Sediment yield in the Patuxent River (Md.) undergoing urbanization, 1968–1969

Water discharge from the Patuxent River into its estuary was near-average (95%) during the water year 1968–1969 although precipitation was only 79% of the average. Suspended-sediment discharge into the estuary, however, was more then double the normal yield (344 metric tons/km² compared to 143 metric tons/km²). These increases in runoff and suspended-sediment yields, despite decreased precipitation, must be attributed to urbanization of the drainage basin.The maximum measured suspended-sediment concentrations in the rural Middle Patuxent basin (Piedmont Province) increased only 40-fold during an increase from “average” to high water runoff (15 mg/l to 600 mg/l). In the portion of the Little Patuxent River basin undergoing urbanization (Piedmont portion), stream concentrations increased by over two orders of magnitude (20 mg/l to 2400 mg/l) as a result of heavy rainfall. The area undergoing urbanization of the Little Patuxent yielded more than twice as much suspended sediment per unit area as the rural Middle Patuxent (620 metric tons/km² versus 290 metric tons/km²). This increase also is interpreted to be the direct result of erosion of soils denuded or disturbed during urban construction.Using the Middle Patuxent as a “standard” for normal erosion rates in rural areas, construction sites contributed about 82% of the suspended sediment discharged by the Patuxent River into its estuary even though such sites represented only 23% of the drainage basin.     

Future Changes in Biological Activity in the North Pacific Due to Anthropogenic Forcing of the Physical Environment

Many studies have examined the physical changes expected in the environment as a result of anthropogenic forcing. These physical changes will have an effect on ecosystems as well. In this study, a nitrogen-phytoplankton-zooplankton (NPZ) model is used to examine the effects of changes in the physical environment on primary productivity in the North Pacific ocean. The physical variables considered are mixed layer temperature and depth, solar insolation, and large-scale upwelling. The changes in these fields by the 2090s are taken from a coupled ocean-atmosphere general circulation model forced by projected atmospheric CO₂ and sulfates, then applied to the NPZ biological model. Theresult is a change in the seasonal cycle of phytoplankton and herbivore concentrations across the subpolar North Pacific, moving from a regime characterized by strong variability with low wintertime values and a spring bloom, to much more constant yearly values. A reduction of yearly-averaged primary productivity accompanies much of this shift to more constant year-round conditions. In other regions, productivity increases as warmer surface waters enable higher growth rates. Changes in mixed layer temperature and depth account for almost all the changes in productivity; model-predicted changes in surface insolation and large-scale upwelling have little impact.     

Flux of particulate matter in the tidal marshes and subtidal shallows of the Rhode River estuary

There was a net influx of suspended particulate matter to the uppermost part of the Rhode River estuary during the several years of this study. Most of the influx was due to episodic discharges of suspended sediment from the watershed during heavy rains. In contrast, tidal exchange of particulate matter was not related to rainstorms. Sediment composition data and historical records indicate that marsh accretion accounts for only 13% of the sediment trapping although marshes occupy 60% of the study area. Influx of particulate matter to the marshes is directly related to the amount of time they are submerged during tidal cycles.     

Response of underwater light transmittance in the Rhode River estuary to changes in water-quality parameters

A year-long study of incident and underwater light transmittance (400–800 nm) in the Rhode River, Maryland, a tidal tributary to Chesapeake Bay, indicated that light transmittance responded in both intensity and spectral quality to changes in the amount and type of dissolved and suspended materials in the water. At times of relatively clear water, transmittance was similar to that previously reported in the literature for coastal waters. With high concentrations of suspended and dissolved materials in the water, attenuation of irradiance was high in the upper part of the water column and different for the various wave bands, depending on the type of material present. At such times, attenuation was higher in the upper part of the water column under sunny, clear skies than on cloudy days. We believe this to be due to higher concentrations of pigments and suspended particles in the water on sunny days, increasing the scattering and adsorption. A second factor was a lower average cosine on cloudy days, decreasing the effect of scattering on the average path length per meter of depth. High attenuation coefficients in the middle of the spectrum are attributed to accessory pigments. Regression of the diffuse attenuation coefficient on eight water-quality parameters explained up to 93% of the variance in the attenuation coefficient. Chlorophylls a and c and mineral suspensates were the three most important variables for data taken under clear skies. In contrast, under cloudy skies, the three most important variables were different for different wavelengths. Models of irradiance attenuation in turbid estuarine waters require the use of more variables than models for open ocean waters.     

The ACPI Project, Element 1: Initializing a Coupled Climate Model from Observed Conditions

A problem for climate change studies with coupled ocean-atmosphere models has been how to incorporate observed initial conditions into the ocean, which holds most of the `memory' of anthropogenic forcing effects. The first difficulty is the lack of comprehensive three-dimensional observations of the current ocean temperature (T) and salinity (S) fields to initialize to. The second problem is that directly imposing observed T and S fields into the model results in rapid drift back to the model climatology, with the corresponding loss of the observed information. Anthropogenic forcing scenarios therefore typically initialize future runs by starting with pre-industrial conditions. However, if the future climate depends on the details of the present climate, then initializing the model to observations may provide more accurate forecasts. Also, this ～ 130 yr spin up imposes substantial overhead if only a few decades of predictions are desired. A new technique to address these problems is presented. In lieu of observed T and S, assimilated ocean data were used. To reduce model drift, an anomaly coupling scheme was devised. This consists of letting the model's climatological (pre-industrial) oceanic and atmospheric heat contents and transports balance each other, while adding on the (much smaller) changes in heat content since the pre-industrial era as anomalies. The result is model drift of no more than 0.2 K over 50 years, significantly smaller than the forced response of 1.0 K. An ensemble of runs with these assimilated initial conditions is then compared to a set spun up from pre-industrial conditions. No systematic differences were found, i.e., the model simulation of the ocean temperature structure in the late 1990s is statistically indistinguishable from the assimilated observations. However, a model with a worse representation of the late 20th century climate might show significant differences if initialized in this way.     

Reconstructing snowmelt in Idaho’s watershed using historic streamflow records

In recent decades, a warming climate likely has accelerated the timing of spring snowmelt in the western United States; however, records of the timing of snowmelt typically only extend to the 1980s. Stream gage data can lengthen records of the timing of snowmelt back to the early 1900s, enhancing understanding of past, current, and future climate change on snowmelt-dominated watersheds and associated ecosystems. We used snowpack telemetry data and historic streamflow records to test reconstructions of final snowmelt dates using Short Time Fourier Transform (STFT) wavelet analysis of hydrographs. STFT reconstructions tested against known final snowmelt dates over the last ~25 years indicate final snowmelt can be determined within ±4 days ~95% of the time and within ±7 days 100% of the time. Comparison of the STFT method with the center of timing method indicates that in addition to reconstructing actual snowmelt dates (as opposed to dates associated with the center of timing of streamflow), the STFT method may limit interpretation errors associated with changes in discharge not related to snowmelt. Reconstructions of final snowmelt dates in the Idaho, U.S. study area show intervals of early snowmelt (1920s–1930s), later and less variable snowmelt (1940s–1970s), and both variable and early snowmelt (~1985–2007). Early and variable snowmelt during the last ~20 years is associated with large wildfires.     

Host galaxy morphologies of X-ray selected AGN: assessing the significance of different black hole fuelling mechanisms to the accretion density of the Universe at z∼ 1.

We use morphological information of X-ray selected active galactic nuclei (AGN) hosts to set limits on the fraction of the accretion density of the Universe at   z ≈ 1  that is not likely to be associated with major mergers. Deep X-ray observations are combined with high-resolution optical data from the Hubble Space Telescope in the All-wavelength Extended Groth strip International Survey, Great Observatories Origins Deep Survey (GOODS) North and GOODS South fields to explore the morphological breakdown of X-ray sources in the redshift interval  0.5 < z < 1.3  . The sample is split into discs, early-type bulge-dominated galaxies, peculiar systems and point sources in which the nuclear source outshines the host galaxy. The X-ray luminosity function and luminosity density of AGN at   z ≈ 1  are then calculated as a function of morphological type. We find that disc-dominated hosts contribute  30 ± 9  per cent to the total AGN space density and  23 ± 6  per cent to the luminosity density at   z ≈ 1  . We argue that AGN in disc galaxies are most likely fuelled not by major merger events but by minor interactions or internal instabilities. We find evidence that these mechanisms may be more efficient in producing luminous AGN     compared to predictions for the stochastic fuelling of massive black holes in disc galaxies.