the appropriate measurement procedures. This paper presents a review of the literature dealing with the determination of hydraulic heads, and aims to provide quantitative guidance on the likely sources of error and when these can be expected to become important. The most common measurement procedures are discussed and the main sources of error are identified, i.e. those related to (1) the measurement instruments, (2) the conversion from pressure to heads, (3) time lag effects, and (4) observation well defects. It is argued that heads should be determined following well-defined guidelines, and that it should become standard practice in hydrogeology to provide quantitative estimates of the measurement error.     

Spatial variability of three‐dimensional Reynolds stresses in a developing channel bend

Experimental results of the mean flow field and turbulence characteristics for flow in a model channel bend with a mobile sand bed are presented. Acoustic Doppler velocimeters (ADVs) were used to measure the three components of instantaneous velocities at multiple cross sections in a 135° channel bend for two separate experiments at different stages of clear water scour conditions. With measurements at multiple cross sections through the bend it was possible to map the changes in both the spatial distribution of the mean velocity field and the three Reynolds shear stresses. Turbulent stresses are known to contribute to sediment transport and the three‐dimensionality inherent to flow in open channel bends presents a useful case for determining specific relations between three‐dimensional turbulence and sediment entrainment and transport. These measurements will also provide the necessary data for validating numerical simulations of turbulent flow and sediment transport. The results show that the magnitude and distribution of three‐dimensional Reynolds stresses increase through the bend, with streamwise‐cross stream and cross stream‐vertical components exceeding the maximum principal Reynolds stress through the bend. The most intriguing observation is that near‐bed maximum positive streamwise‐cross stream Reynolds stress coincides with the leading edge of the outer bank scour hole (or thalweg), while maximum cross stream‐vertical Reynolds stress (in combination with high negative streamwise‐cross stream Reynolds stress near the bend apex) coincides with the leading edge of the inner bank bar. Maximum Reynolds stress and average turbulent kinetic energy appear to be greater and more localized over the scour hole before final equilibrium scour is reached. This suggests that the turbulent energy in the flow is higher while the channel bed is developing, and both lower turbulent energy and a broader distribution of turbulent stresses near the bed are required for cessation of particle mobilization and transport. Copyright © 2010 John Wiley & Sons, Ltd.     

Single and multigrain quartz‐luminescence dating of irrigation‐channel features in Santa Fe,New Mexico

Acequias (irrigation channels and ditches) were used by Spanish settlers, their descendants, and Native Americans in New Mexico. Several such features were recently excavated in Santa Fe, but material for numeric dating was difficult to find. Therefore, for this high‐energy‐deposition irrigation‐feature setting we applied optically stimulated luminescence (OSL) sediment dating methods to determine the timing of last filling of some of these acequias. We report multigrain single‐aliquot quartz (MGSAQ) OSL dating results and the first single‐grain quartz (SGQ) OSL dating results for irrigation features. One sample yielded an average age of 96 ± 13 yr, consistent with the maximum expected age of 127 yr (before 2007). An OSL age of 175 ± 15 yr for another sample delimits a sedimentation event since the first construction of that feature ca. 300 yr ago. A sample known to be younger than 400–450 yr but predating the mid‐19th century gave an SGQ age of 376 ±31 yr. These results indicate that: (1) Regional quartz in New Mexico is highly favorable to OSL dating; (2) in this setting, SGQ OSL dating is preferred to MGSAQ dating; and (3) for the last 500–600 yr, SGQ OSL dating in such settings is preferred to ¹⁴C dating because OSL dating lacks those ambiguities inherent in converting ¹⁴C ages to calendar years. © 2009 Wiley Periodicals, Inc.     

Low water contents in pyroxenes from spinel-peridotites of the oxidized, sub-arc mantle wedge

Pyroxene water contents measured by Fourier transform infrared spectrometry for Mexican and Simcoe (WA, USA) spinel-peridotite xenoliths range from 140 to 528 ppm in clinopyroxenes and 39 to 265 ppm in orthopyroxenes. Correlations between these water contents and major-element compositional data for the pyroxenes, associated spinels, and whole-rock xenoliths demonstrate that these water contents record mantle values that have not been perturbed since the xenoliths were brought to the surface by their host magmas. Broad positive correlations of pyroxene water contents with whole-rock Al₂O₃ are consistent with water behaving as an incompatible element during peridotite melting. The main control on the range of pyroxene water contents, however, appears to be the redox state of the peridotite, because estimates of oxygen fugacity from Mössbauer (Simcoe) and microprobe data (Mexico) on spinels are negatively correlated with water contents. This is consistent with the dominant mechanism of H incorporation into pyroxene, which is dependent on the oxidation-reduction of iron. Metasomatism of sub-arc mantle-wedge peridotites by oxidized fluids or melts rising from the slab raises the oxygen fugacity of the peridotites, and where temperature is high enough, induces them to partially melt. The oxidation, in turn, lowers the solubility of water in the peridotite minerals, causing more than half of the original water to be expelled. That water enters the hydrous partial melts and these ascend through the lithosphere to feed the arc magmatic system in the upper crust. Low water contents in pyroxenes from sub-arc mantle-wedge peridotites, such as those from Simcoe and some western Mexican sites, therefore appear to be complementary to the high water contents that characterize subduction-zone magmas and fuel their explosive eruptions. An estimate of water budget in subduction zones, however, indicates that the amount of water coming from the dehydration of mantle-wedge anhydrous minerals probably accounts for less than 5% of the total water present in subduction-related magmas. The high water contents of arc magmas thus are mainly attributed to fluids or melts from the slab proper. The relatively dry sub-arc mantle wedge appears to be an effective medium through which subducted water is transported from slabs toward the surface.     

Changes in Area and Carbon in Forests of the Middle Zavolgie: A Regional Case Study of Russian Forests

We studied forest land-use and carbon storage over a 40-year period in the Middle Zavolgie region of Russia, an area of approximately 287,000 km². Data were obtained from state forest inventories for 1958 and 1995. In spite of the effects of disturbances and uncontrolled harvesting between 1958 and 1990, the forests of the Middle Zavolgie Region remained a considerable pool of ecosystem carbon (C). Over the study period the total area of forest lands decreased by approximately 2%, while the growing stock increased by 8%. There were significant changes in the age class structure of these forest ecosystems toward a larger proportion of young and middle aged stands. The total amount of carbon in the stem biomass of forests in all regions of Middle Zavolgie increased over the 40-year period and was equal to about 307 TgC in 1995. A regional approach for estimating the C dynamics of forest ecosystems in response to land use in the Middle Zavolgie region can contribute to understanding the potential role of Russian forests in C sequestration. This information is important for implementation of international conventions concerning national carbon budgets and reducing the potential negative impacts of climate change.     

土壤中稀土元素地球化学研究进展

综述了土壤中稀土元素含量、稀土赋存形态、稀土生物有效性、稀土在土壤环境中的分布规律、迁移方式及其影响因子。讨论了稀土微肥应用中应注意的问题以及由此带来的环境效应，并探讨了建立具有普遍适用性的稀土生物有效性方法学的途径等。     

Annotated translation of “Nota in verband met de voorgenomen putboring nabij Amsterdam [Note concerning the intended well drilling near Amsterdam]” by J. Drabbe and W. Badon Ghijben (1889)

The famous report by engineers Drabbe and Badon Ghijben (1889), on an intended well drilling near Amsterdam (the Netherlands), was one of the key documents that contributed to the Ghijben-Herzberg formula, which links water-table elevation to the depth of the freshwater–saltwater interface in coastal aquifers. The report has been often cited but no English translation has appeared in the literature to date. The aim of this annotated translation of the report is to provide the international scientific community with easier access than was hitherto the case, plus electronic access to the original in Dutch. A brief introduction to the report is provided, followed by a translation that follows the original text as closely as possible.