Dispersive Stresses at the Canopy Upstream Edge

The derivation of flow and mass transfer models in canopy and porous media environments involves the spatial-averaging of the flow properties and their subscale equations. The averaging of the momentum equation generates the dispersive stress terms that represent the subscale spatial variations of the unresolved velocity field. While previous studies ignored the dispersive stresses in their flow models, recent evidence indicates that the dispersive stresses may be important. Here we focus our attention on the magnitude of the normal dispersive stresses in the entry region of a ‘forest patch’, where the in-canopy velocities are large and the longitudinal derivatives do not cancel out. Highly detailed particle image velocimetry measurements, at a temporal and spatial resolution of 5 Hz and 1.4 mm, are obtained inside and around a 1-m long model canopy which consists of transparent vertical cylinders 6 mm in diameter and 74.3 mm high (h). The cylinders are randomly distributed to form a relatively sparse forest patch with a leaf area density of 7.56 m⁻¹ and a fluid volume fraction (porosity) of 0.965. We present results of the double averaged flow properties at three different regions of the forest patch; the upstream edge (x ≈ 0), the fully-developed interior region (x ≈ 10h) and the downstream edge (x ≈ 13h). We find that the normal dispersive stresses around the entry region of the forest patch are significantly larger than the normal Reynolds stresses. An order of magnitude analysis of the relevant terms in the momentum equation indicates that the longitudinal derivatives of the dispersive stresses are of the same order of magnitude as that of the drag force and similar to that of the horizontal convection term. The longitudinal derivatives of the Reynolds stresses are smaller, though cannot be ignored. Comparing these results with the characteristic profiles measured in the fully-developed region indicates that the dispersive stresses, which are generated at the forest patch entrance, decrease along an adjustment region while maintaining their profile shape. We find that the dispersive stresses influence the rate at which momentum penetrates into the canopy. These observations suggest that under certain flow conditions, dispersive stresses may dominate the momentum balance and therefore must be considered in future canopy and porous media flow models.     

Surface layer variability in the Ross Sea,Antarctica as assessed by in situ fluorescence measurements

Phytoplankton fluorescence, temperature and salinity were measured from December through February using in situ instruments deployed at two locations in the southern Ross Sea, Antarctica during the austral summers of three consecutive years (2003–2004, 2004–2005, and 2005–2006) to assess the short-term, seasonal and interannual variations in phytoplankton biomass and oceanographic conditions. The seasonal climatologies of physical forcing variables were also determined from satellite measurements, and the data from the two sites compared to the 2000–2009 mean. In situ fluorometers were deployed at three depths at 77°S, 172.7°E and 77.5°S, 180°. Significant differences between the two sites were consistently observed, confirming the anticipated high level of spatial and temporal heterogeneity. Chlorophyll fluorescence was maximal in late December, and generally decreased rapidly to modest levels in January and February. However, during 1 year (2003–2004) a secondary bloom was found, with summer maxima being similar to those observed during spring. Fluorescence displayed a strong diel cycle, with strong quenching during periods of maximum irradiance. The magnitude of this reduction was large (the minimum average fluorescence was 25% of the daily mean) and decreased with depth. Fluorescence varied interannually, with the absolute levels and temporal patterns being different among years. The two sites had different temperature/salinity properties as measured at 24 m, and both variables changed with time. During 2004–2005 we were able to continuously measure the photosynthetic quantum efficiency of PSII (F_v/F_m) at 11 m, which revealed a minimum in December, and an increase in January, whereas the absolute fluorescence (F_o) decreased simultaneously. We suggest that this reflected a mixing event, whereby available irradiance increased, allowing a short period of growth in a more favorable optical environment. While substantial variations from the mean physical forcing were observed, the linkage of these physical variations with fluorescence was not always clear. Short-term (over 24-h) changes in fluorescence occurred, and were likely related to advective events. Wind events altered fluorescence in the surface layer, and these redistributed phytoplankton in the surface. The variability in chlorophyll fluorescence and physical forcing over a variety of scales in the Ross Sea provides insights into temporal–spatial coupling of phytoplankton.     

Coastal eutrophication, land use changes and Ceratium furca (Dinophyceae) blooms in Pago Pago Harbor, American Samoa 2007-2009

The bloom forming dinoflagellate, Ceratium furca, has been linked with coastal eutrophication worldwide in tropical and subtropical locations. During the summer of 2007, an unusual 6-month long bloom of C. furca was observed in Pago Pago Harbor, Tutuila Island, American Samoa. Incidents of dinoflagellate blooms in this area have not been previously reported. The bloom was first reported in May and dissipated in November 2007. In February-March 2009, a similar C. furca bloom was observed. During both blooms,...     

Coastal eutrophication, land use changes and Ceratium furca (Dinophyceae) blooms in Pago Pago Harbor, American Samoa 2007�C2009

The bloom forming dinoflagellate, Ceratium furca, has been linked with coastal eutrophication worldwide in tropical and subtropical locations. During the summer of 2007, an unusual 6-month long bloom of C. furca was observed in Pago Pago Harbor, Tutuila Island, American Samoa. Incidents of dinoflagellate blooms in this area have not been previously reported. The bloom was first reported in May and dissipated in November 2007. In February?CMarch 2009, a similar C. furca bloom was observed. During both blooms, no fish mortality events were reported. Maximum cell counts were observed on September 20, 2007 at 9 200 cell/mL. At this time, total nitrogen was measured at 1.2 mg/L while total phosphate was below detection limits. Changes in land use practices may have been the primary driver of these blooms. Intense fertilization of athletic fields coupled with ineffective management strategies is hypothesized to have a direct link to the increase in nutrients found in the Pago Pago Harbor and may have been the trigger for the initialization of these blooms. During 2008, the fields were not used due to an infestation of the fire ant, Solenopsis geminata. Once controlled, the fields were opened again in 2009 and fertilizers were applied in January, a month before the bloom was observed.     

Testing the Response of Mycenaean Terracotta Figures and Vessels to Earthquake Ground Motions

In the late 1970s, the hypothesis originated that earthquakes played a decisive role in the decline of the Mycenaean civilization at the end of the Late Bronze Age. The late excavator of Tiryns in the Argolid/Greece, Klaus Kilian, presented evidence for earthquake‐related damage to Mycenaean structures and considered four terracotta figures and two ceramic vessels lying on a floor of a potential cult room (later 12th century B.C.) in the Lower Citadel as “earthquake victims.” The broken figures were confined to a small area on the cult room's floor in front of a bench. We assess the archaeological data and test the plausibility of earthquake‐induced toppling of the objects with engineering seismological models. Dimensions and physical properties of the models were taken from the originals. In our simulations the models are virtually placed on a bench, and are exposed to earthquake ground motions based on records from recent Greek earthquakes. We test the artifacts’ toppling behavior and compare the final position of the fallen objects with the original find spots. Statistical analysis of 74,250 model calculations with highly varied ground motion parameters and bench heights reveal only a small probability that the find situation of the objects was caused by an earthquake.     

Impact of a non-Gaussian density field on Sunyaev–Zeldovich observables

The main statistical properties of the Sunyaev–Zeldovich (S–Z) effect – the power spectrum, cluster number counts and angular correlation function – are calculated and compared within the framework of two density fields which differ in their predictions of the cluster mass function at high redshifts. We do so for the usual Press & Schechter mass function, which is derived on the basis of a Gaussian density fluctuation field, and for a mass function based on a  χ²  distributed density field. These three S–Z observables are found to be very significantly dependent on the choice of the mass function. The different predictions of the Gaussian and non-Gaussian density fields are probed in detail by investigating the behaviour of the three S–Z observables in terms of cluster mass and redshift. The formation time distribution of clusters is also demonstrated to be sensitive to the underlying mass function. A semiquantitative assessment is given of its impact on the concentration parameter and the temperature of intracluster gas.     

A tale of two isotopes: differences in hydrograph separation for a runoff event when using δD versus δ18O

It is often assumed that stable water isotopes (δD and δ¹⁸O) provide redundant information for a given sample of water. In this note we illustrate that the choice of isotope used may influence the resultant hydrograph separation. This is especially true in light of the spatial and temporal variability in the isotopic composition of rainfall water at the catchment scale. We present several possible hydrograph separations based on both δD and δ¹⁸O observed in rainfall for a single runoff event occurring in the southwest USA. This study demonstrates the potential of using both stable water isotopes by showing that δD and δ¹⁸O may provide unique information for catchment hydrologists. We also report on the utility of new technology capable of simultaneous measurements of both δD and δ¹⁸O using off‐axis integrated cavity output spectroscopy (OA‐ICOS) methods. This may be of interest to catchment hydrologists seeking to incorporate this type of equipment into their laboratory. Copyright © 2009 John Wiley & Sons, Ltd.     

A Comparison of Otolith Geochemistry and Stable Isotope Markers to Track Fish Movement: Describing Estuarine Ingress by Larval and Post-larval Halibut

Estuarine recruitment of fishes is a potential bottleneck in the life cycle of many coastal species. We investigated patterns of size-at-ingress for larval and post-larval California halibut entering the Punta Banda Estuary (PBE), Mexico, using both otolith geochemistry and carbon stable isotope ratios (SIR). Juvenile halibut (n?=?126; 38–163 mm standard length [SL]) were collected from inside PBE and the adjacent exposed coast during the fall of 2003, and otoliths (geochemistry) and muscle tissues (SIR) were analyzed to reconstruct the environmental histories of individuals. Based on geochemical analyses, nearly all fish collected from PBE were characterized by a non-estuarine signature (e.g., low Mn and Ba) in the otolith growth bands deposited when fish were <30 mm SL. Although fish collected from the coast retained that signature throughout their lives, fish collected within PBE showed elevated concentrations of Mn and Ba in the otolith growth bands deposited once halibut were 30–70 mm SL, thereby recording ingress. Carbon SIR of juvenile halibut prey also differed between the estuary and coast. Muscle δ ¹³C values of halibut captured along the coast were consistent (ca.?15‰), while those captured in the estuary were variable and generally more enriched in ¹³C (?16‰ to ?11‰). Both natural tagging approaches agreed that most halibut (~75 %) enter PBE long after settlement (>?>?8–12 mm SL), although size-at-ingress estimates were significantly larger (mean difference = 27 mm; p?<?0.001) when derived via carbon SIR than with otolith geochemistry. Potential explanations for the differences in size-at-ingress estimates involve the magnitude of isotopic and trace element gradients at this ocean–estuary boundary, the temporal resolution of environmental tags stored within otoliths and soft tissues, and the size-at-capture or somatic growth rate of juvenile halibut. We conclude by discussing the relative merits of otolith geochemistry and SIR as natural tags, and by considering the implications of secondary dispersal into estuaries by post-larval fish.