A changing nutrient regime in the Gulf of Maine

Recent oceanographic observations and a retrospective analysis of nutrients and hydrography over the past five decades have revealed that the principal source of nutrients to the Gulf of Maine, the deep, nutrient-rich continental slope waters that enter at depth through the Northeast Channel, may have become less important to the Gulf's nutrient load. Since the 1970s, the deeper waters in the interior Gulf of Maine (>100 m) have become fresher and cooler, with lower nitrate (NO₃) but higher silicate (Si(OH)₄) concentrations. Prior to this decade, nitrate concentrations in the Gulf normally exceeded silicate by 4–5 μM, but now silicate and nitrate are nearly equal. These changes only partially correspond with that expected from deep slope water fluxes correlated with the North Atlantic Oscillation, and are opposite to patterns in freshwater discharges from the major rivers in the region. We suggest that accelerated melting in the Arctic and concomitant freshening of the Labrador Sea in recent decades have likely increased the equatorward baroclinic transport of the inner limb of the Labrador Current that flows over the broad continental shelf from the Grand Banks of Newfoundland to the Gulf of Maine. That current system now brings a greater fraction of colder and fresher deep shelf waters into the Gulf than warmer and saltier offshore slope waters which were previously thought to dominate the flux of nutrients. Those deep shelf waters reflect nitrate losses from sediment denitrification and silicate accumulations from rivers and in situ regeneration, which together are altering the nutrient regime and potentially the structure of the planktonic ecosystem.     

Transparent Exopolymer Particles (TEP) in a River-Dominated Estuary: Spatial–Temporal Distributions and an Assessment of Controls upon TEP Formation

Transparent exopolymer particle (TEP) distributions were examined in North Carolina’s Neuse River Estuary (NRE) over a 1-year period, and experiments were conducted to examine controls upon TEP formation. TEP-carbon averaged 16% of the total organic carbon pool in the NRE. From May to early August 2007, TEP ranged from 805 to 1,801 μg xanthan gum (XG) L⁻¹. A large phytoplankton bloom peaked in early August and then subsequently declined. Within 2 weeks of the bloom’s peak, TEP concentrations increased to >3,500 μg XG L⁻¹ and remained elevated through mid-December. Decreasing water temperatures and enhanced retention due to drought conditions in Fall 2007 likely aided in the persistence of TEP concentrations during this timeframe. Thereafter, TEP decreased coincident with increased river flow and flushing of the estuary that began in January 2008, and TEP remained low (ranging from 991 to 1,712 μg XG L⁻¹) until the end of April 2008. TEP was positively correlated with salinity, suggesting that cation availability (which co-varies with salinity and stabilizes the structural integrity of TEP) may play a role in estuarine TEP formation. In two cation addition experiments using water from the mesohaline region, TEP concentrations tended to be higher in treatments with vs. without cations added. Statistically significant relationships were also found between in situ TEP concentrations and pH, mixing, and temperature, suggesting that a complex suite of environmental parameters affect TEP formation and its distribution in estuaries.     

Connecting Local to Global: Geographic Information Systems and Ecological Footprints as Tools for Sustainability

Tools that support public engagement with sustainability are essential for local sustainability planning. This research investigates the ability of two geographic information system (GIS)-based tools to promote discussion of sustainability in a suburban context. A local ecological footprint tool and a community environmental atlas (an environmentally themed online mapping system) were created for seven suburban boroughs of Montreal. Variations of both tools have been used to support sustainability efforts, but their use has not been widely evaluated. Working from a public participation GIS (PPGIS) framework that recognizes the powerful influence of data representation, this research uses focus groups to evaluate how well these tools address three criteria that have emerged from the literature on public engagement in sustainability: interdependency across systems, reflexivity about personal and social decision making, and interactions across spatial scales. Whereas the atlas remains advantageous for discussing local spatial specifics, it was found that the ecological footprint helped people see the interconnections among systems, integrate local and global aspects of sustainability, and reflect on the values and assumptions underlying current social and economic structures.     

Natural Buffering of Groundwater Contaminants Related to Development in Fractured Rock

In rural areas of New England groundwater from fractured crystalline and sedimentary bedrock is a critical water resource. Increasingly, studies have shown that development occurring in rural areas is resulting in the impairment of water quality in fractured rock aquifers. The objective of this study was to evaluate the spatial and temporal variations in groundwater quality associated with development and evaluate the extent to which common groundwater contaminants associated with rural development may be naturally buffered. The study entailed a compilation and synthesis of over 2500 reports on domestic water quality that spanned a 30 year period. Focus was placed on the spatial distribution and temporal variations in sodium, chloride, iron, manganese, nitrate, and nitrite. Results indicate that despite significant levels of development, the amount of contamination to the bedrock has been minimal. Of the constituents examined, only the chloride concentration exhibits a systematic increase over time, but the level of chloride remained relatively low. The flux of chloride to the bedrock from deicing appears minimal despite the significant amounts of road salt used in the study area. Sodium concentrations in the bedrock remained relatively constant and appear to be buffered by ion exchange with calcium as suggested by the increase in hardness with time. Iron and manganese were present at relatively low levels but did not show any systematic trends over time. Nitrate and nitrite concentrations were very low and found to be inversely correlated with manganese and iron concentrations. This suggests that the presence of iron and manganese contribute to denitrification. This study indicated that both geochemical and biogeochemical processes are active buffering mechanisms that help shield the bedrock from contaminants associated with development.     

Mapping the Land Cover of Mexico Using AVHRR Time‐Series Data Sets

Abstract

An important methodological and analytical requirement for analyzing spatial relationships between regional habitats and species distributions in Mexico is the development of standard methods for mapping the country's land cover/land use formations. This necessarily involves the use of global data such as that produced by the Advanced Very High Resolution Radiometer (AVHRR). We created a nine‐band time‐series composite image from AVHRR Normalized Difference Vegetation Index (NDVI) bi‐weekly data. Each band represented the maximum NDVI for a particular month of either 1992 or 1993. We carried out a supervised classification approach, using the latest comprehensive land cover/vegetation map created by the Mexican National Institute of Geography (INEGI) as reference data. Training areas for 26 land cover/vegetation types were selected and digitized on the computer's screen by overlaying the INEGI vector coverage on the NDVI image. To obtain specific spectral responses for each vegetation type, as determined by its characteristic phenology and geographic location, the statistics of the spectral signatures were subjected to a cluster analysis. A total of 104 classes distributed among the 26 land cover types were used to perform the classification. Elevation data were used to direct classification output for pine‐oak and coastal vegetation types. The overall correspondence value of the classification proposed in this paper was 54%; however, for main vegetation formations correspondence values were higher (60‐80%). In order to obtain refinements in the proposed classification we recommend further analysis of the signature statistics and adding topographic data into the classification algorithm.     

The application of lead isotope ratios in the Antarctic macroalga Iridaea cordata as a contaminant monitoring tool

Lead (Pb) isotope ratios were measured in the marine macroalga Iridaea cordata collected from four locations in the Windmill Islands, East Antarctica. Based on the masses of thalli collected, samples analysed in this study were likely to be a mixture of one and two year old thalli. For a sample of thalli of various ages (<12 months to 2 years old) from the same site there was no apparent variation in Pb concentration or Pb isotope ratio with thallus mass/age, indicating that contaminant sources had been constant over the lifetime of the thalli sampled. I.cordata samples close to the Thala Valley waste disposal site (Brown Bay Inner) near the Australian Station, Casey, displayed isotopic signatures (²⁰⁸Pb/²⁰⁴Pb 35.99; ²⁰⁶Pb/²⁰⁷Pb 1.066; n = 3; average values shown) trending towards that possessed by major Australian Pb sources (Broken Hill and Mt Isa, ²⁰⁸Pb/²⁰⁴Pb 35.60; ²⁰⁶Pb/²⁰⁷Pb 1.041) suggesting that these samples had been exposed to anthropogenic Pb originating from the Thala Valley waste disposal site. Material collected hundreds of metres from the tip location at Brown Bay Outer had isotopic values (²⁰⁸Pb/²⁰⁴Pb 36.32; ²⁰⁶Pb/²⁰⁷Pb 1.088; n = 10) intermediate between Brown Bay Inner and sites further from the contaminant source at Sparkes Bay and Wilkes (²⁰⁸Pb/²⁰⁴Pb 36.46; ²⁰⁶Pb/²⁰⁷Pb 1.094; n = 4) showing that contaminant transport was predominantly restricted to Brown Bay Inner. This study demonstrates that the isotope ratios of Pb in marine macroalgae can provide valuable information as to the origin and extent of heavy metal flux in a marine environment.     

Plasmaspheric hiss overview and relation to chorus

A brief overview is given of the history of plasmaspheric hiss research, particularly in the context of the recent work by Bortnik et al. (2008) indicating that chorus could be the likely source of plasmaspheric hiss. Previous suggestions given in the literature for this theory are reviewed and then the mechanism itself is outlined, focusing on the characteristic cyclical trajectories executed by typical ray paths that enter into the plasmasphere. A number of directional propagation studies performed in the past are then discussed as well as other work which bears relevance to the present mechanism.     

Determination of High Field Strength Elements, Rb, Sr, Mo, Sb, Cs, Tl and Bi at ng g−1 Levels in Geological Reference Materials by Magnetic Sector ICP-MS after HF/HClO4 High Pressure Digestion

Six low abundance rock reference materials (basalt BIR-1, dunite DTS-1, dolerite DNC-1, peridotite PCC-1, serpentine UB-N and basalt TAFAHI) have been analysed for high field strength elements (Zr, Nb, Hf, Ta, Th and U), Rb, Sr, Mo, Sb, Cs, Tl and Bi at ng g⁻¹ levels (in rock) by magnetic sector inductively coupled plasma-mass spectrometry after HF/HClO₄ high pressure decomposition. The adopted method uses only indium as an internal standard. Detection limits were found to be in the range of 0.08 to 16.2 pg ml⁻¹ in solution (equivalent to 0.08 to 16.2 ng g⁻¹ in rock). Our data for high field strength elements, Rb, Sr, Mo, Sb, Cs, Tl and Bi for the six selected low abundance geological reference materials show general agreement with previously published data. Our Ta values in DTS-1 and PCC-1 (1.3 and 0.5 ng g⁻¹) are lower than in previously published studies, providing smooth primitive mantle distribution patterns. Lower values were also found for Tl in BIR-1, DTS-1 and PCC-1 (2, 0.4 and 0.8 ng g⁻¹). Compared with quadrupole ICP-MS studies, the proposed magnetic sector ICP-MS method can generally provide better detection limits, so that the measurement of high field strength elements, Rb, Sr, Mo, Sb, Cs, Tl and Bi at ng g⁻¹ levels can be achieved without pre-concentration, ion exchange separation or other specialised techniques.     

Assessment of Nitrate—Nitrogen Distribution in Kansas Groundwater, 1990–1998

Nitrate-nitrogen (nitrate-N) in groundwater is a potential problem in many parts of Kansas.From 1990 to 1998, 747 water samples were collected from domestic, irrigation, monitoring,and public water supply wells primarily from the western two-thirds of the state, and analyzedfor nitrate-N by the Kansas Geological Survey. Nitrate-N concentrations of the 747 samplesanalyzed range from 29% with less than or equal to 3 mg/L, 51% between 3 and 10 mg/L,and 20% greater than or equal to 10 mg/L. Factors that show a statistically significantrelationship with the occurrence of nitrate-N in Kansas groundwater in this assessment includegeographic area of the state, depth of well, and age of well. Nitrate-N levels of wells screenedin the High Plains aquifer in south-central Kansas showed a statistically higher concentrationthan samples collected from the High Plains aquifer in the western portion of the state.Comparison of nitrate-N with depth of well indicated that shallower wells throughout the stategenerally have higher nitrate-N values than deeper wells. Irrigation wells older than 1975showed statistically higher nitrate-N concentration than wells installed during and since 1975,possibly related to changes in well-construction practices and regulations that occurred in 1975.     

Modeled Sverdrup flow in the North Atlantic from 11 different wind stress climatologies

In studies of large-scale ocean dynamics, often quoted values of Sverdrup transport are computed using the Hellerman–Rosenstein wind stress climatology. The Sverdrup solution varies, however, depending on the wind set used. We examine the differences in the large-scale upper ocean response to different surface momentum forcing fields for the North Atlantic Ocean by comparing the different Sverdrup interior/Munk western boundary layer solutions produced by a 1/16° linear numerical ocean model forced by 11 different wind stress climatologies. Significant differences in the results underscore the importance of careful selection of a wind set for Sverdrup transport calculation and for driving nonlinear models. This high-resolution modeling approach to solving the linear wind-driven ocean circulation problem is a convenient way to discern details of the Sverdrup flow and Munk western boundary layers in areas of complicated geometry such as the Caribbean and Bahamas. In addition, the linear solutions from a large number of wind sets provide a well-understood baseline oceanic response to wind stress forcing and thus, (1) insight into the dynamics of observed circulation features, by themselves and in conjunction with nonlinear models, and (2) insight into nonlinear model sensitivity to the choice of wind-forcing product.The wind stress products are evaluated and insight into the linear dynamics of specific ocean features is obtained by examining wind stress curl patterns in relation to the corresponding high-resolution linear solutions in conjunction with observational knowledge of the ocean circulation. In the Sverdrup/Munk solutions, the Gulf Stream pathway consists of two branches. One separates from the coast at the observed separation point, but penetrates due east in an unrealistic manner. The other, which overshoots the separation point at Cape Hatteras and continues to flow northward along the continental boundary, is required to balance the Sverdrup interior transport. A similar depiction of the Gulf Stream is commonly seen in the mean flow of nonlinear, eddy-resolving basin-scale models of the North Atlantic Ocean. An O(1) change from linear dynamics is required for realistic simulation of the Gulf Stream pathway. Nine of the eleven Sverdrup solutions have a C-shaped subtropical gyre, similar to what is seen in dynamic height contours derived from observations. Three mechanisms are identified that can contribute to this pattern in the Sverdrup transport contours. Along 27°N, several wind sets drive realistic total western boundary current transport (within 10% of observed) when a 14 Sv global thermohaline contribution is added (COADS, ECMWF 10 m re-analysis and operational, Hellerman–Rosenstein and National Centers for Environmental Prediction (NCEP) surface stress re-analysis), a few drive transport that is substantially too high (ECMWF 1000 mb re-analysis and operational and Isemer–Hasse) and Fleet Numerical Meteorology and Oceanography Center (FNMOC) surface stresses give linear transport that is slightly weaker than observed. However, higher order dynamics are required to explain the partitioning of this transport between the Florida Straits and just east of the Bahamas (minimal in the linear solutions vs. 5 Sv observed east of the Bahamas). Part of the Azores Current transport is explained by Sverdrup dynamics. So are the basic path of the North Atlantic Current (NAC) and the circulation features within the Intra-Americas Sea (IAS), when a linear rendition of the northward upper ocean return flow of the global thermohaline circulation is added in the form of a Munk western boundary layer.