Contribution of estuaries to commercial fisheries in temperate Western Australia and the concept of estuarine dependence

Fisheries catch statistics for temperate Western Australia are considered in conjunction with life cycle data to elucidate the importance of estuaries to the commercial and recreational fisheries in this region. The data are used to discuss whether the term estuarine-dependent is strictly applicable to all species of finfish found in abundance in estuaries. Between 1976 and 1984, 96 species of finfish, 7 species of crustaceans and 12 species of mollusks contributed to the large commercial fishery in estuaries, protected coastal areas and open marine waters of temperate Western Australia. The mean annual weight and monetary value (in 1984 terms) of this fishery was 21,355 t and $A151.3×10⁶. The contribution of the weight (4,340 t) and value ($A3.7×10⁶) of the estuarine-dependent species to the total fishery was 20.3 and 2.4%, respectively. Estuarine-dependent marine species frequently use protected inshore waters in temperate Western Australia, and have to do so when they occur in subtropical regions in Western Australia where there are no permanent estuaries. Even the semi-anadromous Perth herring and some species which are estuarinesensu stricto in south-western Australia complete their life cycle within the marine waters of this latter more northern region. Since virtually none of the commercially important marine species in temperate Western Australia can be considered to be entirely dependent on estuaries, and a similar conclusion is valid for many species of marine teleosts found in abundance in estuaries in temperate waters elsewhere in the world, these marine species would be best regarded as estuarine opportunists rather than estuarine dependents.     

Patterns in use of estuarine habitat by juvenile English sole (Pleuronectes vetulus) in four Eastern North Pacific estuaries

English sole (Pleuronectes vetulus) is one of a few commercially important marine fishes on the Pacific coast of North America that use estuarine areas as nurseries for juvenile stages. Trawl surveys of four United States Pacific Northwest estuaries were conducted to determine spatial patterns of juvenile English sole residence in estuaries during 1998–2000. Additional data from 1983–1988 were also analyzed. Two size classes of juvenile English sole were identified during surveys, with densities of small (Total length [TL] <50 mm) sole ranging from 0 to 11,300 fish ha^?1 across all sites, and densities of large (TL 50–150 mm) sole ranging from 0 to 33,000 fish ha^?1 across all sites. Principal components analysis of static habitat data collected at each trawl survey site was used to define habitat types within each estuary, and discriminant function analysis was used to test the resulting classification scheme. Both small and large cohort English sole used lower side channel locations at significantly higher densities than other estuarine areas. Small English sole also showed significant relationships with both bottom temperature and depth. These patterns in habitat use were consistent across all estuaries and indicate that English sole used shallow depth areas surrounded by extensive tidal flats, where temperatures were optimal for growth. The analysis also suggested a carrying capacity may exist for large English sole in nursery estuaries.     

Clay minerals in the major Chinese coastal estuaries and their provenance implications

A sea-survey was carried out along the Chinese coast from the Bohai Sea to South China Sea, and 15 surficial samples were taken from major coastal estuaries, i.e., from north to south, the Yellow, abandoned Yellow, Yangtze, Qiantang, Ou, Min, Han, and Pearl River estuaries. On the basis of clay mineral analysis and published data collection, this paper discusses the characteristics of clay minerals of each coastal river and potentials of implications of sediment sources. The results show that the change of diagnostic clay minerals from smectite to kaolinite from the Yellow to the Pearl River estuaries is southward, as clearly shown by the ratios of smectite to chlorite (S/Ch) and kaolinite to illite (K/I). S/Ch decreases southward, while K/I moves reversely, implying change in climate setting from cool/dry to humid/hot with intensified weathering processes. Besides, these two indicators in the estuaries are also linked with the derivation of parent-rock of each river-basin and coastal hydrodynamics. The disproportional higher ratio of K/I in the Yellow and abandoned Yellow River estuaries is ascribed to their sediment sources with abundant loess component in the north-central China. The K/I decrease offshore from the Pearl River estuary is primarily due to hydrodynamic sorting. Therefore, the S/Ch and K/I of the present study comprehensively reflect the controls of climate, parent-rock, and sediment transport along the coast.     

Formation and seasonal evolution of Atlantic menhaden juvenile nurseries in coastal estuaries

A hypothesis on the formation and seasonal evolution of Atlantic menhaden (Brevoortia tyrannus) juvenile nurseries in coastal estuaries is described. A series of cruises were undertaken to capture postmetamorphic juvenile menhaden and to characterize several biological and physical parameters along estuarine gradients. The two study systems, the Neuse and Pamlico rivers in North Carolina, contain important menhaden nursery grounds. Juvenile menhaden abundance was found to be associated with gradients of phytoplankton biomass as evidenced by chlorophylla levels in the upper water column. Fish abundances were only secondarily associated with salinity gradients as salinity was a factor that moderated primary production in the estuary. The persistence of spatial and temporal trends in the distribution of phytoplankton in the Neuse and Pamlico estuaries was reviewed. The review suggested that postmetamorphic juvenile menhaden modify their distribution patterns to match those created by phytoplankton biomass, which in turn makes them most abundant in the phytoplankton maxima of estuaries. Because the location of these maxima varies with the mixing and nutrient dynamics of different estuaries, so will the location of the nursery.     

Microphytobenthos contribution to nutrient-phytoplankton dynamics in a shallow coastal lagoon

Nutrient fluxes and primary production were examined in Lake Illawarra (New South Wales, Australia), a shallow (Z_mean=1.9 m) coastal lagoon with a surface area of 35 km², by intensive measurement of dissolved nutrients and oxygen profiles over a 22-h period. Rates of primary production and nutrient uptake were calculated for the microphytobenthos, seagrass beds, macroalgae, and pelagic phytoplankton. Although gross nutrient release rates to the water column and sediment pore waters were potentially high, primary production by microphytobenthos rapidly sequesters the re-mineralized nutrients so that net releases, averaged over times longer than a day, were low. Production in the water column was closely coupled with the relatively low sediment net nutrient release rates and detrital decomposition in the water column. Dissolved inorganic nitrogen and silica concentrations in the water column are drawn down at the beginning of the day. The system did not appear to be light limited so photosynthesis occurs as fast as the nutrients become available to the phytoplankton and microphytobenthos. We conjecture that microphytobenthos are the dominant primary producers and, as has been shown previously, that the nutrient uptake occurs in phase with the various stages of the diatom growth.     

Challenges and strategies for better use of scientific information in the management of coastal estuaries

Numerous studies have concluded that better use of scientific information could improve the quality of coastal and estuarine environmental management. Approaches for effecting such a change include ecosystem-based, integrated, and adaptive management, but such basic re-orientation of estuarine and coastal management has proved difficult to achieve. Even environmental indicators, seemingly straightforward ways of injecting scientific information into decision making, have achieved broad on-the-ground use in relatively few instances—principally the largest estuary management programs. A conceptual framework useful for examining environmental management systems affecting the five PNCERS (Pacific Northwest coastal Ecosystems Regional Study) estuaries conceives of environmental managers, researchers, and interested and affected parties in the public as interacting through the multi-layered institutional arrangements that currently promote the utilization, management, or protection of coastal and estuarine resources. Considerable variation exists in the approach and effectiveness of the region's environmental management organizations. Interaction between science and management in the region appears to be limited to an extent by high transaction costs; a cultural divide between environmental scientists and environmental managers is perceived by members of both groups who work with the PNCERS estuaries as inhibiting communications between them. Mechanisms that both groups identify as useful for improving the flow of information between science and management are little used, perhaps as a result. The two groups have very different patterns of information dissemination and acquisition, and though both chose agency archives and databases as their top methods for disseminating information, neither group relies much on these vehicles for information they seek. Both residents' and practitioners' perceptions of threats to the PNCERS estuaries show patterns of estuary-to-estuary variation. One theme that emerges is that problems associated with poor land management in adjacent uplands are common to most of these estuaries, potentially providing a sense of commonality through which a more regional approach to estuary management could emerge. A common set of estuarine environmental indicators implemented for all estuaries could help instigate such a regional approach, but resource constraints, especially at the local level, will have to be overcome for that to occur. There is currently substantial lack of common vision among coastal practitioners as to the purpose and desirability of indicators, and relatively little experience or knowledge of their use, particularly at the local level. Use of estuarine science in the management of these estuaries appears to be greatest during periods in which the largest programmatic shifts in environmental management approaches occur, an observation consistent with other studies that have concluded that the use of environmental science in environmental management tends to be episodic.     

Particle trapping in stratified estuaries: Application to observations

Estuarine turbidity maxima (ETM) retain suspended particulate matter (SPM) through advection, settling, aggregation, and nonlinearities in bed processes, but the relative importance of these processes varies strongly between systems. Observations from two strongly advective systems (the Columbia and Fraser Rivers) are used to investigate seasonal cycles of SPM retention and the effects of very high flows. Results for the Fraser and Columbia plus literature values for 13 other estuaries illustrate the applicability of scaling parameters and the response of ETM phenomena to a range of river flow (U _r ) levels and tidal forcing. The most efficient trapping (represented by Trapping EfficiencyE, the ratio of maximum ETM concentration to the source SPM concentration) occurs for low ratios of river flow to tidal current amplitude (U_T), represented by low values of the Supply number S_r.E in the Columbia is found to be maximal in a null zone where advection or tidal asymmetry (represented by Advection numberA) is weak(A ∼ 0). The ratio of aggregation to disaggregation (the Floc number Θ) is maximal on neap tides, while the ratio of erosion to deposition (the Erosion number P) is maximal on spring tides. The ratio of settling velocity to vertical mixing (Rouse numberP) is relatively constant in the Columbia ETM(P ∼ 0.7), because particle settling velocity and turbulence levels adjust together. Assuming that this result applies broadly, scaling variables and data are combined to express ETM properties in terms of the friction velocity (U_*),U _r , andU _T , allowing a considerable simplification of the parameters used to describe ETM.     

Climate shifts and coastal changes in a geological perspective. A contribution to integrated coastal zone management

This Special Issue deals with the impact of climate change on western European coastal systems. Notwithstanding the inherent problems of studying geological data in terms of climate shifts, the results show that on the meso- and the macroscale of time, climatic forcing is a major drive for coastal change. However, its impact is largely influenced by other factors. Sediment availability plays a dominant role in the evolution of coastal systems and it can be considered one of the most important thresholds at the land-ocean interface. Sea-level changes are expected to have a significant impact on most European coasts. There is particular concern for the tidally influenced flats and marshes, and for those coastal areas known to have already a net sediment deficit and to be threatened by erosion. Areas where isostatic uplift has countered sea-level rise until now, are expected to become subject to coastal erosion in the near future under an accelerated sea-level rise scenario. The sensitivity and vulnerability of coastal systems to climate shifts is shown to be largely controlled by storm magnitude and fetch. A particular case of vulnerability is the impact of tsunamis. Finally, the consequences of human interference have been demonstrated in many cases. The implementation of geoscientific studies for rational, comprehensive and cost-effective strategies on a regional or national level of integrated coastal zone management is reviewed.     

Eutrophication,water management,and the functioning of Dutch estuaries and coastal lagoons

A number of major European rivers (especially the Rhine) have a prevailing influence on the nutrient cycling of most Dutch estuaries. Owing to the increased loading of the estuaries with nitrogen and phosphorus compounds, effects of eutrophication on the biological communities are most evident in the tidal Western Wadden Sea and in a nontidal brackish lagoon, Veerse Meer. Whether the relation between changed nutrient loadings and changed biomass and production of primary and secondary producers in the turbid tidal Dutch ecosystems should be considered as a causal relation is questionable. The very widespread practice of lagoon modification confuses the effects of nutrient loading.     

Oceanography of the U.S. Pacific Northwest Coastal Ocean and estuaries with application to coastal ecology

Ocean processes are generally large scale on the U.S. Pacific Northwest coast; this is true of both seasonal variations and event-scale upwelling-downwelling fluctuations., which are highly energetic. Coastal upwelling supplies most of the macronutrients available for production, although the intensity of upwelling-favorable wind forcing increases southward while primary production and chlorophyll are higher in the north, off the Washington coast. This discrepancy could be related to several mesoscale features: the wider, more gently sloping shelf to the north, the existence of numerous submarine canyons to the north, the availability of Columbia River plume water and sediment north of the river mouth, and the existence of a semi-permanent eddy offshore of the Strait of Juan de Fuca. We suggest that these features have important effects on the magnitude and timing of macronutrient or micronutrient delivery to the plankton. These features are potentially important as well to transport pathways and residence times of planktonic larvae and to the development of harmful algal blooms. The coastal plain estuaries, with the exception of the Columbia River, are relatively small, with large tidal forcing and highly seasonal direct river inputs that are low to negligible during the growing season. Primary production in these estuaries is likely controlled not by river-driven stratification but by coastal upwelling and exchange with the ocean. Both baroclinic mechanisms (the gravitational circulation) and barotropic ones (lateral stirring by tide and, possibly, wind) contribute to this exchange. Because estuarine hydrography and ecology are so dominated by ocean signals, the coastal estuaries, like the coastal ocean, are largely synchronous on seasonal and event time scales, though, intrusions of the Columbia River plume can cause strong asymmetries between Washington and Oregon estuaries especially during spring downwelling conditions. Water property correlation increases between spring and summer as wind forcing becomes more spatially coherent along the coast. Estuarine habitat is structure not only, by large scale forcing but also by fine scale processes in the extensive intertidal zone, such as by solar heating or differential advection by tidal, curents.     

Surface complexation effects on phosphate adsorption to ferric iron oxyhydroxides along pH and salinity gradients in estuaries and coastal aquifers

Non-conservative behavior of dissolved inorganic phosphate (DIP) in estuaries is generally ascribed to desorption from iron and aluminum (hydr)oxides with increasing salinity. Here, we assess this hypothesis by simulating the reversible adsorption of phosphate onto a model oxide (goethite) along physico-chemical gradients representative of surface and subsurface estuaries. The simulations are carried out using a surface complexation model (SCM), which represents the main aqueous speciation and adsorption reactions of DIP, plus the ionic strength-dependent coulombic interactions in solution and at the mineral-solution interface. According to the model calculations, variations in pH and salinity alone are unlikely to explain the often reported production of DIP in surface estuaries. In particular, increased aqueous complexation of phosphate by Mg²⁺ and Ca²⁺ ions with increasing salinity is offset by the formation of ternary Mg-phosphate surface complexes and the drop in electrical potential at the mineral-water interface. However, when taking into account the downstream decrease in the abundance of sorption sites, the model correctly simulates the observed release of DIP in the Scheldt estuary. The sharp increase in pH accompanying the admixing of seawater to fresh groundwater should also cause desorption of phosphate from iron oxyhydroxides during seawater intrusion in coastal aquifers. As for surface estuaries, the model calculations indicate that significant DIP release additionally requires a reduction in the phosphate sorption site density. In anoxic aquifers, this can result from the supply of seawater sulfate and the subsequent reductive dissolution of iron oxyhydroxides coupled to microbial sulfate reduction.     

Sources of nitrogen to estuaries in the United States

The purpose of this study was to quantify the nitrogen (N) inputs to 34 estuaries on the Atlantic and Gulf Coasts of the United States. Total nitrogen (TN) inputs ranged from 1 kg N ha⁻¹ yr⁻¹ for Upper Laguna Madre, Texas, to 49 kg N ha⁻¹ yr⁻¹ for Massachusetts Bay, Massachusetts. TN inputs to 11 of the 34 estuaries were dominated by urban N sources (point sources and septic systems) and nonpoint source N runoff (5% of total); point sources accounted for 36–86% of the TN inputs to these 11 urban-dominated estuaries. TN inputs to 20 of the 34 estuaries were dominated by agricultural N sources; N fertilization was the dominant source (46% of the total), followed by manure (32% of the total) and N fixation by crops (16% of the total). Atmospheric deposition (runoff from watershed plus direct deposition to the surface of the estuary) was the dominant N source for three estuaries (Barnegat Bay, New Jersey: 64%; St. Catherines-Sapelo, Georgia: 72%; and Barataria Bay, Louisiana: 53%). Six estuaries had atmospheric contributions ≥30% of the TN inputs (Casco Bay, Maine: 43%; Buzzards Bay, Massachusetts: 30%; Great Bay, New Jersey: 40%; Chesapeake Bay: 30%; Terrebonne-Timbalier Bay, Louisiana: 59%; and Upper Laguna Madre: 41%). Results from our study suggest that reductions in N loadings to estuaries should be accomplished by implementing watershed specific programs that target the dominant N sources.     

Estuarine production of juvenile dungeness crab (cancer magister) and contribution to the Oregon-Washington coastal fishery

Estuaries provide nursery habitat for juvenile stages of several commercial decapod crustaceans worldwide, and those in the Northeastern Pacific are viewed as providing this function for Dungeness crab,Cancer magister. It is difficult to ascertain the degree to which such estuarine production of juveniles eventually contributes to coastal adult populations and fisheries since there are no direct surveys of adult abundance. As other authors have done, we used fishery landings data to compute the long-term average contribution of 1 + juvenile crab populations reared in estuaries to future coastal fisheries. We focused on Oregon and Washington states, but grouped landings in two large geographic zones by combining fishery ports as adjacent to Large Estuarine Zones (LEZ; Grays Harbor and Willapa Bay, Washington, and both sides of the Columbia River) and Small Estuarine Zones (SEZ; all other ports in Oregon). Mortality estimates were used to reduce 1 + crab abundance to surviving legal males, and portrayed as percent of the fisheries. Trends in the SEZ indicate that an average of only about 5–7% of estuarine production adds to the coastal adult population and contributes about $0.7 million to the fishery. The contribution is 25–30% in the LEZ (but may be higher since interannual density varies up to 5 times) and is worth about $3.9 million based on present ex-vessel value. Analyses of crab distribution and density indicate that the majority of an estuarine population (50–80%) is located in lower side channels (LSC) in spring and summer where temperature is higher and prey within and on adjacent intertidal flats is high. The potential average dollar value of equivalent legal male crab produced from the juvenile population is about $180 ha^?1 in LSC (but $280 ha^?1 in Grays Harbor where long-term density is highest), and lower in other estuarine habitats ($50–100 ha^?1). Estuarine juvenile production provides a relatively stable source of recruits to coastal adult populations, and large systems in the LEZ are important nurseries. Since direct coastal settlement of larvae does occur but is highly variable, the estuarine contribution may be especially important when physical forcing or unusual events lead to low survival of the coastal 0+ cohort. An unusually long period of very low landings in the LEZ from 1981–1987 is interpreted in light of the Mount St. Helens eruption (1980) and subsequent transport and deposition of very fine silt fractions over much of the LEZ nearshore shelf that may have adversely affected several year classes of small, early benthic phase juveniles at that time.     

Effects of hypoxia, and the balance between hypoxia and enrichment, on coastal fishes and fisheries

A reduction in dissolved oxygen concentration is one of the most important direct effects of nutrient over-enrichment of coastal waters on fishes. Because hypoxia can cause mortality, reduced growth rates, and altered distributions and behaviors of fishes, as well as changes in the relative importance of organisms and pathways of carbon flow within food webs, hypoxia and anoxia can lead to large reductions in the abundance, diversity, and harvest of fishes within affected waters. Nutrient enrichment, however, typically increases prey abundance in more highly oxygenated surface waters and beyond the boundaries of the hypoxic zone. Because of this mosaic of high and low oxygen areas within a system, not only the actual oxygen concentration of bottom waters, but the spatial arrangement, predictability, and persistence of highly oxygenated, high productivity habitats, and the ability of fishes to locate and use those favorable habitats, will determine the ultimate effect of low oxygen on fish populations. Negative effects of hypoxia on fish, habitat, and food webs potentially make both fish populations and entire systems more susceptible to additional anthropogenic and natural stressors.     

Application of a multimetric fish index to assess the environmental condition of South African estuaries

A multimetric fish index, the Estuarine Fish Community Index (EFCI) of Harrison and Whitfield (2004), was applied to data collected for 190 South African estuaries. Estuaries spanned three biogeographic regions and included three distinct estuarine typologies. The EFCI is based on 14 metrics or measures that represent four broad fish community attributes: species diversity and composition, species abundance, nursery function, and trophic integrity. Metric reference conditions and scoring criteria were developed separately for each estuary type within each zoogeographic region. The final EFCI was applied to each estuary by comparing its fish community with the appropriate reference. Index values ranged between 18 (very poor) and 66 (very good). A comparison of the EFCI with independent measures of estuarine condition revealed that the index was able to effectively differentiate between poor and good quality sites. Applying the EFCI to estuaries in which multiple samples were taken also showed that the index is reproducible. The EFCI is both a robust and sensitive method for assessing the ecological condition of estuarine systems; it is also an effective communication tool for converting ecological information into an easily understood format for managers, policy makers, and the general public.     

A framework for coupling shoals and shallow embayments with main channels in numerical modeling of coastal plain estuaries

A simple computational framework is developed to include shoals and shallow embayments (SSE) and their interaction with main channels in estuarine modeling. The scheme, treating SSE as temporary storage areas, accounts for the water and material exchanges between SSE and main channels as the tide rises and falls, and for the biogeochemical processes affecting nonconservative substances such as water-quality parameters in SSE. The scheme, because of its simple nature, can be easily incorporated into one-, two- or three-dimensional models of estuaries with substantial SSE. The concept and the model implementation of the scheme are explained using a vertical two-dimensional model of estuarine hydrodynamics and water quality. The model application to the tidal Rappahannock River, a western shore tributary of Chesapeake Bay, demonstrates the scheme is simple and physically reasonable, and the importance of SSE in estuarine modeling. The inclusion of SSE in a water-quality model not only provides a framework, for computing water-quality conditions therein but also accounts for the interaction between SSE and the main channel. It is shown that significant errors may result if the effects of SSE are not properly accounted for in modeling of an estuary with extensive SSE.     

Modelling potential impacts of bottom trawl fisheries on soft sediment biogeochemistry in the North Sea

Bottom trawling causes physical disturbance to sediments particularly in shelf areas. The disturbance due to trawling is most significant in deeper areas with softer sediments where levels of natural disturbance due to wave and tidal action are low. In heavily fished areas, trawls may impact the same area of seabed more than four times per year. A single pass of a beam trawl, the heaviest gear routinely used in shelf sea fisheries, can kill 5–65% of the resident fauna and mix the top few cm of sediment. We expect that sediment community function, carbon mineralisation and biogeochemical fluxes will be strongly affected by trawling activity because the physical effects of trawling are equivalent to those of an extreme bioturbator, and yet, unlike bioturbating macrofauna, trawling does not directly contribute to community metabolism. We used an existing box-model of a generalised soft sediment system to examine the effects of trawling disturbance on carbon mineralisation and chemical concentrations. We contrasted the effects of a natural scenario, where bioturbation is a function of macrobenthos biomass, with an anthropogenic impact scenario where physical disturbance results from trawling rather than the action of bioturbating macrofauna. Simulation results suggest that the effects of low levels of trawling disturbance will be similar to those of natural bioturbators but that high levels of trawling disturbance prevent the modelled system from reaching equilibrium due to large carbon fluxes between oxic and anoxic carbon compartments. The presence of macrobenthos in the natural disturbance scenario allowed sediment chemical storage and fluxes to reach equilibrium. This is because the macrobenthos are important carbon consumers in the system whose presence reduces the magnitude of available carbon fluxes. In soft sediment systems, where the level physical disturbance due to waves and tides is low, model results suggest that intensive trawling disturbance could cause large fluctuations in benthic chemical fluxes and storage.     

Analysis of flood vulnerability and assessment of the impacts in coastal zones of Bangladesh due to potential sea-level rise

The 4th IPCC report highlights the increased vulnerability of the coastal areas from floods due to sea-level rise (SLR). The existing coastal flood control structures in Bangladesh are not adequate to adapt these changes and new measures are urgently necessary. It is important to determine the impacts of SLR on flooding to analyse the performance of the existing structures and corresponding impact to plan for suitable adaptation and mitigation measures to reduce the impacts of floods on coastal zone. The study aims to develop a comprehensive understanding of the possible effects of SLR on floods in the coastal zone of Bangladesh. A hydrodynamic model, which is a combination of surface and river parts, was utilized for flood simulation. The tool was applied under a range of future scenarios, and results indicate both spatial variability of risk and changes in flood characteristics between now and under SLR. Estimated impact on population, infrastructure and transportation is also exposed. These types of impact estimation would be of value to flood plain management authorities to minimize the socio-economic impact.     

Uranium isotopes in rivers,estuaries and adjacent coastal sediments of western India: their weathering,transport and oceanic budget

The two major river systems on the west coast of India, Narbada and Tapti, their estuaries and the coastal Arabian sea sediments have been extensively studied for their uranium concentrations and ${}^{238}\text{U}{}^{238}\text{U}$ activity ratios.The ²³⁸U concentrations in the aqueous phase of these river systems exhibit a strong positive correlation with the sum of the major cations, σ Na + K + Mg + Ca, and with the HCO₃^? ion contents. The abundance ratio of dissolved U to the sum of the major cations in these waters is similar to their ratio in typical crustal rocks. These findings lead us to conclude that ²³⁸U is brought into the aqueous phase along with major cations and bicarbonate. The strong positive correlation between ²³⁸U and total dissolved salts for selected rivers of the world yield an annual dissolved ²³⁸U flux of 0.88 × 10¹⁰g/yr to the oceans, a value very similar to its removal rate from the oceans, 1.05 × 10¹⁰g/yr, estimated based on its correlation with HCO₃^? contents of rivers.In the estuaries, both ²³⁸U and its great-grand daughter ²³⁴U behave conservatively beyond chlorosities 0.14 g/l. These data confirm our earlier findings in other Indian estuaries. The behavior of uranium isotopes in the chlorosity zone 0.02–0.14 g/l, was studied in the Narbada estuary in some detail. The results, though not conclusive, seem to indicate a minor removal of these isotopes in this region. Reexamination of the results for the Gironde and Zaire estuaries (Martin et al., 1978a and b) also appear to confirm the conservative behavior of U isotopes in unpolluted estuaries. It is borne out from all the available data that estuaries beyond 0.14 g/l chlorosities act neither as a sink nor as a source for uranium isotopes, the behavior in the low chlorosity zones warrants further detailed investigation.A review of the uranium isotope measurements in river waters yield a discharge weighted-average ²³⁸U concentration of 0.22 μg/l with a ${}^{234}\text{U}{}^{238}\text{U}$ activity ratio of 1.20 ± 0.06ismissing. The residence time of uranium isotopes in the oceans estimated from the ²³⁸U concentration and the ${}^{234}\text{U}{}^{238}\text{U}$ A. R. of the rivers yield conflicting results; the material balance of uranium isotopes in the marine environment still remains a paradox. If the disparity between the results is real, then an additional ²³⁴U flux of about 0.25 dpm/cm₂·10³ yr into the oceans (about 20% of its river supply) is necessitated.     

Prediction of tidal excursion length in estuaries due to the environmental changes

Tidal excursion is an important parameter that indicates hydraulic and mixing characteristics of estuarine environments. Prediction of the tidal excursion length provides a proper tool for environmental management of estuaries. In this study, the governing equations of the salinity transport were scaled first to recognize the effective dimensionless parameters of tidal excursion length. Then, a laterally averaged two-dimensional numerical model called CE-QUAL-W2 was used as a virtual laboratory to simulate the salinity intrusion length. Existing field data of Limpopo estuary, as a case study, was used for calibration and verification of the model and reasonable agreement was observed between the model results and the field data. Finally, the verified model was used to assess the influences of the governing parameters. The results showed that simple power functions can be used to describe the effects of dimensionless parameters obtained by scaling of the governing equations. As a result, a new formula in form of a power function was derived to predict the tidal excursion length based on the geometric and hydrodynamic characteristics of alluvial estuaries. Comparison of the computed tidal excursion lengths using the derived formula with the observed measurements in several estuaries showed the robustness of the developed formula.