Nitrogen isotope ratios in estuarine biota collected along a nutrient gradient in Narragansett Bay, Rhode Island, USA     

Pruell RJ  Taplin BK  Lake JL  Jayaraman S 《Marine pollution bulletin》2006,52(6):612-620

Stable nitrogen isotope ratios were used to study the incorporation of anthropogenically-derived nitrogen into the food webs of salt marsh systems along a contamination gradient in Narragansett Bay. Nitrogen isotope ratios (delta(15)N) were measured in six estuarine species collected from three marshes along this gradient, monthly from June to October between 1997 and 1999. A significant decrease in delta(15)N was found with distance along the estuary for four of the six species. Significant differences were found among monthly isotope ratios for some species. Nitrogen isotope ratios in sea lettuce (Ulva lactuca) increased during the summer season with highest delta(15)N values measured during September and October. This trend was most pronounced at the station receiving the highest nutrient inputs. Elevated delta(15)N values at this station appeared to correlate with seawater ammonia/nitrate concentration ratios. The temporal variations in delta(15)N suggest that care should be taken in species selection and the design of sampling schemes of studies using delta(15)N for monitoring anthropogenic nutrients in aquatic systems. Sampling programs designed to determine long-term trends should consider species that do not show rapid fluctuations in isotope ratios. The mud snail, Nassarius obsoletus, responded this way in the present study. Studies designed to measure short-term changes should include species such as U. lactuca, which rapidly respond to isotope changes. The results from this study also help to establish a baseline for nitrogen isotope values in Narragansett Bay. This information can be used to monitor future trends in nitrogen inputs to this estuary.  相似文献   

Studies on turbulent diffusion processes and evaluation of diffusivity values from hydrodynamic observations in Corpus Christi Bay   总被引：1，自引：0，他引：1  

Temitope O. Ojo  James S. Bonner  Cheryl Page 《Continental Shelf Research》2006,26(20):2629-2644

The physical process of dispersion which can be attributed to turbulence (turbulent diffusion) or shear (shear-augmented diffusion) within the flow field is very important as it ultimately governs the distribution of constituents of interest within the environment. A series of diffusion experiments were conducted in Corpus Christi Bay, TX with the purpose of characterizing turbulent diffusion through dispersion coefficients or turbulent diffusivity, K_i (i=x, y, z) dependent on the degree of randomness or turbulence intensity, I.Measured with a boat-mounted acoustic doppler current profiler (ADCP), the Eulerian velocity time-series of fluid particles in random motion, u_i was used in the evaluation of the Eulerian time-scale of turbulence, T^E based on the velocity correlation function, R^E with T^E being related to the Lagrangian time-scale T^L through a scaling parameter, β(=T^L/T^E). Surface currents were obtained with high frequency (HF) Radar equipment deployed over the study area from which the horizontal velocity gradients were determined.Within the spatial scale of the experiment (1000 m), the observed low horizontal gradients (10⁻⁴ s⁻¹) allowed for the generation of velocity time-series from an ADCP mounted on a moving platform. A numerical scheme for evaluating turbulent diffusivity values was developed on the basis of Eulerian current measurements and calibrated through the statistics of an evolving dye patch for the scaling parameter β which in this scheme was found to be in the range 1–3.  相似文献   

Assessing selected natural and anthropogenic impacts on freshwater lens morphology on small barrier Islands: Dog Island and St. George Island, Florida, USA   总被引：3，自引：2，他引：1  

James C. Schneider  Sarah E. Kruse 《Hydrogeology Journal》2006,14(1-2):131-145

Abstract The freshwater lens morphologies of the barrier islands Dog Island and St. George Island on the panhandle coast of Florida (FL), USA, are controlled to varying degrees by both natural and anthropogenic factors. Variable-density groundwater flow models confirm that spatial variability of recharge values can account for the observed lens asymmetry on these islands. The depth to the base of the lens does not vary significantly seasonally. Human development has altered recharge patterns in some areas, locally thinning the freshwater lens. Aqueduct water supply to St. George Island represents 7–25% of natural recharge; higher recharge rates are required to simulate the lens on St. George Island than on Dog Island. On both islands, coastal erosion rates are sufficiently rapid that the freshwater lens may not be in equilibrium with current boundary conditions.

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Resumen Las morfologías de los lentes de agua dulce en las islas de barrera: Islas Dog y St. George, en la costa alargada y estrecha de Florida, EUA, están controladas en diferentes grados por factores antropogénicos y naturales. Los modelos de flujo con densidad variable del agua subterránea, confirman que la variabilidad espacial de los valores de recarga, pueden ser la causa de la asimetría observada en los lentes de estas islas. La profundidad hasta la base de los lentes no varía significativamente con las estaciones climáticas. El desarrollo humano ha alterado las tendencias de recarga en algunas áreas, causando adelgazamiento local de los lentes de agua dulce. El abastecimiento de agua del acueducto en la Isla St. George, representa entre 7% y 25% de la recarga natural; por otro lado se necesitan cantidades mayores de recarga para simular los lentes en la Isla St. George que en la Isla Dog. En ambas islas, las tasas de erosión costera son tan rápidas, que las lentes de agua dulce podrían no estar en equilibrio con las condiciones actuales de frontera.

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Résumé La morphologie des lentilles deau douce des îles de Dog et de St. George, situés dans le district côtier de Florida-Etats Unis est contrôlé dans une mesure variable par des facteurs naturelles et humaines. Les modèles découlement à densité variable des eaux souterraines confirment que la variation spatiale de la recharge peut expliquer les asymétries observées des lentilles des cettes îles. La profondeur des lentilles ne présente pas des variations saisonnières importantes. Les activités humaines ont modifié la recharge dans certaines zones en amincissant localement les lentilles deau douce. Lalimentation en eaux de lîle de St. George représente 7–25% de la recharge naturelle; pour simuler les lentilles de lîle de Dog il a été nécessaire dintroduire des valeurs plus élevées de la recharge par rapport aux ceux utilisés pour lîle de St. George. Pour les deux îles lérosion côtière est assez rapide ainsi que les lentilles de eau douce ne sont pas en équilibre avec les conditions à la frontière.

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A paleolimnological record of rainfall and drought from East Java, Indonesia during the last 1,400?years     

Jessica R. Rodysill  James M. Russell  Satria Bijaksana  Erik T. Brown  La Ode Safiuddin  Hilde Eggermont 《Journal of Paleolimnology》2012,47(1):125-139

Variations in the location and strength of convection in the Western Pacific Warm Pool (WPWP) have a profound impact on the distribution and amount of global rainfall. Much of the variability in WPWP convection is attributed to variations in the El Ni?o-Southern Oscillation, for which the long-term trends and forcing mechanisms remain poorly understood. Despite the importance of WPWP convection to global climate change, we have very few paleohydrological reconstructions from the region. Here we present a new paleolimnologic and paleohydrologic record spanning the past 1,400 years using a multi-proxy dataset from Lake Logung, located in East Java, Indonesia that provides insights into centennial-scale trends in warm pool hydrology. Organic matter δ¹³C data indicate that East Java became wetter over the last millennium until ca. 1800 Common Era (CE), consistent with evidence for the southward migration of the Intertropical Convergence Zone (ITCZ) during this time. Superimposed on this long-term trend are four decade- to century-scale droughts, inferred from organic matter δ¹³C and calcite abundance data. They are centered at 1030, 1550, 1830, and 1996 CE. The three more recent droughts correlate with hydrologic anomalies documented in other proxy records from the WPWP region on both sides of the equator, and the two most recent droughts correlate in time with historically documented periods of multiple, intense El Ni?o events. Thus, our record provides strong evidence that century-scale hydrologic variability in this region relates to changes in the Walker Circulation. Human activity within the lake catchment is apparent since 1860 CE.  相似文献   

A multi-proxy record of the Last Glacial Maximum and last 14,500?years of paleoenvironmental change at Lone Spruce Pond, southwestern Alaska     

Darrell S. Kaufman  Yarrow Axford  R. Scott Anderson  Scott F. Lamoureux  Daniel E. Schindler  Ian R. Walker  Al Werner 《Journal of Paleolimnology》2012,48(1):9-26

Sediment cores from Lone Spruce Pond (60.007°N, 159.143°W), southwestern Alaska, record paleoenvironmental changes during the global Last Glacial Maximum (LGM), and during the last 14,500 calendar years BP (14.5?cal?ka). We analyzed the abundance of organic matter, biogenic silica, carbon, and nitrogen, and the isotope ratios of C and N, magnetic susceptibility, and grain-size distribution of bulk sediment, abundance of alder shrub (Alnus) pollen, and midge (Chironomidae and Chaoboridae) assemblages in a 4.7-m-long sediment sequence from the depocenter at 22?m water depth. The basal unit contains macrofossils dating to 25?C21?cal?ka (the global LGM), and is interpreted as glacial-lacustrine sediment. The open water requires that the outlet of the Ahklun Mountain ice cap had retreated to within 6?km of the range crest. In addition to cladocerans and diatoms, the glacial-lacustrine mud contains chironomids consistent with deep, oligotrophic conditions; several taxa associated with relatively warm conditions are present, suggestive of relative warmth during the global LGM. The glacial-lacustrine unit is separated from the overlying non-glacial lake sediment by a possible disconformity, which might record a readvance of glacier ice. Non-glacial sediment began accumulating around 14.5?cal?ka, with high flux of mineral matter and fluctuating physical and biological properties through the global deglacial period, including a reversal in biogenic-silica (BSi) content during the Younger Dryas (YD). During the global deglacial interval, the ??¹³C values of lake sediment were higher relative to other periods, consistent with low C:N ratios (8), and suggesting a dominant atmospheric CO₂ source of C for phytoplankton. Concentrations of aquatic faunal remains (chironomids and Cladocera) were low throughout the deglacial interval, diversity was low and warm-indicator taxa were absent. Higher production and air temperatures are inferred following the YD, when bulk organic-matter (OM) content (LOI 550?°C) increased substantially and permanently, from 10 to 30?%, a trend paralleled by an increase in C and N abundance, an increase in C:N ratio (to about 12), and a decrease in ??¹³C of sediment. Post-YD warming is marked by a rapid shift in the midge assemblage. Between 8.9 and 8.5?cal?ka, Alnus pollen tripled (25?C75?%), followed by the near-tripling of BSi (7?C19?%) by 8.2?cal?ka, and ??¹⁵N began a steady rise, reflecting the buildup of N and an increase in denitrification in soils. Several chironomid taxa indicative of relatively warm conditions were present throughout the Holocene. Quantitative chironomid-based temperature inferences are complicated by the expansion of Alnus and resulting changes in lake nutrient status and production; these changes were associated with an abrupt increase in cladoceran abundance and persistent shift in the chironomid assemblage. During the last 2,000?years, chironomid-assemblage changes suggest cooler temperatures, and BSi and OM values were generally lower than their maximum Holocene values, with minima during the seventh and eighth centuries, and again during the eighteenth century.  相似文献   

A late glacial Antarctic climate teleconnection and variable Holocene seasonality at Lake Pupuke, Auckland, New Zealand     

Thomas Stephens  Daniel Atkin  Paul Augustinus  Philip Shane  Andrew Lorrey  Alayne Street-Perrott  Andreas Nilsson  Ian Snowball 《Journal of Paleolimnology》2012,48(4):785-800

We present the first continuous paleolimnological reconstruction from the North Island of New Zealand (37°S) that spans the last 48.2?cal kyr. A tephra- and radiocarbon-based chronology was developed to infer the timing of marked paleolimnological changes in Lake Pupuke, Auckland, New Zealand, identified using sedimentology, magnetic susceptibility, grain size and geochemistry (carbon, nitrogen and sulphur concentrations and fluxes, carbon and nitrogen stable isotopes). Variable erosional influx, biomass and benthic REDOX conditions are linked to changing effective precipitation and seasonality within three inferred broad intervals of climatic change: (1) the Last Glacial Coldest Phase (LGCP) of reduced effective precipitation and cooler temperatures, from 28.8 to 18.0?cal kyr BP, (2) the Last Glacial Interglacial Transition (LGIT) of increasing effective precipitation and warmer conditions, from 18.0 to 10.2?cal?kyr?BP, and (3) a Holocene interval of high effective precipitation, beginning with a warm period of limited seasonality from 10.2?cal?kyr?BP and followed by increasing seasonality from 7.6?cal?kyr?BP. The LGCP and LGIT also contain millennial-scale climate events, including the coldest inferred glacial conditions during the LGCP from 27.8 to 26.0 and 22.0?C19.0?cal?kyr?BP, and a climate reversal in the LGIT associated with lower lake level, from 14.5 to 13.8?cal?kyr?BP, coeval with the Antarctic Cold Reversal. The onset of seasonal thermal stratification occurred at 5.7?cal?kyr?BP and was linked to natural eutrophication of Lake Pupuke, which produced enhanced organic sedimentation.  相似文献   

On the long-term context of the 1997–2009 ‘Big Dry’ in South-Eastern Australia: insights from a 206-year multi-proxy rainfall reconstruction     

Joëlle Gergis  Ailie Jane Eyre Gallant  Karl Braganza  David John Karoly  Kathryn Allen  Louise Cullen  Rosanne D’Arrigo  Ian Goodwin  Pauline Grierson  Shayne McGregor 《Climatic change》2012,111(3-4):923-944

This study presents the first multi-proxy reconstruction of rainfall variability from the mid-latitude region of south-eastern Australia (SEA). A skilful rainfall reconstruction for the 1783–1988 period was possible using twelve annually-resolved palaeoclimate records from the Australasian region. An innovative Monte Carlo calibration and verification technique is introduced to provide the robust uncertainty estimates needed for reliable climate reconstructions. Our ensemble median reconstruction captures 33% of inter-annual and 72% of decadal variations in instrumental SEA rainfall observations. We investigate the stability of regional SEA rainfall with large-scale circulation associated with El Niño–Southern Oscillation (ENSO) and the Inter-decadal Pacific Oscillation (IPO) over the past 206 years. We find evidence for a robust relationship with high SEA rainfall, ENSO and the IPO over the 1840–1988 period. These relationships break down in the late 18th–early 19th century, coinciding with a known period of equatorial Pacific Sea Surface Temperature (SST) cooling during one of the most severe periods of the Little Ice Age. In comparison to a markedly wetter late 18th/early 19th century containing 75% of sustained wet years, 70% of all reconstructed sustained dry years in SEA occur during the 20th century. In the context of the rainfall estimates introduced here, there is a 97.1% probability that the decadal rainfall anomaly recorded during the 1998–2008 ‘Big Dry’ is the worst experienced since the first European settlement of Australia.  相似文献   

Understanding and partitioning future climates for Australian regions from CMIP3 using ocean warming indices     

Ian Geoffrey Watterson 《Climatic change》2012,111(3-4):903-922

The patterns of large-scale climate change over the 21st century simulated by 23 CMIP3 global climate models are analyzed to provide understanding of the range of projected temperature T and precipitation P changes for Australia published in 2007. Means of change, standardized by the global warming, within each of 11 regions are calculated for each model. Correlations between regions across the 23 models indicate that the changes are rather coherent across much of the mainland. The all-Australian average changes are also well correlated with a pattern of tropical sea surface temperatures. A Pacific-Indian Dipole index, representing this pattern, correlates strongly with Australian P. It also correlates well with variables in Southeast Asia. The global warming itself correlates well with Australian warming. These two indices of large-scale ocean warming are used to partition the 23 models into four representative future climates. For Australia overall, these can be described as much warmer and drier, much warmer, warmer and drier, and warmer. The four climates span much of the range of the earlier Australian projections over most of the continent. Further, they may be reproduced by a downscaling model forced with the SST anomalies. An assessment of the realism of the ocean pattern changes has the potential to reduce the uncertainty of projections, both for Australia and beyond.  相似文献   

An empirical framework for tropical cyclone climatology     

Nam-Young Kang  James B. Elsner 《Climate Dynamics》2012,39(3-4):669-680

An empirical approach for analyzing tropical cyclone climate is presented. The approach uses lifetime-maximum wind speed and cyclone frequency to induce two orthogonal variables labeled “activity” and “efficiency of intensity”. The paired variations of activity and efficiency of intensity along with the opponent variations of frequency and intensity configure a framework for evaluating tropical cyclone climate. Although cyclone activity as defined in this framework is highly correlated with the commonly used exponent indices like accumulated cyclone energy, it does not contain cyclone duration. Empirical quantiles are used to determine threshold intensity levels, and variant year ranges are used to find consistent trends in tropical cyclone climatology. In the western North Pacific, cyclone activity is decreasing despite increases in lifetime-maximum intensity. This is due to overwhelming decreases in cyclone frequency. These changes are also explained by an increasing efficiency of intensity. The North Atlantic shows different behavior. Cyclone activity is increasing due to increasing frequency and, to a lesser extent, increasing intensity. These changes are also explained by a decreasing efficiency of intensity. Tropical cyclone trends over the North Atlantic basin are more consistent over different year ranges than tropical cyclone trends over the western North Pacific.  相似文献   

Perturbed physics ensemble using the MIROC5 coupled atmosphere–ocean GCM without flux corrections: experimental design and results     

Hideo Shiogama  Masahiro Watanabe  Masakazu Yoshimori  Tokuta Yokohata  Tomoo Ogura  James D. Annan  Julia C. Hargreaves  Manabu Abe  Youichi Kamae  Ryouta O’ishi  Rei Nobui  Seita Emori  Toru Nozawa  Ayako Abe-Ouchi  Masahide Kimoto 《Climate Dynamics》2012,39(12):3041-3056

In this study, we constructed a perturbed physics ensemble (PPE) for the MIROC5 coupled atmosphere–ocean general circulation model (CGCM) to investigate the parametric uncertainty of climate sensitivity (CS). Previous studies of PPEs have mainly used the atmosphere-slab ocean models. A few PPE studies using a CGCM applied flux corrections, because perturbations in parameters can lead to large radiation imbalances at the top of the atmosphere and climate drifts. We developed a method to prevent climate drifts in PPE experiments using the MIROC5 CGCM without flux corrections. We simultaneously swept 10 parameters in atmosphere and surface schemes. The range of CS (estimated from our 35 ensemble members) was not wide (2.2–3.2?°C). The shortwave cloud feedback related to changes in middle-level cloud albedo dominated the variations in the total feedback. We found three performance metrics for the present climate simulations of middle-level cloud albedo, precipitation, and ENSO amplitude that systematically relate to the variations in shortwave cloud feedback in this PPE.  相似文献