Species composition,abundance and biomass of microphytoplankton in the KwaZulu-Natal Bight on the east coast of South Africa

Nearshore marine environments are influenced by an array of variables that can either be land-derived or of marine origin, and nearshore phytoplankton communities may differ in their taxonomic composition and biomass in response to such variables. The KwaZulu-Natal Bight (hereafter referred to as ‘the bight’) is an oligo-mesotrophic, nearshore oceanic environment, that is influenced by both terrestrial run-off and upwelling. A microphytoplankton survey of the bight conducted over several stations and depths and two seasons was conducted in order to ascertain species composition, abundance and biomass. Microphytoplankton abundance was generally low (a maximum of 180 000 cells l–1 was recorded) but differed considerably between sites and seasons. A total of 99 taxa of mainly Bacillariophyceae and some Dinophyceae, Prymnesiophyceae and Cyanophyceae were identified in the present study. In the central bight, higher abundance and biomass were measured in February (wet season), which may be a possible consequence of terrestrial nutrient inputs. In the northern and southern bight we measured higher abundance and biomass in August (dry season). Upwelling was not detected during the study, but an influence of terrestrial nutrient sources was detected at the coastal stations. Turbid conditions were specific to the site near the Thukela River mouth and possibly influenced abundance, biomass and species composition at this site. Historic data on microphytoplankton composition are scarce, but comparisons with surveys from the 1960s reveal that around 60% of the common diatoms recorded then also occurred in the present study. Small taxa [20–200 µm] dominated the microphytoplankton community. Community composition was fairly uniform throughout the bight in both seasons, dominated in general by Chaetoceros species, and on occasion co-dominated by Thalassionema nitzschioides and Dactyliosolen fragilissimus.     

Hydrologic metrics for status‐and‐trends monitoring in urban and urbanizing watersheds

Local governmental agencies are increasingly undertaking potentially costly “status‐and‐trends” monitoring to evaluate the effectiveness of stormwater control measures and land‐use planning strategies or to satisfy regulatory requirements. Little guidance is presently available for such efforts, and so we have explored the application, interpretation, and temporal limitations of well‐established hydrologic metrics of runoff changes from urbanization, making use of an unusually long‐duration, high‐quality data set from the Pacific Northwest (USA) with direct applicability to urban and urbanizing watersheds. Three metrics previously identified for their utility in identifying hydrologic conditions with biological importance that respond to watershed urbanization—T_Qmean (the fraction of time that flows exceed the mean annual discharge), the Richards‐Baker Index (characterizing flashiness relative to the mean discharge), and the annual tally of wet‐season day‐to‐day flow reversals (the total number of days that reverse the prior days' increasing or decreasing trend)—are all successful in stratifying watersheds across a range of urbanization, as measured by total contributing area of urban development. All metrics respond with statistical significance to multidecadal trends in urbanization, but none detect trends in watershed‐scale urbanization over the course of a single decade. This suggests a minimum period over which dependable trends in hydrologic alteration (or improvement) can be detected with confidence. The metrics also prove less well suited to urbanizing watersheds in a semi‐arid climate, with only flow reversals showing a response consistent with prior findings from more humid regions. We also explore the use of stage as a surrogate for discharge in calculating these metrics, recognizing potentially significant agency cost savings in data collection with minimal loss of information. This approach is feasible but cannot be implemented under current data‐reporting practices, requiring measurement of water‐depth values and preservation of the full precision of the original recorded data. With these caveats, however, hydrologic metrics based on stage should prove as or more useful, at least in the context of status‐and‐trends monitoring, as those based on subsequent calculations of discharge.     

The effects of thermal stress and fluid pressure on induced seismicity during stimulation to production within fractured reservoirs

We use a continuum model of reservoir evolution to explore the interaction of coupled thermal, hydraulic and chemical processes that influence the evolution of seismicity within a fractured reservoir from stimulation to production. Events occur from energy release on seeded fractures enabling moment magnitude, frequency and spatial distribution to be determined with time. Event magnitudes vary in the range ?2 to +2 with the largest event size (~2) corresponding to the largest fracture size (~500 m) and a prescribed stress drop of 9 MPa. Modelled b‐values (~0.6–0.7) also correspond to observations (~0.7–0.8) for response in the Cooper Basin (Australia). We track the hydrodynamic and thermal fronts to define causality in the triggering of seismicity. The hydrodynamic front moves twice as fast as the thermal front and envelops the triggered seismicity at early time (days to month) – with higher flow rates correlating with larger magnitude events. For later time (month to years), thermal drawdown and potentially chemical influences principally trigger the seismicity, but result in a reduction in both the number of events and their magnitudes.     

Re‐orientation of the extension direction and pure extensional faulting at oblique rift margins: comparison between the Main Ethiopian Rift and laboratory experiments

In this study, we draw on a unique combination of well‐resolved fault‐slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending Main Ethiopian Rift, East Africa. These data show that both boundary and internal faults – oblique and orthogonal to the plate divergence (PD) respectively – exhibit almost pure dip‐slip motion, and indicate significant local deflection in orientation of the extension direction at rift margins. Scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep‐seated, pre‐existing weakness – oblique to the direction of PD – that is able to cause a local rotation in the orientation of the extension direction at rift margins. Minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems, as supported by existing and new palaeomagnetic data from the rift.     

In situ cosmogenic exposure ages from the Isle of Skye,northwest Scotland: implications for the timing of deglaciation and readvance from 15 to 11 ka

We present 10 in situ cosmogenic exposure ages from two moraines on the Isle of Skye. The Strollamus medial moraine was deposited during deglaciation of the Devensian ice sheet and yields a mean exposure age from five samples of 14.3 ± 0.9 ka. The moraine age indicates that a significant ice mass existed on Skye at the time of a regional readvance recorded in Wester Ross, northwest Scotland. Taken at face value the ages suggest that deglaciation did not occur until well into Greenland Interstade 1. The Slapin moraine represents the local limit of the Loch Lomond Readvance (LLR) and yields a mean exposure age from five samples of 11.5 ± 0.7 ka, which is consistent with deposition relating to the LLR. These ages suggest that the maximum extent may have been reached late in the stadial and that some glaciers may have remained active until after the climatic amelioration that marks its end. This scenario is considered unlikely given the nature of the climate during this period, which leads us to call for a locally calibrated production rate. Copyright © 2011 John Wiley & Sons, Ltd.     

Fibre reinforced sands: from experiments to modelling and beyond

Based on hypotheses derived directly from experimental observations of the triaxial behaviour, a constitutive model for fibre reinforced sands is built in this paper. Both the sand matrix and the fibres obey their own constitutive law, whereas their contributions are superimposed using a volumetric homogenization procedure. The Severn‐Trent sand model, which combines well‐known concepts such as critical state theory, Mohr‐Coulomb like strength criterion, bounding surface plasticity and kinematic hardening, is adopted for the sand matrix. Although the fibres are treated as discrete forces with defined orientation, an equivalent continuum stress for the fibre phase is derived to allow the superposition of effects of sand and fibres. The fibres are considered as purely tensile elements following a linear elastic constitutive rule. The strain in the fibres is expressed as a fraction of the strain in the reinforced sample so that imperfect bonding is assumed at the sand‐fibre interface. Only those fibres oriented within the tensile strain domain of the sample can mobilize tensile stress—the orientation of fibres is one of the key ingredients to capture the anisotropic behaviour of fibre reinforced soil that is observed for triaxial compression and extension loading. A further mechanism of partition of the volume of voids between the fibres and the sand matrix is introduced and shown to be fundamental for the simulation of the volumetric behaviour of fibre‐reinforced soils. Copyright © 2012 John Wiley & Sons, Ltd.