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1.
K. G. Sellner D. C. Brownlee M. H. Bundy S. G. Brownlee K. R. Braun 《Estuaries and Coasts》1993,16(4):859-872
During summer, bloom-forming cyanobacteria, including Anacystis, Aphanizomenon, and Microcystis aeruginosa, dominate tidal-fresh waters of the upper Potomac River estuary with densities exceeding 108 cells l?1. In an attempt to determine the importance of these high cyanobacteria densities to planktonic herbivory in the system, short-term grazing experiments were conducted in July and August 1987. Using size-fractionated river phytoplankton assemblages, zooplankton grazing rates were determined for dominant or subdominant planktonic microzooplankton and mesozooplankton feeding on 14C-labeled river assemblages, 14C-labeled river assemblages enriched with unlabeled cyanobacteria, and unlabeled river assemblages enriched with 14C-labeled cyanobacteria. Grazing rates were estimated for the rotifers Polyarthra remata, Hexarthra mira, Asplanchna brightwelli, Brachionus angularis, Epiphanes sp., Trichocerca similis, and the cyclopoid copepod Cyclops vernalis. Neither rotifers nor the copepod grazed heavily on Microcystis. Rotifer grazing rates on labeled cyanobacteria ranged from 4 to 1,650 nl· [individual · h]?1 while copepod rates ranged from undetectable to 135 μl · [copepod · h]?1. Grazing rates on labeled river phytoplankton assemblages were 4–100 times higher than noted for zooplankton feeding on cyanobacteria. The addition of the colonial alga to labeled river phytoplankton assemblages resulted in mixed zooplankton responses, that is, lower and higher grazing rates than observed on river assemblages with no added cyanobacteria. Total zooplankton demand for cyanobacteria and river phytoplankton assemblages was estimated for the study period July–August 1987. Rotifer plus C. vernalis herbivory would have removed 1–5% and 49%, respectively, of the standing stock of the two autotroph pools each day. Literature-derived clearance rates for Bosmina indicate, however, that herbivory by this cladoceran could increase demand to 24% and 60%, respectively, in bloom and nonbloom assemblages. These data suggest that the majority of cyanobacterial production remains ungrazed and may be transported to the lower estuary for salinity-induced aggregation and sedimentation. 相似文献
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Alida Bundy Johanna J. Heymans Lyne Morissette Claude Savenkoff 《Progress in Oceanography》2009,81(1-4):188
The facts: four Northwest Atlantic ecosystems, three cod stock collapses 15 years ago (plus one severely depleted), seals now top predator in all ecosystems, all had cod as a top predator before collapse, groundfish declines in all areas, forage base increased in most systems. No recovery in any system. Have these ecosystems fundamentally changed? Why? The challenge: compare and contrast these four ecosystems. The answer: using mass balance models, empirical data and a suite of ecosystem indicators, we explore how and why these systems have changed over time. At the ecosystem and community level, we see broad similarities between ecosystems. However, structurally and functionally these systems have shifted to an alternate state, with changes in predator structure, trophic structure and flow. 相似文献
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Ignacio Trujillo † C. J. Conselice Kevin Bundy M. C. Cooper P. Eisenhardt Richard S. Ellis 《Monthly notices of the Royal Astronomical Society》2007,382(1):109-120
Using the combined capabilities of the large near-infrared Palomar/DEEP-2 survey, and the superb resolution of the Advanced Camera for Surveys HST camera, we explore the size evolution of 831 very massive galaxies ( M ⋆ ≥ 1011 h −2 70 M⊙ ) since z ∼ 2 . We split our sample according to their light concentration using the Sérsic index n . At a given stellar mass, both low ( n < 2.5) and high ( n > 2.5) concentrated objects were much smaller in the past than their local massive counterparts. This evolution is particularly strong for the highly concentrated (spheroid like) objects. At z ∼ 1.5 , massive spheroid-like objects were a factor of 4 (±0.4) smaller (i.e. almost two orders of magnitudes denser) than those we see today. These small sized, high-mass galaxies do not exist in the nearby Universe, suggesting that this population merged with other galaxies over several billion years to form the largest galaxies we see today. 相似文献
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The coast of southeast Africa is dominated by sandy beaches that tend to be confined within log‐spiral or headland‐bound embayments. Investigations using serendipitous air imagery data set have been previously undertaken and conclusions drawn about the stability of the coast. We show that conclusions drawn from this data, with respect to the high water mark (HWM) position are fraught with errors, which include tidal state, pressure regime, beach slope, high‐swell erosion, seasonal and multi‐annual changes. We highlight and discuss these sources of error, together with their magnitudes. The most significant of these are the high‐swell, seasonal and multi‐annual variations. From case studies we show that the seasonal beach rotation and long‐term beach width variation are responsible for tens of metres of unaccounted HWM variation, 30 to 50 m is common, with maximums reaching 60 to 100 m. Overall the southeast African coastline appears to be in a state of long‐term dynamic equilibrium. There is no evidence of any sea‐level rise‐forced transgression in the coastal sediment budget, despite sea‐level rise (SLR). If such a signal is, in fact present, it is lost within the beach width variation. Some southeast African coastal reaches are suffering chronic erosion, but these are related to anthropogenic impacts. The extreme difficulty of placing a HWM, with any temporal validity on this coast precludes the routine use of the Bruun Rule. Although no transgressive signature is found, there is evidence of a decreasing coastal sand budget as a result of anthropogenic or natural climate change, or both. This decrease in the coastal sand volume is likely to result in increased future erosion. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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Michael C. Cooper Jeffrey A. Newman Benjamin J. Weiner Renbin Yan Christopher N. A. Willmer Kevin Bundy Alison L. Coil † Christopher J. Conselice Marc Davis S. M. Faber Brian F. Gerke Puragra Guhathakurta David C. Koo Kai G. Noeske 《Monthly notices of the Royal Astronomical Society》2008,383(3):1058-1078
Using galaxy samples drawn from the Sloan Digital Sky Survey and the DEEP2 Galaxy Redshift Survey, we study the relationship between star formation and environment at z ∼ 0.1 and 1. We estimate the total star formation rate (SFR) and specific star formation rate (sSFR) for each galaxy according to the measured [O ii ]λ 3727 Å nebular line luminosity, corrected using empirical calibrations to match more robust SFR indicators. Echoing previous results, we find that in the local Universe star formation depends on environment such that galaxies in regions of higher overdensity, on average, have lower SFRs and longer star formation time-scales than their counterparts in lower density regions. At z ∼ 1 , we show that the relationship between sSFR and environment mirrors that found locally. However, we discover that the relationship between total SFR and overdensity at z ∼ 1 is inverted relative to the local relation. This observed evolution in the SFR–density relation is driven, in part, by a population of bright, blue galaxies in dense environments at z ∼ 1 . This population, which lacks a counterpart at z ∼ 0 , is thought to evolve into members of the red sequence from z ∼ 1 to ∼0. Finally, we conclude that environment does not play a dominant role in the cosmic star formation history at z < 1 : the dependence of the mean galaxy SFR on local galaxy density at constant redshift is small compared to the decline in the global SFR space density over the last 7 Gyr. 相似文献
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The occurrence rate of type III solar bursts in the frequency range 4.9 MHz to 30 kHz is analyzed as a function of burst intensity and burst arrival direction. We find that (a) the occurrence rate of bursts falls off with increasing flux, S, according to the power law S
–1.5, and (b) the distribution of burst arrival directions at each frequency shows a significantly larger number of bursts observed west of the Earth-Sun line than east of it. This western excess in occurrence rate appears to be correlated with the direction of the average interplanetary magnetic field, and is interpreted as beaming of the observed burst radiation along the magnetic field direction.Presently at the University of Maryland, College Park, Maryland. 相似文献
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Sarah J. Bundy Kirkpatrick 《Natural Hazards》2018,90(1):365-389
Citizens have historically become involved in response to disasters by helping both themselves and others. Recently, the idea has emerged of individuals providing this assistance in the response period using bicycles. Community events have been organized by bicycling enthusiasts in US cities to demonstrate how bicycles could potentially be of use in disaster situations. Yet, there has been no empirical research around the idea of citizen bicyclists in disaster response. This study explored the potential use of bicycles and their citizen riders in disaster events in the USA—specifically considering what role, if any, citizen bicyclists could play in such scenarios. Data were initially collected through 21 in-depth, telephone interviews with emergency management officials and bicycling advocates from bicycle-friendly cities in ten different states. Grounded theory was used to conceptualize the overall research design and analyze the data. Based on theoretical and snowball sampling, an additional six interviews were completed with individuals who had requisite knowledge and experiences applicable to the research question. Participants indicated that there are a variety of tasks and activities citizen bicyclists could undertake in disaster response; however, it would have to be an event of significant scope and magnitude for bicycle usage to be widespread—an unlikely occurrence for many jurisdictions. Concerns about training and integration with the formal emergency management structure were also identified. Implications for potential citizen bicyclists—and citizen responders more broadly—are discussed. 相似文献
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Suspended sediment did not significantly affect Eurytemora affinis, which is the numerically dominant, late winter, early spring mesozooplankton taxon in Chesapeake Bay. In preliminary analyses of survival, broods per female and nauplii development for suspended sediment concentrations from 0 to 350 mg l−1, few significant differences were observed between populations exposed to no suspended sediment, and those in 50, 100 and 350 mg l−1. However, in every case, highest suspended sediment levels reduced physiological or reproductive parameters in the copepod. These results suggest that current levels of suspended sediment in the Chesapeake Bay should not reduce population success of the copepod. However, copepod production could decline at slightly higher suspended sediment concentrations resulting from urban population growth and development in the watershed, as well as at levels which are typical of several European estuaries. 相似文献