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Annual Scale (annual + scale)
Selected AbstractsFrom small-scale habitat loopholes to decadal cycles: a habitat-based hypothesis explaining fluctuation in pelagic fish populations off PeruFISH AND FISHERIES, Issue 4 2004Arnaud Bertrand Abstract The Peru-Humboldt Current system (HCS) supports the world's largest pelagic fisheries. Among the world's eastern boundary current systems, it is the most exposed to high climatic stress and is directly affected by El Niño and La Niña events. In this volatile ecosystem, fish have been led to develop adaptive strategies in space and time. In this paper, we attempt to understand the mechanisms underlying such strategies, focusing on the El Niño 1997,98 in Peru from which an extensive set of hydrographic, capture and acoustic survey data are available. An integrated analysis of the data is crucial, as each has substantial shortcomings individually; for example, both catch data and acoustic surveys may easily lead to wrong conclusions. Existing hypotheses on anchovy and sardine alternations lead us to a ,habitat-based' synthetic hypothesis. Using our data, an integrated approach evaluated how fish responded to habitat variation, and determined the consequences in terms of fish-population variability. Various factors occurring at a range of different spatio-temporal scales were considered: interdecadal regime (warm ,El Viejo'/cool ,La Vieja' decadal scale); strength and the duration of the El Niño Southern Oscillation event (interannual scale); population condition before the event (interannual scale); fishing pressure and other predation (annual scale); changes in reproductive behaviour (intra-annual scale); presence of local upwelling (local scale). During El Niño 1997,98, anchovy was able to exploit a small-scale temporal and spatial ,loophole' inside the general unfavourable conditions. Moreover, sardine did not do better than anchovy during this El Niño and was not able to take advantage of the ,loophole' opened by this short-term event. Our results question the traditional view that El Niño is bad for anchovy and good for sardine. [source] The Sahelian standardized rainfall index revisitedINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2009Abdou Ali Abstract The Standardized Precipitation Index (SPI) is usually defined as the arithmetic mean of the normalized precipitation recorded at several stations over a region of interest where the standard deviation computed at each station over a period of reference is used as the normalizing factor. It is common to use this index in order to diagnose whether the Sahelian region can be considered as wet or dry for a given year. There are several key factors interfering with the relevance of the SPI as a measure of how rainy is a season over the Sahel. The strong spatial variability of the Sahelian rainfall at the annual scale, the uneven distribution of the raingauge network and the mean interannual climatological gradients across the region are the most important of these factors, and their influence is studied in detail here. Using an optimal interpolation algorithm to compute the SPI, the effects of various sampling schemes are first studied showing that the SPI computed as a single mean value over the whole Sahelian region is relatively robust with respect to these effects. However, the central key question remains that computing a single mean SPI over the Sahelian region hides the strong underlying spatial variability of this index. For instance, 2006 was a significantly dry season over the Sahel as a whole, but working at the 0.5° × 0.5° resolution shows that in fact only 28% of the area was significantly dry, while 15% of the Sahel was significantly wet. From conditional empirical distributions a distribution function is proposed to determine the spatial distributions of the 0.5° × 0.5° SPI values for a given mean regional SPI value. Studying in detail the space,time pattern of the SPI over the 1950,2006 period, also shows that recent years are characterized by a greater interannual variability than the previous 40 years, and by a contrast between the western Sahel remaining dry and the eastern Sahel returning to wetter conditions. Copyright © 2008 Royal Meteorological Society [source] Abandonment of farmland and vegetation succession following the Eurasian plague pandemic of ad 1347,52JOURNAL OF BIOGEOGRAPHY, Issue 4 2007Dan Yeloff Abstract Aim, This paper reviews the available documentary, archaeological and palaeoecological evidence for the abandonment of agricultural land and consequent regeneration of the forest in Europe after the Black Death. Location, Western and northern Europe. Methods, This review is the result of an exhaustive search of the historical, archaeological and palaeoecological literature for evidence indicating agricultural decline and forest regeneration in Eurasia during the 14th century. The available evidence for landscape change can be divided into two categories: (1) documentary and archaeological sources, and (2) palaeoecological reconstructions of past vegetation. In the past few years, several pollen diagrams from north-west Europe have been reported with precise chronologies (decadal and even annual scale) showing the abandonment of farmland and consequent ecological change in the late medieval period. Results and main conclusions, There is strong evidence of agricultural continuity at several sites in Western Europe at the time of the Black Death. The effects of the Black Death on the European rural landscape varied geographically, with major factors probably including the impact of the plague on the local population, and soil quality. At two sites in western and northern Ireland, the late medieval decline in cereal agriculture was probably a direct result of population reduction following the Black Death. In contrast, the decline in cereal production began at sites in Britain and France before the Black Death pandemic of ad 1347,52, and was probably due to the crisis in the agricultural economy, exacerbated by political instability and climate deterioration. Much of the abandoned arable land was probably exploited for grazing during the period between the decline in cereal farming and the Black Death. In the aftermath of the Black Death, grazing pressure was greatly reduced owing to reductions in the grazing animal population and a shortage of farmers. Vegetation succession on the abandoned grazing land resulted in increased cover of woody tree species, particularly Betula and Corylus, by the late 14th century. The cover of woodland was greatest at c.ad 1400, before forest clearance and agriculture increased in intensity. [source] Seasonal pattern of depth selection in smallmouth bassJOURNAL OF ZOOLOGY, Issue 2 2009C. D. Suski Abstract The current study used a stationary acoustic telemetry array to monitor the depth selection of adult smallmouth bass Micropterus dolomieu in a large, oligotrophic lake in Ontario, Canada. At an annual scale, smallmouth bass demonstrated regular, seasonal changes in inhabited depths: fish occupied shallow water during the summer (2,5 m depth) and descended to deeper water (12,15 m depth) during winter under ice. Smallmouth bass remained above the thermocline in the summer, seasonal depth patterns did not vary across fish size and movements to and from seasonal depths were closely linked to the development and degradation of the thermocline. At finer time scales, smallmouth bass exhibited diel vertical migrations in summer, with fish moving to <2 m at night, and then descending to 3,5 m during the day. This pattern remained constant during the summer period examined, and varied with size such that larger fish remained deeper than smaller fish. During winter, depth did not vary across the 3-month monitoring period (c. 14 m), but small (<2 m) changes in depth were observed periodically, suggesting limited movements were occurring. Results are further discussed in the context of climate change and reproductive success for this species. [source] Ecohydrology of a semi-arid forest: partitioning among water balance components and its implications for predicted precipitation changesECOHYDROLOGY, Issue 2 2010Naama Raz Yaseef Abstract The distribution of precipitation inputs into different hydrological components of water-limited forest ecosystems determines water availability to trees and consequently forest productivity. We constructed a complete hydrological budget of a semi-arid pine forest (285 mm annual precipitation) by directly measuring its main components: precipitation (P), soil water content, evapotranspiration (ET, eddy covariance), tree transpiration (sap flux), soil evaporation (soil chambers), and intercepted precipitation (calculated). Our results indicated that on average for the 4-year study period, ET accounted for 94% of P, varying between 100% when P < 250 mm and 85% when P > 300 mm (with indications for losses to subsurface flow and soil moisture storage in wetter years). Direct measurements of the components of the ET flux demonstrated that both transpiration and soil evaporation were significant in this dry forest (45% and 36% of ET, respectively). Comparison between ecosystem ET (eddy covariance measurements) and the sum of its measured components showed good agreement on annual scales, but up to 30% discrepancies (in both directions) on shorter timescales. The pulsed storm pattern, characteristics of semi-arid climates, was sufficient to maintain the topsoil layer wet during the whole wet season. Only less often and intensive storms resulted in infiltration to the root zone, increasing water availability for uptake by deeper roots. Our results indicate that climate change predictions that link reduced precipitation with increased storm intensity may have a smaller effect on water availability to forest ecosystems than reduced precipitation alone, which could help forests' survival and maintain productivity even under drier conditions. Copyright © 2009 John Wiley & Sons, Ltd. [source] Simulating soil-water movement under a hedgerow surrounding a bottomland reveals the importance of transpiration in water balanceHYDROLOGICAL PROCESSES, Issue 5 2008Z. Thomas Abstract The objective of this study was to quantify components of the water balance related to root-water uptake in the soil below a hedgerow. At this local scale, a two-dimensional (2D) flow domain in the x,z plane 6 m long and 1·55 m deep was considered. An attempt was made to estimate transpiration using a simulation model. The SWMS-2D model was modified and used to simulate temporally and spatially heterogeneous boundary conditions. A function with a variable spatial distribution of root-water uptake was considered, and model calibration was performed by adjusting this root-water uptake distribution. Observed data from a previous field study were compared against model predictions. During the validation step, satisfactory agreement was obtained, as the difference between observed and modelled pressure head values was less than 50 cm for 80% of the study data. Hedge transpiration capacity is a significant component of soil-water balance in the summer, when predicted transpiration reaches about 5·6 mm day,1. One of the most important findings is that hedge transpiration is nearly twice that of a forest canopy. In addition, soil-water content is significantly different whether downslope or upslope depending on the root-water uptake. The high transpiration rate was mainly due to the presence of a shallow water table below the hedgerow trees. Soil-water content was not a limiting factor for transpiration in this context, as it could be in one with a much deeper water table. Hedgerow tree transpiration exerts a strong impact not only on water content within the vadose zone but also on the water-table profile along the transect. Results obtained at the local scale reveal that the global impact of hedges at the catchment scale has been underestimated in the past. Transpiration rate exerts a major influence on water balance at both the seasonal and annual scales for watersheds with a dense network of hedgerows. Copyright © 2007 John Wiley & Sons, Ltd. [source] Hydrochemical behaviour of dissolved nitrogen and carbon in a headwater stream of the Canadian Shield: relevance of antecedent soil moisture conditionsHYDROLOGICAL PROCESSES, Issue 3 2008Julie M. L. Turgeon Abstract This paper examines the impact of contrasting antecedent soil moisture conditions on the hydrochemical response, here the changes in dissolved nitrogen (NO3,, NH4+ and dissolved organic nitrogen (DON)) and dissolved organic carbon (DOC) concentrations, of a first-order stream during hydrological events. The study was performed in the Hermine, a 5 ha forested watershed of the Canadian Shield. It focused on a series of eight precipitation events (spring, summer and fall) sampled every 2 or 3 h and showing contrasted antecedent moisture conditions. The partition of the eight events between two groups (dry or wet) of antecedent moisture conditions was conducted using a principal component analysis (PCA). The partition was controlled (first axis explained 86% of the variability) by the antecedent streamflow, the streamflow to precipitation ratio Q/P and by the antecedent groundwater depth. The mean H+, NO3,, NH4+, total dissolved nitrogen and DOC concentrations and electrical conductivity values in the stream were significantly higher following dry antecedent conditions than after wetter conditions had prevailed in the Hermine, although the temporal variability was high (17 to 138%). At the event scale, a significantly higher proportion of the changes in DON, NO3,, and DOC concentrations in the stream was explained by temporal variations in discharge compared with the seasonal and annual scales. Two of the key hydrochemical features of the dry events were the synchronous changes in DOC and flow and the frequent negative relationships between discharge and NO3,. The DON concentrations were much less responsive than DOC to changes in discharge, whereas NH was not in phase with streamflow. During wet events, the synchronicity between streamflow and DON or NO3, was higher than during dry events and discharge and NO3, were generally positively linked. Based on these observations, the hydrological behaviour of the Hermine is conceptually compatible with a two-component model of shallow (DON and DOC rich; variable NO3,) and deep (DON and DOC poor; variable NO3,) subsurface flow. The high NO3, and DOC levels measured at the early stages of dry events reflected the contribution from NO3, -rich groundwaters. The contribution of rapid surface flow on water-repellent soil materials located close to the stream channel is hypothesized to explain the DOC levels. An understanding of the complex interactions between antecedent soil moisture conditions, the presence of soil nutrients available for leaching and the dynamics of soil water flow paths during storms is essential to explain the fluxes of dissolved nitrogen and carbon in streams of forested watersheds. Copyright © 2007 John Wiley & Sons, Ltd. [source] The structure of the lunar semi-diurnal pressure tide L2THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 606 2005Stephen W. Goulter Abstract The Hough structure of the lunar semi-diurnal tide L2(p) in surface pressure is estimated using joint weightings by area and probable-error structure, on annual and Lloyd seasonal scales, with new data. Global representations, on seasonal and annual scales of the L2 wave, in both spherical harmonic and Hough function forms, are presented for the Haurwitz,Cowley dataset enlarged by the more recent tidal determinations of Hutchings and Palumbo. These are mainly for southern hemiphere island locations. The asymmetric Hough eigenfunction terms H32 are considerably larger than previously estimated, both annually and seasonally, a possible artefact of the earlier data analysis technique. They are consistent and identified with the well-known difference in phasing and amplitude of L2 by hemisphere. The dependence of the Hough structure on the order of the fitting and on the more extreme residuals is examined. It is stable for the smallest models (spherical harmonic terms up to degree 6 and order 3), to removal of the largest residuals from a first fitting, and to the two largest datasets. Seasonal changes in the Hough structure are discussed. The asymmetric results appear consistent with early frictional/thermal interpretations on the seasonal variation of L2. But the sensitivity of L2 to upper-air temperature structure is not consistently shown in the analyses for the main mode H(2, 2). Copyright © 2005 Royal Meteorological Society. [source] | ||||||||||