We studied the occurrence of large high frequency temperature fluctuations and their potential association with settlement of intertidal invertebrates during the spring/summer period 1999/2000 at Las Cruces, on the coast of central Chile. Our results showed the existence of internal tidal bores, characterized by sharp drops in water temperature at the surface and near the bottom, and subsequent temperature increases, which occurred with a semidiurnal periodicity. Measured currents support the hypothesis of alternating onshore and offshore movement of warm-water fronts. The frequency of the events varied through the summer and their amplitude seems to be modulated by onshore winds. The strongest events were observed when strong onshore winds occurred in late afternoon hours and the entire water column showed a semidiurnal temperature signal. Highest values of chlorophyll concentration in the intertidal zone and daily settlement of bivalves, gastropods and crustaceans were observed at times when conditions were favorable for occurrence of internal tidal bores. Results suggest that internal tidal bore warm fronts could play an important role in the transport of neustonic larval invertebrates and the delivery of phytoplankton to at least some intertidal sites along the coast of Chile. 相似文献
We studied the seasonal change of the spatial distribution of nitrite (NO-2), nitrate (NO-3), reactive phosphate (PO3-4), and silicate (SiO2) in the Colorado River Delta. We also generated 24-h time series at one location to study their short-period variability. The delta is a negative estuary. During summer, salinity may be as high as 40. Amplitude of spring tides is as large as 9 m, and this causes great water turbidity by sediment resuspension. Nutrient concentrations were high throughout the whole year, with lower values towards the oceanic region. Maximum nutrient values in the river delta were 15, 53, 11·5 and 92 μM, for NO-2, NO-3, PO3-4, and SiO2, respectively. Most values were under 2, 40, 5, and 60 μM, for NO-2, NO-3, PO3-4, and SiO2, respectively. Our nutrient data show no clear seasonal pattern. Possibly, high NO-3 values in the delta are due to groundwater input, mostly at the internal extreme, and high NO-2, PO3-4, and SiO2 values are due to resuspension of sediments and mixing of porewaters with the water column, caused mainly during spring tides. In the case of NO-2, oxidation of NH+4 in the water column would be part of the mechanism. This would explain the high negative correlation between NO-3 and sea-level, and the relatively low correlation between the other nutrients and sea-level, for the time series generated at a single location. 相似文献
We have estimated the spatial variability of phytoplankton specific absorption coefficients (a*ph) in the water column of the California Current System during November 2002, taking into account the variability in pigment
composition and phytoplankton community structure and size. Oligotrophic conditions (surface Chl < 0.2 mg m−3) dominated offshore, while mesotrophic conditions (surface Chl 0.2 to 2.0 mg m−3) where found inshore. The specific absorption coefficient at 440 [a*ph(440)] ranged from 0.025–0.281 m2mg−1 while at 675 nm [a*ph(675)] it varied between 0.014 and 0.087 m2mg−1. The implementation of a size index based on HPLC data showed the community structure was dominated by picoplankton. This
would reduce the package effect in the variability of a*ph(675). Normalized aph curves were classified in two groups according to their shape, separating all spectra with peaks between 440 and 550 nm as
the second group. Most samples in the first group were from surface layers, while the second group were from the deep chlorophyll
maximum or deeper. Accessory photoprotective pigments (APP) tended to decrease with depth and accessory photosynthetic pigments
(APS) to increase, indicating the importance of photoprotective mechanisms in surface layers and adaptation to low light at
depth. Samples with higher ratios of APP:APS (>0.4) were considered as phytoplankton adapted to high irradiances, and lower
ratios (<0.26) as adapted to low irradiances. We found a good relationship between APP:APS and a*ph(440) for the deeper layer (DCM and below), but no clear evidence of the factors causing the variability of a*ph(440) in the upper layer. 相似文献
The propagation of linear sound and thermal waves in a reacting fluid, in which the heating and cooling processes can be represented by a heat-loss functionL(, T, is studied. A complex dispersion relation is found, from which the phase velocity and the scale length for damping (or amplification), of the above two-wave mode are calculated Wave amplification may occur in reacting locally stable fluids. Results are applied to a hydrogen plasma model assumed to be heated at a non-specified constant rate and cooled by recombination, excitation, and ionization by collisions, and free-free transitions. The phase velocity
, the scale-length for damping
, and the relevant relaxation times are calculated as functions of the dimensionless frequency
, for temperatures ranging from those at which the hydrogen plasma is neutral to those at which it becomes completely ionized. 相似文献
A new Late Jurassic assemblage of “conchostracans”, ostracods, bivalves and caddisfly cases from the locality “Estancia La Sin Rumbo”, Chubut Province (Patagonia, Argentina) is recorded. The fossils occur in the upper part of an outcropping 45 m thick volcaniclastic lacustrine sequence of yellowish tuffs and tuffites of the Puesto Almada Member, which is the upper member of the Cañadón Asfalto Formation with U/Pb age of 161 ± 3 Ma. The sequence represents one sedimentary cycle composed of a (lower) hemicycle of expansion and a (higher) hemicycle of contraction of the water body. The invertebrates lived in small freshwater bodies during the periods of expansion of the lake. The occurrence of a great number of small spinicaudatans, associated with mud-cracks, is evidence of dry climatic conditions and suggests several local mortality events. The spinicaudatan record of the fushunograptid–orthestheriid (component of the Eosestheriopsis dianzhongensis fauna) and the presence of Congestheriella rauhuti Gallego and Shen, suggest a Late Jurassic (Oxfordian to Tithonian) age. Caddisfly cases are recorded for the first time in the Cañadón Asfalto Basin. 相似文献
We have analyzed Long-Period (LP) seismic events of Volcán de Colima (Mexico) recorded by four three-component broad-band
seismometers during January 2006. Frequency-domain analysis shows spectra mainly peaked in the range 0.3–2 Hz and characterized
by a monotonic decreasing envelope, as those produced in a cylindrically-symmetric self-oscillating cavity interacting with
a confined jet. Independent Component Analysis, a time-series decomposition method, extracts two or three nonlinear oscillation
modes depending on the station, with the fundamental one peaked at 0.4–0.5 Hz. These decomposed waves have a well defined
spectral content and are self-sustained oscillations with low dimensionality, as shown by dynamic system reconstruction methods.
Moreover, they show radial polarization in near field and transverse polarization in far field in North-West South-East direction.
Finally, we hypothesize a branched plumbing system with two principal conduits having alignments orthogonal to one another. 相似文献
Our understanding of how groundwater mediates evapotranspiration/streamflow partitioning is still fragmented and catchment studies under changing vegetation conditions can provide a useful frame for integration. We explored this partition in a flat sedimentary dry catchment in central Argentina in which the replacement of native vegetation with rainfed crops was accompanied by the abrupt formation of groundwater-fed streams by subsurface erosion (i.e., sapping) episodes. Historical records indicated widespread water table rises (~0.3 m y−1 on average). Groundwater level and stream baseflow fluctuated seasonally with minima in the warm rainy season, indicating that evaporative discharge rather than rainfall shapes saturated flows. Diurnal groundwater level fluctuations showed that plant uptake was widespread where water tables are shallow (<3 m) but restricted to deep-rooted Prosopis forests where they are deep (7–10 m). MODIS and LANDSAT NDVI revealed a long-term greening for native vegetation, new wetlands included, but not for croplands, suggesting more limited evapotranspiration-groundwater level regulation under agriculture. Close to the deepest (20 m) and most active incisions, groundwater level and greenness declined and stream baseflow showed no seasonal fluctuations, hinting decoupling from evapotranspiration. Intense ecological and geomorphological transformations in this catchment exposed the interplay of five mechanisms governing evapotranspiration/streamflow partition including (a) unsaturated uptake and both (b) riparian and (c) distributed uptake from the saturated zone by plants, as well as (d) deepening incisions and (e) sediment deposits over riparian zones by streams. Acknowledging the complex interplay of these mechanisms with groundwater is crucial to predict and manage future hydrological changes in the dry plains of South America. 相似文献
Sudden avulsions, unexpected channel migrations and backfilling phenomena are autogenic phenomena that can considerably change the propagation patterns of sediment-laden flows on alluvial fans. Once the initial and boundary conditions of the hazard scenario with a given return period are determined, the assessment of the associated exposed areas is based on one numerical, essentially deterministic, process simulation which may not adequately capture the underlying process variability. We generated sediment-laden flows on an experimental alluvial fan by following a “similarity-of-process concept”. Specifically, we considered a convexly shaped alluvial fan model layout featuring a curved guiding channel. As loading conditions, we defined a reference, an increased and a reduced level for the released water volume and the predisposed solid fraction, respectively. Further, we imposed two different stream power regimes and accomplished, for each factor combination, eight experimental runs. The associated exposure areas were recorded by video and mapped in a GIS. We then analysed exposure data and determined exposure probability maps superposing the footprints of the eight repetitions associated with each experimental loading condition. The patterns of exposure referred to the specific loading conditions showed a noticeable variability related to the main effects of the total event volume, the solid fraction, the interactions between them, and the imposed stream power in the feeding channel. Our research suggests that adopting a probabilistic notion of exposure in risk assessment and mitigation is advisable. Further, a major challenge consists in adapting numerical codes to better reflect the stochastics of process propagation for more reliable flood hazard assessments.