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1.
The hydroclimatic conditions of water runoff formation and the hydrography of Parana and Uruguay river basins in the South America are considered. A survey of the recent studies of the hydrological regime of these rivers is given. Observation data are used to evaluate the long-term average values of water runoff and suspended sediment yield in the Parana and Uruguay and their variations along the rivers. Characteristics of many-year runoff variations in the rivers were evaluated. A climate-induced increase was identified in the Parana and Uruguay water runoff, and the corresponding present-day trends in river runoff variations in both rivers were evaluated. The total water runoff and suspended sediment yield of the Parana and Uruguay into La Plata estuary were calculated. Water balance of the drainage basin of La Plata estuary was characterized.  相似文献   

2.
The Río de la Plata, one of the most important South American estuarine environments, is characterized by a bottom salinity front that generates an ecotone between the river and the estuary. Based on bottom trawls and costal sampling we describe the distribution, types, and amount of debris found in the bottom and shoreline across this front. Plastics and plastic bags were the main debris types in both areas. Concentrations of total debris upriver the front were always significantly higher than downriver the front showing that the front acts as a barrier accumulating debris. Moreover, a large part of debris end ups accumulated in the coastal area upriver the frontal position. This area is particularly sensitive because the coastline encompasses an UNESCO Man and the Biosphere Reserve and a Ramsar site, and due to the ecological significance of the front for many valuable species.  相似文献   

3.
The seasonal pattern of size-fractionated phytoplankton biomass, primary production and respiration was investigated along the longitudinal axis of the Nervión–Ibaizabal estuary (Bay of Biscay) from April 2003 to September 2004. Environmental factors influencing phytoplankton dynamics were also studied. Chlorophyll a biomass showed a longitudinal pattern of increase from the outer Abra bay to the inner estuary. On a seasonal scale, in the intermediate and inner estuary phytoplankton biomass maxima were registered in summer, the warmest and driest season, whereas in the outer bay chlorophyll a peaks occurred in May 2004, but were delayed to August 2003, likely due to a very rainy spring. Data suggest that river flow exerts a marked influence on the timing of phytoplankton biomass maxima in this estuary, decreased river flows providing a lowering of turbidity and an increase in water residence time needed for chlorophyll a to build up. Nutrient concentrations were high enough not to limit phytoplankton growth throughout the annual cycle, except silicate and occasionally phosphate in the outer bay during summer. Silicate concentration correlated positively with river flow, whereas ammonium and phosphate maximum values were generally measured in the mid-estuary, suggesting the importance of allochthonous anthropogenic sources. In the intermediate and inner estuary phytoplankton biomass was generally dominated by >8 μm size-fraction (ca. 60%), but in August 2003 <8 μm size-fraction increased its contribution in the intermediate estuary. It is argued that the lower nutrient concentrations measured in August 2003 than in August 2004 could have played a role. This is the first study in which phytoplankton primary production rates have been measured along the longitudinal axis of the Nervión–Ibaizabal estuary. Throughout the annual cycle these rates ranged from 0.001 to 3.163 g C m?3 d?1 and were comparable to those measured in nearby small estuaries of the Basque coast and other larger estuaries on the Bay of Biscay. Surface plankton community respiration rate maxima were measured during the spring 2004 chlorophyll a peak in the Abra bay and in summer months at the mid and inner estuary, coinciding with chlorophyll a biomass and primary production maxima. In general, respiration rates showed a positive correlation with temperature. In order to compare results from the Nervión–Ibaizabal estuary with other nearshore coastal and estuarine ecosystems within the Bay of Biscay a review of existing information on phytoplankton biomass and primary production dynamics was performed.  相似文献   

4.
Motivated by field studies of the Ems estuary which show longitudinal gradients in bottom sediment concentration as high as O(0.01 kg/m4), we develop an analytical model for estuarine residual circulation based on currents from salinity gradients, turbidity gradients, and freshwater discharge. Salinity is assumed to be vertically well mixed, while the vertical concentration profile is assumed to result from a balance between a constant settling velocity and turbulent diffusive flux. Width and depth of the model estuary are held constant. Model results show that turbidity gradients enhance tidally averaged circulation upstream of the estuarine turbidity maximum (ETM), but significantly reduce residual circulation downstream, where salinity and turbidity gradients oppose each other. We apply the condition of morphodynamic equilibrium (vanishing sediment transport) and develop an analytical solution for the position of the turbidity maximum and the distribution of suspended sediment concentration (SSC) along a longitudinal axis. A sensitivity study shows great variability in the longitudinal distribution of suspended sediment with the applied salinity gradient and six model parameters: settling velocity, vertical mixing, horizontal dispersion, total sediment supply, fresh water flow, and water depth. Increasing depth and settling velocity move the ETM upstream, while increasing freshwater discharge and vertical mixing move the ETM downstream. Moreover, the longitudinal distribution of SSC is inherently asymmetric around the ETM, and depends on spatial variations in the residual current structure and the vertical profile of SSC.  相似文献   

5.
An island in a stratified region of the shelf seas creates a local increase in tidal mixing. The influences of the enhanced mixing on both the physical structure and phytoplankton biomass distribution have been assessed in a detailed survey of the Scilly Isles region of the Celtic Sea. Marked asymmetries in the observed pattern of stratification and sea surface temperature are in accord with the h/u3 distribution which predicts low stability regions occurring on the sides of the island. Displacement of the low stability regions relative to the h/u3 minima is consistent with a northward mean flow.Levels of biomass and primary productivity were found to be increased by a factor ～5 over a large region (～20 ×island area) surrounding the islands with particularly intense concentrations of phytoplankton in the pycnocline where chlorophyll levels up to 30 mg m?3 were observed. The location of these maxima at some distance from the islands is suggestive of an intrusive flow of mixed water into the pycnocline. Estimates of nitrate flux, associated with the production of mixed water by stirring, are of the right order to sustain the observed levels of production.  相似文献   

6.
Satellite ocean color and surface salinity data are used to characterize the space–time variability of the Río de la Plata plume. River outflow and satellite wind data are also used to assess their combined effect on the plume spreading over the Southwestern South Atlantic continental shelf. Over the continental shelf satellite-derived surface chlorophyll-a (CSAT) estimated by the OC4v4 SeaWiFS retrieval algorithm is a good indicator of surface salinity. The log (CSAT) distribution over the shelf presents three distinct modes, each associated to: Subantarctic Shelf Water, Subtropical Shelf Water and Plata Plume water. The log (CSAT) 0.4–0.8 range is associated with a sharp surface salinity transition across the offshore edge of the Plata plume from 28.5 to 32.5. Waters of surface salinity <31, derived from mixtures of Plata waters with continental shelf waters, are associated to log (CSAT)>0.5. In austral winter CSAT maxima extend northeastward from the Plata estuary beyond 30°S. In summer the high CSAT waters along the southern Brazil shelf retreat to 32°S and extend south of the estuary to about 37.5°S, only exceeding this latitude during extraordinary events. The seasonal CSAT variations northeast of the estuary are primarily controlled by reversals of the along-shore wind stress and surface currents. Along-shore wind stress and CSAT variations in the inner and mid-shelves are in phase north of the estuary and 180° out of phase south of the estuary. At interannual time scales northernmost Plata plume penetrations in winter (∼1200 km from the estuary) are associated with more intense and persistent northeastward wind stress, which in the period 2000–2003, prevailed over the shelf south of 26°S. In contrast, in winter 1999, 2004 and 2005, characterized by weaker northeastward wind stress, the plume only reached between 650 and 900 km. Intense southwestward plume extensions beyond 38°S are dominated by interannual time scales and appear to be related to the magnitude of the river outflow. The plume response to large river outflow fluctuations observed at interannual time scales is moderate, except offshore from the estuary mouth, where outflow variations lead CSAT variations by about 2 months.  相似文献   

7.
Rostherne Mere is a small but relatively deep, turbid and highly eutrophic lake. In recent years, the productivity and dynamics of its phytoplankton have been particularly sensitive to the interaction between light income and thermal stability. Analysis reveals that interannual differences in the size and duration of phytoplankton crops, as well as in the predominating species, are not random but that there is a reproducible coherence with weather-generated external forcing. Severe light-limitation restricts phytoplankton development outside the stratified period, though delayed stratification in spring may promote relatively large diatom crops. In summer,Microcystis will usually dominate provided its recruitment period coincides with high water clarity. OtherwiseCeratium generally dominates. Examples of extremely stable stratification leading toScenedesmus dominance and of persistent episodes of summer mixing favouringOscillatoria dominance are found to agree well with previous matrix models.  相似文献   

8.
Suspended sediments form an integral part of shelf sea systems, determining light penetration for primary production through turbidity and dispersion of pollutants by adsorption and settling of particles. The settling speed of suspended particles depends on their size and on turbulence. Here a method of determining particle size via remote sensing measurements of ocean colour and brightness has been applied to a set of monthly satellite images of the Irish Sea covering a full year (2006). The suspended sediment concentration was calculated from the ratio between green (555 nm) and red (665 nm) wavelengths in MODIS imagery. Empirical formulae were employed to convert suspended sediment concentrations and irradiance reflectance in the red part of the spectrum into specific scattering by mineral particles and floc size. A geographical pattern was evident in all images with shallow areas with fast currents having high year-average suspended sediment concentrations (7.6 mg l−1), high specific scattering (0.225 m2 g−1) and thus small particle sizes (143 μm). The reverse is true for deeper areas with slower currents, e.g. the Gyre southwest of the Isle of Man where turbidity levels are lower (3.3 mg l−1), specific scattering is lower (0.081 m2 g−1) and thus particle sizes are larger (595 μm) on average over a year. Temporal signals are also seen over the year in these parameters with minimum seasonal amplitudes (a factor 3.5) in the Turbidity Maximum and maximum seasonal amplitudes twice as large (a factor 7) in the Gyre. In the Gyre heating overcomes mixing in summer and stratification occurs allowing suspended sediments to settle out and flocs to grow large. The size of aggregated flocs is theoretically proportional to the Kolmogorov scale. This scale was calculated using depth, current, and wind speed data and compared to the size of flocculated particles. The proportionality changes through the year, indicating the influence of biological processes in summer in promoting larger flocs.  相似文献   

9.
An idealised model is presented and analysed to gain more fundamental understanding about the dynamics of phytoplankton blooms in well-mixed, suspended sediment dominated estuaries. The model describes the behaviour of subtidal currents, suspended sediments, nutrients and phytoplankton in a channel geometry. The initial growth of phytoplankton and its spatial distribution is calculated by solving an eigenvalue problem. The growth rates depend on the position in the estuary due to along-estuary variations in nutrient concentration and suspended sediment concentration. The model yields an insight into how the onset of blooms in the model depends on physical and biological processes (turbulent mixing, fresh water discharge, light attenuation, imposed nutrient concentrations at the river and sea side). In particular, the model demonstrates that the joint action of spatial variations in turbidity and in nutrients causes the maximum phytoplankton concentrations to occur seaward of the estuarine turbidity maximum.  相似文献   

10.
This study investigated the spatial variability of a common faecal indicator organism, Escherichia coli, in an urban salt-wedge estuary in Melbourne, Australia. Data were collected through comprehensive depth profiling in the water column at four sites and included measurements of temperature, salinity, pH, dissolved oxygen, turbidity, and E. coli concentrations. Vertical variability of E. coli was closely related to the salt-wedge dynamics; in the presence of a salt-wedge, there was a significant decrease in E. coli concentrations with depth. Transverse variability was low and was most likely dwarfed by the analytical uncertainties of E. coli measurements. Longitudinal variability was also low, potentially reflecting minimal die-off, settling, and additional inputs entering along the estuary. These results were supported by a simple mixing model that predicted E. coli concentrations based on salinity measurements. Additionally, an assessment of a sentinel monitoring station suggested routine monitoring locations may produce conservative estimates of E. coli concentrations in stratified estuaries.  相似文献   

11.
A spatial and temporal study on data collected along the longitudinal gradient of the Principal Channel of Bahía Blanca estuary, Argentina, was carried out during 1992–1993. At nine stations, phytoplankton abundance, chlorophyll a (Chl-a) concentration, inorganic nutrient levels, Secchi disk depth, euphotic depth:mixing depth ratio (Zeu:Zm), salinity and temperature were recorded. Phytoplankton abundance, Chl-a concentration and nutrient levels decreased towards the outer zone of the estuary. The inner zone (stations 1 and 2), which was characterized by high turbidity, high nutrient concentrations and high Zeu:Zm (>0.16, [critical mixing ratio]), registered the highest phytoplankton abundance and Chl-a concentrations. Temporal variability of data was also noteworthy in this zone. The highest biomass values thus corresponded to June, July, August and the beginning of spring (18 μg Chl-a L−1 and 9×106 cells L−1) concomitantly with a diatom bloom. In the middle zone (stations 3–6), a strong phytoplankton biomass decrease was observed and it coincided with both deep-mixed depths and low Zeu:Zm (<0.16). The outer zone (stations 7–9), which was characterized by low phytoplankton biomass values and low nutrient levels all along the year, was the area mostly influenced by waters from the adjacent continental shelf. In view of the above, it can be concluded that the most important primary production in the Bahía Blanca would be produced in the shallow inner zone during winter, being the spatial reach of the phytoplankton biomass principally limited to estuarine waters. Presumably, less than 5% of such biomass may reach the coastal area of the estuary.  相似文献   

12.
Major river systems discharging into continental shelf waters frequently form buoyant coastal currents that propagate along the continental shelf in the direction of coastal trapped wave propagation (with the coast on the right/left, in the northern/southern hemisphere). The combined flow of the Uruguay and Paraná Rivers, which discharges freshwater into the Río de la Plata estuary (Lat. ∼36°S), often gives rise to a buoyant coastal current (the ‘Plata plume’) that extends northward along the continental shelf off Uruguay and Southern Brazil. Depending upon the prevailing rainfall, wind and tidal conditions, the Patos/Mirim Lagoon complex (Lat. ∼32°S) may also produce a freshwater outflow plume that expands across the inner continental shelf. Under these circumstances the Patos outflow plume can be embedded in temperature, salinity and current fields that are strongly influenced by the larger Plata plume. The purpose of this paper is to present observations of such an embedded plume structure and to determine the dynamical characteristics of the ambient and embedded plumes.  相似文献   

13.
In many coastal plain estuaries light attenuation by suspended sediments confines the photic zone to a small fraction of the water column, such that light limitation is a major control on phytoplankon production and turnover rate. For a variety of estuarine systems (e.g. San Francisco Bay, Puget Sound, Delaware Bay, Hudson River plume), photic-zone productivity can be estimated as a function of phytoplankton biomass times mean irradiance of the photic zone. Net water column productivity also varies with light availability, and in San Francisco Bay net productivity is zero (estimated respiratory loss of phytoplankton balances photosynthesis) when the ratio of photic depth (Zp) to mixed depth (Zm) is less than about 0.2. Thus whenever Zp: Zm < 0.2, the water column is a sink for phytoplankton production.Much of the spatial and temporal variability of phytoplankton biomass or productivity in estuaries is explained by variations in the ratio of photic depth to mixed depth. For example, phytoplankton blooms often coincide with stratification events that reduce the depth of the surface mixed layer (increase Zp: Zm). Shallow estuarine embayments (high Zp: Zm) are often characterized by high phytoplankton biomass relative to adjacent channels (low Zp: Zm). Many estuaries have longitudinal gradients in productivity that mirror the distribution of suspended sediments: productivity is low near the riverine source of sediments (low Zp: Zm) and increases toward the estuary mouth where turbidity decreases. Some of these generalizations are qualitative in nature, and detailed understanding of the interaction between turbidity and estuarine phytoplankton dynamics requires improved understanding of vertical mixing rates and phytoplankton respiration.  相似文献   

14.
Phytoplankton cells in estuary waters usually experience drastic changes in chemical and physical environments due to mixing of fresh and seawaters. In order to see their photosynthetic performance in such dynamic waters, we measured the photosynthetic carbon fixation by natural phytoplankton assemblages in the Jiulong River estuary of the South China Sea during April 24-26 and July 24-26 of 2008, and investigated its relationship with environmental changes in the presence or the absence of UV radiation. Phytoplankton biomass (Chl a) decreased sharply from the river-mouth to seawards (17.3-2.1 μg L−1), with the dominant species changed from chlorophytes to diatoms. The photosynthetic rate based on Chl a at noon time under PAR-alone increased from 1.9 μg C (μg Chl a)−1 L−1 in low salinity zone (SSS < 10) to 12.4 μg C (μg Chl a)−1 L−1 in turbidity front (SSS within 10-20), and then decreased to 2.1 μg C (μg Chl a)−1 L−1 in mixohaline zone (SSS > 20); accordingly, the carbon fixation per volume of seawater increased from 12.8 to 149 μg C L−1 h−1, and decreased to 14.3 μg C L−1 h−1. Solar UVR caused the inhibition of carbon fixation in surface water of all the investigated zones, by 39% in turbidity area and 7-10% in freshwater or mixohaline zones. In the turbidity zone, higher availability of CO2 could have enhanced the photosynthetic performance; while osmotic stress might be responsible for the higher sensitivity of phytoplankton assemblages to solar UV radiation.  相似文献   

15.
Turbulence measurements in fine-scale phytoplankton layers (∼1 to ∼10 m) in the Gulf of Aqaba (Red Sea) were used to evaluate mechanisms of layer formation, maintenance, and breakdown. Simultaneous profiles of chlorophyll a (Chl a) fluorescence and temperature microstructure were measured in the upper 40 m of a 430 m water column over a 16-d period, using a Self Contained Autonomous MicroProfiler (SCAMP). Layers of concentrated phytoplankton were identified in 95 of the 456 profiles. The layers were situated in density stratified regions between 15 and 38 m depth and were characterized by intensities of 0.1 to 0.35 μg Chl a L−1 (as much as two times background concentrations) and an average thickness of 10 m. We show that turbulent mixing and isopycnal displacements associated with internal waves modulated the thickness of the layers. Variations in mixing rates within layers were connected to the vertical structure of the stratified turbulence and the stage of layer development. The breakdown of a persistent phytoplankton layer was tied to strong turbulent mixing at the base of the surface mixed layer, which encroached on the layer from above. Hydrographic observations and scaling analysis suggest that the layers most likely formed in horizontal intrusions from the adjacent coastal region. The cross-shore propagation of phytoplankton-rich intrusions may have important implications for the trophic state of offshore planktonic communities.  相似文献   

16.
The Pearl River Estuary is among the largest estuaries in the subtropical areas of the world. Along the salinity and turbidity gradient between the freshwater reach of the Pearl River and the marine water of the South China Sea, the spatial and temporal composition and abundance of phytoplankton was examined in relation to physic-chemical variables during the dry and wet seasons of 2009. Water samples for phytoplankton and environmental parameters were collected from 18 stations during two seasons along a transect from upper estuary to estuarine and marine sectors. A total of 162 species belonging to 7 phyla were identified, with diatoms dominated in both seasons while dinoflagellates proliferated in autumn. Two main clusters and three sub-clades under each main cluster corresponding to seasons and water sectors were defined with multivariate analysis (cluster and nMDS). Based on the species composition and abundance of phytoplankton, both seasonal and spatial variability were observed at a significant level (ANOSIM: season effect, R=0.896, P<0.01; station effect, R=0.463, P<0.01). The correlation analysis between biotic and abiotic variables indicated that instead of the “proverbial” anthropogenic nutrients loading and salinity gradient, the best 2-variable combination (temperature and turbidity) showed a significant effect on the pattern of phytoplankton assemblages (ρw=0.49, BIOENV analysis) between wet and dry seasons in the Pearl River Estuary. This result suggests that physical disturbance either natural or manmade is a more important factor in regulating the phytoplankton community structure within the hydrologically distinct zone of estuaries.  相似文献   

17.
The relationship between microbenthic communities, the habitat characteristics, and physical and chemical gradients was studied in the Rio de la Plata estuary. Five replicates of the surface layer were collected seasonally, in 10 sampling sites influenced by different land uses. The distribution of microbenthic communities was governed by two gradients, the first one determined by anthropic factors, related to pollution, and the second one to conductivity and turbidity. The higher densities of producers were observed in sites characterized by fine sediments. During winter, spring, and summer cyanophytes were abundant, while in autumn the diatoms, particularly birraphideans, dominated. The ciliates were the most abundant group among consumers, particularly in winter, and their spatial distribution was influenced by the turbidity. The whole study area has an eutrophic condition. Turbidity and the enrichment with nutrients and organic matter explained 50% of the variability in the species’ distribution.  相似文献   

18.
2009年7月至2010年6月,以每月一次的频率对百花湖(水库)麦西河河口浮游植物群落结构和环境因子进行调查.监测到浮游植物66种(属),浮游植物主要由绿藻、硅藻和蓝藻组成,夏秋季湖泊假鱼腥藻(Pseudanabaena limnetica)为优势浮游植物,而冬春季梅尼小环藻(Cyclotella meneghinia...  相似文献   

19.
Faroe Shelf Water (FSW) is the water mass that occupies the shallow parts of the Faroe Shelf, surrounding the Faroe Islands (62°N, 7°W). Intensive tidal mixing induces a high degree of homogeneity and the circulation system allows a partial isolation from surrounding waters. This water mass, therefore, supports a unique ecosystem of great importance for commercial fish stocks and studies have shown a clear dependence of the ecosystem on the physical processes that maintain this system and control the exchange between the FSW and the off-shelf waters. In order to identify and quantify these processes, a large observational dataset has been analysed and related to alternative theories. From this analysis, the extent and properties of the FSW have been quantified and the degree of stratification explained in terms of the Simpson–Hunter theory. The residual clockwise circulation system, which is responsible for the partial isolation from off-shelf waters, is shown to be mainly generated by tidal rectification. The typical exchange rate of water between the FSW and the off-shelf regimes has been determined by the use of simple models based on the heat and the salt budgets but the actual exchange rate is found to vary considerably in time and space. These results support earlier suggestions that this exchange is the main limiting factor for the phytoplankton spring bloom on the Faroe Shelf and that variations in exchange rate are responsible for the large inter-annual variation in spring bloom timing and intensity. The observations indicate that the on-shelf/off-shelf exchange intensity is not symmetrically distributed around the shelf, but rather concentrated around the narrow southern tip of the Faroe Shelf, where off-shelf waters during intensive exchange events may be imported all the way to the shore.  相似文献   

20.
The long-term variability of the non-tidal circulation in Southampton Water, a partially mixed estuary, was investigated using 71-day acoustic Doppler current profiler (ADCP) time series. The data show evidence that the spring–neap tidal variability of the turbulent mixing modulates the strength of the non-tidal residual circulation, with subtidal neap tide surface flows reaching 0.12 m s–1 compared to <0.05 m s–1 at spring tides. The amplitude of the neap-tide events in this non-tidal circulation is shown to be related to a critical value of the tidal currents, illustrating the strong dependence on tidal mixing. The results suggest that the dominant mechanism for generating these neap-tide circulation events is the baroclinic forcing of the horizontal density gradient, rather than barotropic forcing associated with ebb-induced periodic stratification. While tidal turbulence is thought to be the dominant control on this gravitational circulation, there is evidence of the additional effect of wind-driven mixing, including the effects of wind fetch and possibly wave development with along-estuary winds being more efficient at mixing the estuary than across-estuary winds. Rapid changes in atmospheric pressure also coincided with fluctuations in the gravitational circulation. The observed subtidal flows are shown to be capable of rapidly flushing buoyant material out of the estuary and into the coastal sea at neap tides.Responsible Editor: Iris Grabemann  相似文献   

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