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The Derwent and Huon Rivers are two relatively small river systems in the southeast of Tasmania (Australia). They terminate in estuaries that are very similar in structure and function. Historically, runoff from their adjoining catchments has been very dilute, but coloured by dissolved organic matter. Their location in cool, temperate latitudes results in a maritime climate that is changeable, but delivers regular rainfall, and therefore river flow, throughout the year. Marked seasonal cycles in nutrient levels and biological activity are seen in neighbouring coastal waters. Discharge from both rivers does not have the same seasonal signature; it is consistently enriched in dissolved organic matter (including nitrogenous forms) and depleted in inorganic nitrogen and phosphorus. Small variations in chemistry of the riverine end-members seem to ensue from intra-catchment differences in geology, soils and vegetation influenced by localised rainfall patterns. Silicon manifestly displays this behaviour in the Huon system.

Both the Derwent and Huon estuaries are drowned river valleys, with a strongly stratified (salt-wedge) water column at their head, tending toward partially mixed at their mouth. They are in a microtidal region. The Derwent estuary conforms to a wave-dominated system; the Huon estuary is intermediate between wave-dominated and tide-dominated. Currents are generally weak for both estuaries (≤0.2 m s), and weaker in subsurface waters.

With the arrival of Europeans two centuries ago, the Derwent and Huon estuary departed from the same course. The capital city of Hobart established on the western bank of the Derwent estuary, and the catchment was soon modified by agriculture and tree felling, then by damming for irrigation and hydroelectricity generation. During the 20th century, the estuary became more industrialised, and the urban zone around its shores grew to support a population of 190 000, as did the discharges from these activities to the waterway. Development in the Huon catchment was slower and more constrained. Much less of the catchment was taken up with agriculture, and the population has only attained 13 000 in scattered small towns. The only intensive industry currently in the Huon catchment is aquaculture, active in the waters of the lower estuary. However, downstream processing of timber from catchment forests is planned to recommence. The consequences of these disparate recent histories are that the conditions of the two estuaries are very different. The Derwent estuary has had heavy inputs of organic matter, suspended solids (wood fibre), nutrients, heavy metals and other toxicants. Of all these, the metals (especially zinc, cadmium, lead and mercury) residing in the estuarine sediments pose the greatest threat. They contaminate at levels as severe as seen anywhere worldwide. It might be fortuitous that nutrient inputs do not threaten eutrophication, because the estuary flushes rapidly (∼15 days), and phytoplankton appear to be light-limited in the middle estuary, where most nutrients enter. Organic toxicants cause localised concerns, but need to be more fully investigated. In contrast, the Huon estuary has been modified, but its environmental quality remains high with almost all monitoring results below the Australian guidelines. It serves as a useful baseline, though not pristine, against which the contamination of the Derwent estuary can be evaluated. Nutrients from aquaculture and the lower catchment, causing increased phytoplankton biomass and possibly depleted levels of dissolved oxygen in bottom waters, appear to be the only existing challenges for environmental management of the Huon estuary.

This is a first account of the physical and biogeochemical characteristics of the tropical São Francisco (SF) estuarine system, East Brazil, western South Atlantic. The estuary (Lat. 10°36′S Long. 36° 23′W) is fed by the humid to semiarid SF river basin (AB = 634 × 10 km, L = 2700 km), the second largest of Brazil's territory. Since the 1950s, SF has evolved into a unique system almost solely impacted by a cascade of dams, which now control 98% of the basin and reduced discharge to the estuary by 35%. The recent Xingó dam, operating since 1994 at 180 km from the coast, regulated the formerly unimodal seasonal discharge (range 800 to 8000 m s) to a constant flow of around 2000 m s. The formerly turbid river waters have become transparent and oligotrophic due to drastic material retention by the dams. The young Xingó reservoir exerted significant changes in the relative composition of inorganic and organic dissolved and particulate constituents being delivered to the estuarine mixing zone and thus also to the composition and sustenance of phytoplankton biomass and production. The more or less constant river flow eliminated the former seasonal migration pattern of the estuarine mixing zone and its lower saline portion (S>5 to <15) is now largely positioned over its pro-delta shoals. Relict muddy deposits beyond the river mouth, eroded and resuspended by intense wave mixing, have become the main source of suspended matter to the mixing zone and maintain the coastal plume more turbid than the river. These processes seem to control the behavior of several dissolved inorganic constituents, except for dissolved silicate, which behaves conservatively proportional to the mixing of fresh and marine waters. The extremely low chlorophyll  concentrations indicate that nutrient uptake by primary production along the mixing zone is minor. The coastal plume generally disperses southwestwards at an oblique angle to the coast. Its oligotrophic conditions are maintained by both the low material yields of the basin and efficient flushing by the oceanic South Equatorial Current (SEC), which impinges directly upon the coast.

The Amazon River supplies more freshwater to the ocean than any other river in the world. This enormous volume of freshwater forces the estuarine mixing out of the river channel and onto the continental shelf. On the continental shelf, the estuarine mixing occurs in a very dynamic environment unlike that of a typical estuary. The tides, the wind, and the boundary current that sweeps the continental shelf have a pronounced influence on the chemical and biological processes occurring within the estuary. The dynamic environment, along with the enormous supply of water, solutes and particles makes the Amazon estuary unique. This chapter describes the unique features of the Amazon estuary and how these features influence the processes occurring within the estuary. Examined are the supply and cycling of major and minor elements, and the use of naturally occurring radionuclides to trace processes including water movement, scavenging, sediment-water interaction, and sediment accumulation rates. The biogeochemical cycling of carbon, nitrogen, and phosphorus, and the significances of the Amazon estuary in the global mass balance of these elements are examined.

The Mackenzie Estuary is a seasonally ice covered, deltaic estuary. It receives over 300 km of freshwater and 125 × 10 t of sediment annually in a strongly modulated seasonal cycle. Ice cover plays a crucial role in the physical setting by limiting air--sea interaction (energy and gas exchange), reducing mixing, and withdrawing freshwater from the estuary while leaving behind the bulk of the dissolved components. Few studies have been conducted on estuarine processes occurring in this estuary and, although we can project from temperate estuaries what the important conservative and nonconservative processes are likely to be, the winter encroachment by ice sufficiently alters the physical, chemical, and biological processes that projections from other estuaries will likely be wrong. Here we discuss how the estuary evolves through the seasonal cycles of temperature, ice cover, river inflow, particle loadings, and winds, and review what is known of the biogeochemical cycling within the estuary. Given that the Arctic is exceptionally vulnerable to change, especially in the marginal seas, it is safe to predict that remote, pristine estuaries of the Arctic are as much at risk in the future as estuaries more directly impacted by human encroachment.

The many contrasting environments one finds in the St. Lawrence estuary make it attractive for the study of estuarine biogeochemical processes. The landward part of the estuary is relatively shallow with a partially mixed poorly productive water column and high turbidity. The seaward portion of the estuary is deep with a permanently stratified water column. It is substantially more productive and the turbidity is low. In recent years, research on the St. Lawrence estuary has focussed on quantification of sedimentation by means of radionuclides; speciation and reactivity of trace metals in the water column; sources and composition of organic matter; sediment redox chemistry and early diagenesis of trace elements; contamination by persistent organochlorine pollutants; and stable lead isotope ratios as tracers of anthropogenic inputs.

The River Nile, the most famous river of the ancient world, is the dominant geographic feature of northeastern Africa and the longest river on Earth. At the point of discharge of the Nile into the Mediterranean, the great Nile delta has formed and furnishes the most fertile area for cultivation in the Egyptian territory. The delta is embraced by two large branches of the Nile (the Rosetta and Damietta branches and their promontories), as the northward flowing river bifurcates near the city of Cairo. Both the Rosetta and Damietta branches discharge freshwater directly and indirectly into the Mediterranean Sea to form the Nile estuary (also known as the Nile delta coastal area). Fluctuations in both quantity and quality of the Nile water reaching the Mediterranean, especially as a result of the Aswan High Dam (AHD) construction in 1965, have profoundly influenced the morphometry and hydrology of the Nile, and the ecological characteristics of the river and the surrounding marine environment. This chapter intends to highlight the range of characteristics of the Nile estuary and the main factors influencing them since the AHD construction. To this effect, the geography, hydrology, and ecology of this river-delta-estuary-coastal marine system will be described and illustrated, and recent numerical simulations of its hydrodynamics and ecosystem features will be discussed. The concluding remarks forecast future trends in the development of the Nile estuary and its vital role in the ecology of the Mediterranean Sea.

The Po River collects the discharges of the most populated and industrialized area of Northern Italy and enters the Adriatic Sea with a mean flow of 1470 m s spreading in nine branches along the final stretch and forming a delta originating about 50 km from the sea. According to the last systematic survey carried out in 1989--1990, the Po delta waters suffer a low--moderate pollution from heavy metals and organic micropollutants. However, the Po River carries a nutrient load high enough to cause a severe marine eutrophication problem south of its delta. Provisional models have shown that even a substantial reduction on civil and industrial waste water discharges coupled with an optimal use of fertilizers in agriculture would not be sufficient to solve the problem. The Po River represents an important source also, for heavy metals and marine sediments collected at the river mouth are more polluted than are those offshore, especially for , Zn and Hg. On the basis of the European guidelines the delta sediments have a medium to high contamination as far as the concentration of Ni and DDT are concerned, while data on persistent organic compounds indicate a moderate ecological risk for the biota living in the delta. These same conclusions can be obtained using the “sediment quality benchmark” procedure proposed by the NOAA in the United States.

The Po di Volano canal–Sacca di Goro lagoon is a small hydrographic system partially located in the southern part of the Po River Delta. The total surface area is 830 km for the watershed and 26 km for the lagoon, respectively. The watershed is exploited for agriculture, whilst the coastal lagoon is one of the most important European sites for clam () farming. The lagoon and small inland zones are also included in the Po River Delta Regional Park. Since the mid 1980s, in the Sacca di Goro lagoon abnormal macroalgal blooms occurred, mainly due to the proliferation of the green seaweed . The enormous macroalgal biomass production was often followed by summer anoxia and dystrophic crises.

In this paper, a review of the main studies concerning altered nutrient cycling and water and sediment pollution is presented. Special attention is paid to the discussion of the different aspects of the watershed–coastal lagoon interactions: main features of the watershed and its evolution, anthropogenic pressures, river runoff influence, nutrient and other contaminant cycles, shellfish farming, and macroalgal blooms. Finally, a brief presentation of possible scenarios is given in an ecological economics perspective.

The Danube is the second largest major European river, with a huge estuary located in two countries: Ukraine and Romania. The Danube watershed embraces 15 highly industrialized European countries that produce a high level of anthropogenic pressure. During the last 30 years they have influenced the river, estuary, and Black Sea. At present the Danube runoff is totally regulated by dams. This factor changed the hydrological regime. Another factor that has changed the hydrochemical regime is the oversupply of nutrients, primarily nitrogen and phosphorus. This affected the environment of the river, estuary, and northwestern shelf of the Black Sea. Eutrophication, “water blooming”, and near-bottom hypoxia as a result of this process are developing in the northwestern part of the Black Sea. In the estuary, both water quality and bottom sediments have deteriorated, and the fish catch and biodiversity have decreased. At present a new source of eutrophication is bottom sediment in the shore zone of the sea.

The role of particles in the fate and speciation of trace metals in macrotidal estuaries was studied using a surface complexation model (MOCO). Cadmium was selected as the target metal contaminant due to its reactivity in estuaries: cadmium behavior is mainly controlled by heterogeneous processes (sorption/desorption) related to salinity and suspended matter gradients.

Various scenarios of suspended matter distribution according to salinity were simulated. The impact of surface properties (specific surface area, density of surface sites, acido-basic properties, and complexation constant) was evaluated using data collected on particles from the Gironde, Loire, and Seine estuaries.

Our results show that particle surface properties, evaluated on the basis of various parameters, are instrumental in “non-conservative” contaminant speciation in the estuarine environment. Their evaluation enables us to understand and simulate, to a large extent, the fate of “Cd-type” contaminants (whose behavior is controlled by competition between sorption and desorption processes). The natural variations of these properties can be responsible for significant modifications of the Cd speciation in the macrotidal estuaries where salinity and SM gradients are very strong.