Short-term effects of a low dissolved oxygen event on estuarine fish assemblages following the passage of hurricane Charley

Hurricane Charley, a category 4 storm, passed directly over the Charlotte Harbor estuary and Peace River watershed on August 13, 2004. Following the storm's passage, dissolved oxygen in the Peace River fell below 1 mg l⁻¹ and hypoxic conditions (<2mgl⁻¹) extended into Charlotte Harbor. A long-term fisheries-independent monitoring program (1989 to present) was already in place in Charlotte Harbor, and sampling was intensified during the month after the storm. Changes in fish assemblages that resulted from the hypoxic event were determined using nonmetric multidimensional scaling and similarity percentages analysis. At the mouth of the Peace River and upper Charlotte Harbor, fish abundance decreased dramatically after the hurricane, and typical estuarine fish assemblages were replaced by those dominated by a few resilient estuarine and freshwater species, including the nonindigenous brown hoplo (Hoplosternum littorale) and sailfin catfish (Pterygoplichthys spp.). Fish assemblages in the estuarine portion of the Maykka River, located only a few kilometers west of the Peace River, were unaffected. The hypoxic event was short lived; dissolved oxygen and estuarine fish assemblages in the Peace River and upper Charlotte Harbor recovered within a month. The results of this study are consistent with other hurricane-related hypoxic events in the literature which have reported acute effects to estuarine systems in the short term, rapid recoveries, and long-term resilience.     

Mechanisms and duration of non-tectonically assisted magma emplacement in the upper crust: The Black Mesa pluton, Henry Mountains, Utah

A new study of Black Mesa pluton (Henry Mountains, Colorado Plateau, Utah, USA) indicates that it is a classic example of a small upper-crustal pluton assembled over a few years by incremental amalgamation of discrete magma pulses. The results of our petrostructural study of the pluton interior allow us to constrain the geometry, kinematics and timing of the processes. The symmetric internal fabric is interpreted as an evidence for a feeding by below and not laterally. The observed rotation of the lineation, from WNW–ESE on the very top to NNE–SSW below, lead us to propose that the fabric at the base of the pluton is a record of magma infilling process, and the fabric at the very top is a record of the strain due to the relative movement between magma and wallrocks. A consequence is that except at the contact between pluton and wallrocks (top and margins), the stretching direction, recorded by the lineation, is not parallel to the flow direction of the magma i.e. displacement. The Black Mesa pluton is a sheeted laccolith on its western edge and a bysmalith on its eastern edge. This E–W asymmetry in pluton geometry/construction and the symmetrical internal fabric indicates that the apparently different west and east growth histories could have occurred simultaneously. Our field data indicate pluton growth through an asymmetric vertical stacking of sill-like horizontal magma sheets.One-dimensional thermal models of the pluton provide maximum limits on the duration of its growth. We have constrained the number, the thickness, and the frequency of magma pulses with our structural observations, including: (1) the emplacement of the pluton by under-accretion of successive magma pulses, (2) the absence of solid-state deformation textures at internal contacts, and (3) the apparent absence of significant recrystallization in the wallrocks. Our results suggest that the emplacement of the Black Mesa pluton was an extremely rapid event, with a maximum duration on the order of 100 years, which requires a minimum vertical displacement rate of the wallrocks immediately above the pluton greater than 2 m/yr. Finally, our data show that the rates of plutonic and volcanic processes could be similar, a significant result for interpretation of magma transfer in arc systems.     

Cosmogenic radionuclide evidence for the limited extent of last glacial maximum glaciers in the Tanggula Shan of the central Tibetan Plateau

Cosmogenic radionuclide (CRN) exposure ages provide evidence for the limited extent of last glacial maximum glaciers in the Tanggula Shan, central Tibetan Plateau. The most extensive advances occurred during or before marine oxygen isotope stage 6 (MIS-6) based on previous CRN exposure ages. The second most extensive advance occurred during or before MIS-4 based on previous ages and new ages of 41,400 ± 4300, and 66,800 ± 7100 ¹⁰Be yr. A MIS-2 advance of less than 3 km occurred between 31,900 ± 3400 and 16,000 ± 1700 ¹⁰Be yr.     

Why rehabilitate urban river systems?

This paper addresses the philosophical question: 'why rehabilitate urban river systems?' within an Australian context. Rehabilitation of river systems has become an important objective of many local, state and national governments around the world, who allocate substantial investment into various river projects. An understanding of the various factors influencing stream condition and potential rehabilitation options is essential in order to determine how the process is undertaken, and how success is measured. This paper examines the triple bottom line (economic, social and environmental) factors that influence decisionmaking with respect to urban stream rehabilitation and management and considers their relative value and importance.     

Empirical Estimation of Nearshore Waves From a Global Deep-Water Wave Model

Global wind-wave models such as the National Oceanic and Atmospheric Administration WaveWatch 3 (NWW3) play an important role in monitoring the world's oceans. However, untransformed data at grid points in deep water provide a poor estimate of swell characteristics at nearshore locations, which are often of significant scientific, engineering, and public interest. Explicit wave modeling, such as the Simulating Waves Nearshore (SWAN), is one method for resolving the complex wave transformations affected by bathymetry, winds, and other local factors. However, obtaining accurate bathymetry and determining parameters for such models is often difficult. When target data is available (i.e., from in situ buoys or human observers), empirical alternatives such as artificial neural networks (ANNs) and linear regression may be considered for inferring nearshore conditions from offshore model output. Using a sixfold cross-validation scheme, significant wave height H_s and period were estimated at one onshore and two nearshore locations. In estimating H_s at the shoreline, the validation performance of the best ANN was r=0.91, as compared to those of linear regression (0.82), SWAN (0.78), and the NWW3 H_s baseline (0.54)     

Does hydrological connectivity improve modelling of coarse sediment delivery in upland environments?

Modelling the delivery of landslide-generated sediment to channel networks is challenging due to uncertainty in the magnitude–frequency distribution of failures connected to the channel network. Here, we investigate a simplified treatment of hydrological connectivity as a means for improving identification of coarse sediment delivery to upland rivers. Sediment generation from hillslopes and channel banks and its delivery to the channel network are modelled based on a modified form of SHALSTAB coupled to a network index version of TOPMODEL. The network index treatment has two important hydrological effects: (a) it only allows saturated areas to connect to the hydrological network when there is full saturation along the associated flow path; and (2) overland flow associated with unconnected but saturated zones is assumed to remain within the catchment and to contribute to a reduction in the catchment-averaged saturation deficit. We use this hydrological treatment to restrict sediment delivery to situations where there is surface hydrological connection (i.e. saturation) along the complete flow path that connects failure areas to the drainage network. This represents an extreme restriction on the possibility of connected failure as it does not allow for failed material to connect if failures are associated with partial saturation or where delivery involves runout across areas where hydrological connection is not maintained. The impact of this restriction is assessed by comparing model predictions with field mapping of connected failures and data from continuously recording coarse sediment sensors, for two storm events. The hydrological connection requirement restricted connected failures to zones closer to the drainage network and resulted in a better level of agreement with the field mapped failures. Simulations suggested that in the study catchment the majority of sediment inputs occur from hydrologically-connected areas close to the channel network during moderate sized rainstorms that occur relatively frequently.     

Temperature trends and interannual variability in the Strait of Georgia,British Columbia

A continuous 36 year long record of semi-monthly temperature profiles from the central Strait of Georgia, British Columbia is used to examine low frequency variability and trends through the water column. Decomposition of temperature anomalies into empirical orthogonal functions shows that the dominant mode accounts for 78% of the variance, while the principal component associated with this mode (PC1) is dominated by fluctuations on interannual time scales. To relate the variability within the Strait to that occurring over the northeast Pacific, PC1 is compared with anomalies in local air temperature, sea surface temperatures off the west coast of Vancouver Island, and upper ocean temperatures along Line-P. These comparisons suggest that much of the interannual variability observed in the Strait of Georgia occurs in response to large-scale atmospheric forcing over the northeast Pacific. However, following tropical El Niño events there are significant anomalies associated with processes occurring along the coastal oceanic wave guide. The strongest event in the entire record, the remarkable negative temperature anomaly of winter 1978/1979, appears to be associated with a deep water intrusion that was forced locally.