SCOPE 35 - Scales and Global Change

ABSTRACT

Estuaries are defined as embayments where freshwater and seawater mix. During mixing, chemical reactions mayor may not occur, i.e. certain components mix conservatively from fresh to salt water, while others undergo complicated reactions, exchanges, sedimentation, or are consumed or liberated by biological activity. Estuaries are under constant change: this follows from tidal, diurnal, seasonal, or glacial-interglacial cycles. However, anthropogenic change is of more importance today. Not only is the chemistry, suspended matter load, and water discharge of the input to the estuary modified, but dredging and damming has changed many estuaries completely since the beginning of industrialization. Estuaries are extremely diverse phenomena, defying generalization. Thus one example was chosen to illustrate anthropogenic impact on an estuary: The Elbe River and the inner German Bight. Here the behaviour of nutrients, organic substances, and CO₂ pressure is discussed in more detail for the inner mixing zone and the long-term effects on processes and sediments in the adjacent North Sea are highlighted.

INTRODUCTION

Estuaries are of prime importance for mankind. Since the development of sea going ships in the second millennium B.C. they provided linkage of national and international commerce. In turn, the settlements near such harbours developed to become centres of power and wealth, such as Rome, Venice, Lisboa, Amsterdam, London, New York. Later, industrialization called for places of easy access where large amounts of goods could be stored, processed, and distributed. Thus many of the industrial and population centres of the world are found at estuaries today and economic forces have shaped almost any estuary.

For the the natural scientist estuaries are gigantic mixing vessels for waters of various biological, thermal, hydrochemical, and suspended matter characteristics. Nowhere else do waters of principally different origins mix as thoroughly and rapidly as in estuaries. The number of estuaries fringing the continents is large, every river outlet to the sea qualifies. Cameron and Pritchard (1963) define an estuary as a 'semi-enclosed coastal body of water which has a free connection with the open sea and within which seawater is measurably diluted with fresh water derived from land drainage' .

One can classify estuaries according to their geological history, their morphology, their hydro physical mixing pattern, their climatic situation, their biological activity, their tidal response, their degree of pollution, or to other criteria depending on one's interest.

GENERAL CONSIDERATIONS

The shape of present day estuaries is essentially determined by the late Glacial and Holocene sea level history and by the amount of sediment the river or rivers feeding the respective estuaries carry (Figure 13.1). Rivers with relatively small sediment yields and high tidal energy will tend to form open channels which allow seawater to ingress far inland. Rivers with high sediment yields or which are exposed to low tides tend to form deltas. In deltas the shoreline is built outwards by newly deposited sediment much faster than coastal currents or waves can erode it. Also local subsidence of crust plays a role in determining the kind of estuary a river forms: rising areas will sustain delta growth, subsiding areas may cause ingression of seawater even to a point to drown former deltas.

The ratio of estuary volume to the headwater discharge determines the residence time of freshwater in the estuary. Residence times range from a few hours (large river deltas) to years (fjords with small tributary areas) and may vary during the seasons by an order of magnitude or more.

The physical mixing of seawater with fresh water in the estuary may produce various forms of density gradients (Figure 13.2). Depending on the height of the tidal wave, the depth of the channel and the river discharge, the estuary may either be vertically mixed, more or less stratified, or of a salt wedge character .

Mixing of dissolved or particulate components is called conservative, if their concentration changes in direct proportion to the increasing salinity between their fresh water and seawater concentrations (Figure 13.3, top). Deviation from this conservative mixing line indicates either subtraction from or addition to the phase under consideration. These additions or subtractions may be caused by thermodynamically governed mineral equilibria, by adsorption or desorption processes, by sedimentation, erosion or by biological activity. The same compound may behave differently in different estuaries. It may even alter its behaviour throughout the seasons in the same estuary. Such a compound is, for example, dissolved SiO₂ which in estuaries of large residence times and reasonable clear water may be depleted in summer by diatoms or silicoflagellates while it shows a truly conservative behaviour during the cold season or in estuaries which are too turbid to sustain a significant phytoplankton activity (Boyle et al. , 1974).

Figure 13.1 In geomorphology distributary river mouths are termed deltas. Those consisting only of a single channel are termed estuaries. Geochemically any enclosed reach in which river and seawater mix is an estuarine environment, thus deltas consist of many individual estuaries

Concentration of suspended matter (SM) generally does not behave conservatively. Seawater carries much lower SM-concentrations than most river waters, thus its concentration should be diluted seawards (Figure 13.3, bottom). As the estuary widens up towards the open sea, one may expect a decrease in flow velocity and thus a settling of particles. Furthermore, the increase of salinity, as has been shown by many laboratory experiments, should cause the flocculation of clay minerals very early in the mixing process (Krone, 1962; Whitehouse et al., 1960). However, measurements very often show a turbidity maximum stradelling the point up to where a salinity increase can be measured. As Meade (1972) points out, salinity flocculation, even though often quoted as being the cause of the turbidity maximum, cannot explain this observed SM-concentration pattern. Also, rivers heavily polluted by waste salts should not show any turbidity maximum at all. The Weser in Germany is such a river which carries a salinity of 4% but nevertheless displays a distinct turbidity maximum (Wellershaus, 1982). There is a very good reason why salt flocculation is not observed in the real estuary. This is due to the ubiquity of organic coatings. Rivers do not carry pure clay mineral suspensions, but rather the suspended material is lumped together in flocs. The glue of these flocs may be organic molecules such as mucopolysaccharides. Floc sizes can measure up to 1 mm and may be limited by the size of turbulence (Eisma, 1988). Flocs can be made visible by in-situ photography or by careful in-situ filtration avoiding shock of samples (Eisma et al. , 1983). Laboratory grainsize analysis therefore never gives useful information about size distribution of SM under natural conditions.

Figure 13.2 Salinity/depth diagram (left) and longitudinal salinity sections (right) for various hydrodynamic types of estuaries, illustrated for four sampling stations (after Pickard, 1975)

Figure 13.3 Top: General scheme of conservative and non-conservative mixing of a component carried by freshwater in a higher concentration than in seawater (the alternative being also possible). Bottom: The actually measured distribution of suspended matter (surface of York River, Virginia, 24 March 1960, modified after Nelson, 1960, quoted from Meade, 1972) does not follow any of the mixing lines, indicating that its concentration distribution is governed by hydrodynamical processes

The causes of the turbidity maximum seem to be hydrodynamic in nature. Details of the hydrodynamics of the estuary with regards to SM-concentrations are given, for example, by Officer (1976, 1981). Because the fresh water barotropically flows out on top of the baroclinically intruding salt water , material which settles from the fresh water finds itself going inland again (Postma, 1967). Thus a sort of horizontal eddy is formed trapping both fresh water and seawater suspensions (Figure 13.4). Numerical modelling has indeed shown that this process can cause an enrichment of suspended matter at the tip of the salt wedge (Festa and Hansen, 1978; Officer and Nichols, 1980). At the turbidity maximum sedimentation of fines is to be expected. However, things seem to be even more complicated in tidally ifluenced estuaries. Observations show (Eisma et al., 1983; 1985) that marine suspended matter moves upstream far beyond the tip of the salt wedge into the fresh water tidal reach. One reason for this transport is the asymmetry in flow velocity of the tides and the effect of neap and spring tide cycles (Postma, 1961; Officer, 1981; Allen et al., 1980; Castaing and Allen, 1981). In Figure 13.5 the prolongation of the ebb period is shown as one proceeds upstream. Consequently the flood period becomes shorter but its flow velocity increases. Thus bottom sediments become more effectively eroded during flood than during ebb phases causing a successive deplacement of sediment upstream.

Figure 13.4 Scheme of hyrodynamic conditions in an estuary showing density, water flow, suspended matter concentration, and suspended matter transport (after NEDECO, 1965)

Figure 13.5 Scheme of asymmetry effect of tides in an estuary with high tidal wave on upstream transport of suspended matter and its accumulation in a turbidity maximum (from Allen et al., 1980)

Even though suspension concentration behaves non conservatively throughout the estuarine mixing process, composition of SM may behave conservatively. The SM composition with regard to its mineral matter is mostly explained by a mixture of a fresh water and a marine source (Eisma, 1988). Clay minerals pass the estuary with hardly any changes. Also composition of coatings with regard to heavy metals and their speciation does not undergo marked changes apart from the mixing of two different sources (Schoer and Eggersgluess, 1982). These conclusions are substantiated by a survey of the sediments of the Delaware estuary (Bopp and Biggs, 1981). Factor analysis of 18 parameters measured on 119 samples (size fraction below 63 µm) indicated three sources of sediments: Factor 1 (50.9% of total variance) composed of Fe, Mg, K, Al, and Li signals the presence of Holocene marsh deposits. Factor II (24.1%) contains Sr, Mg, Ca, and Na and represents the marine fine material increasingly dominating seawards. Finally, Factor III groups Cu, Cr, Hg, Pb, and organic matter linked to river input into the bay. Total concentration of heavy metals (natural and anthropogenic) is depending on the grain size distribution (Schoer et al. , 1982). Most heavy metals are transported bound to clay surfaces or associated with grain coatings and are concentrated almost quantitatively in the size fraction below 6.3 µm. Thus, comparison of total concentrations is meaningful only if restricted to certain grain size fractions or normalized to certain background elements (Al, Sc).

Flocculation of dissolved iron has been observed both in the laboratory as well as by field studies in many estuaries causing non-conservative concentration curves indicating second order kinetics (see Fox and Wofsy, 1983, for further quotations). However, amounts thus removed from solution are small compared to the amount of iron present as grain coatings and do not alter the composition of the SM measurable (Schoer and Eggersgluess, 1982). Similar findings are reported for other elements such as Mn, Cu, Zn, Al (compare Sholkovitz, 1976).

Laboratory flocculation of organic matter is reported to amount to 3-11% for Scottish rivers which are rich in humic colloids (Sholkovitz, 1976). For most rivers flocculation of dissolved organic material seems to playa minor role only, i.e. its effect may be so small that it is masked by the accuracy of the dissolved organic carbon (DOC) determination. The mixing curve for DOC thus appears to be conservative (compare Figure 13.14) (Eisma et al., 1982; Ittekkot et al. , 1982). However, the role of organic matter flocculation for removal of dissolved inorganic trace constituents may not be neglected:

Once the estuarine water, now of a brackish, intermediate salinity, has left the morphological confinements of the estuary it mixes into the waters of the adjacent sea. Depending on the volume of the river under consideration, depth and width of the shelf, wind conditions and intensity of coastal currents, and tides the plume of the river may vary considerably in size and structure. Ship-based studies can define such a plume only very roughly because it is a highly dynamic object, but satellite and aircraft images can give real time information of high resolution (Szekielda, 1982; Szekielda et al. , 1983a). It appears that mixing occurs by interfingering of water masses, formation of patches, and upwelling. The patches seem to show a continuous size distribution from entities smaller than 100 m to more than 50 km across with observed life times of up to several days (Szekielda and McGinnis, 1985a). Long life times of these plume structures enable phytoplankton to thrive. Thus one can discern in many plumes two zones, an inner one where sediment load from the river plays the more important role for the reflectivity of the water and an outer one where the chlorophyll of living plankton causes turbidity. Mathematical methods are now available to extract these optical properties from the reflectivity data of aircraft and satellite measurements and translate them into more common units like mg SM/l (Fischer, 1983). Such a double zonation has been shown to exist in the plume of the Amazon (Szekielda et al. , 1983a; Szekielda, 1985) and the Rio de la Plata (Szekielda et al., 1983b). Satellite investigations can also yield estimates of overall seasonal variation of plumes and their relative productivity (Szekielda and McGinnis, 1985a, b).

SYNOPSIS OF CHANGES

Any density and velocity field in an estuary is but of very transient nature. Tidal forces induce a 12 hour cycle, seasons modify the mixing process with relation to discharge, temperature and chemical composition of inflow, and kind and magnitude of biological activity. Further, interannual and long-term processes occur such as erosion and deposition, sea level changes, natural changes in the hydrological and climatic settings, and anthropogenic changes with regard to dredging, construction of dams and harbours, and pollution of various sorts. If we could gather a data record of all possible measurable parameters in the estuarine system spaced at short term intervals for the life time of that estuary (i.e. in cases for large tropical rivers up to several million years) and then run a frequency analysis, we would obtain a diagram similar to Figure 13.6. This diagram illustrates that processes of very short-term frequencies and processes of long-term frequencies shape the estuaries. Most estuaries are not older than the Holocene sea level rise, but some of the large tropical rivers occupy the same outlet over consecutive glacial cycles. During Glacial times, they cut deep canyons which are filled up during Interglacial times (Amazon, Parana, Orinoco, Zaire, Indus, etc.). Thus more material reaches the shelf slope during the Glacials than during the Interglacials per unit of time. In the Figure the Holocene sea level rise is given with a dashed curve because it is for most estuaries a sort of mono event. This is especially true for those estuaries which formed in areas formerly covered by ice and where drainage patterns were established anew by the glaciation. Also, the impact of human activity which since about 150 years is felt to a larger extent in estuaries is not a cyclic event and thus dashed in the graph. True cyclic events are seasons, neap-spring tides, day-night cycles, tides, and velocity changes caused by tides. Storms also play an important role if one thinks about the catastrophes which struck the North Sea coast in former centuries drowning islands and cutting large bays within a few days. The same is true for tropical storms which devastate low-lands with a certain seasonal regularity (Bangladesh, 1985, for example). Tsunamis are another event which shape estuaries. They, however, do not occur at any preset periodicity, but strike irregularly. At the lower end of our scale we find waves which oscillate at the order of seconds. They are important for beach transport and for the gas exchange (Broecker et al. , 1978; Broecker et al. , 1980).

Figure 13.6 Synopsis of periods of short-term and long-term estuarine processes

ANTHROPOGENIC CHANGES OF ESTUARIES

Changes of sea level

During the past century the sea level rose by about 12 cm. The long-term Holocene sea level trend amounts to 2 cm, which suggests that modern sea level rise is significantly different from natural background (Gornitz et al. 1982) (Figure 13.7). This fact causes serious concern as one suspects that the rise is associated with the slight global warming observed for the same period. If model calculations prove to be correct, then the anthropogenic input of CO₂ and other trace gases to the atmosphere may cause a rise in temperature of about 2^oC until the middle of the next century (Grassl et al. , 1984). Gornitz et al. (1982) suggest that the sea level may rise by about 40 to 60 cm until then. Half of this increase may be attributed to the thermal expansion of the ocean warm surface layer, half of it may be due to the melting of glaciers.

Figure 13.7 Global mean sea level rise since 1880 (Gornitz et al., 1982)

A rise of half a metre or more would cause serious problems in many low-land estuaries. The first counter measure to such an increase would be raising dykes. Industrialized countries, which already have substantial dykes along their shore lines, even defending agricultural areas below sea level successfully, will probably be able to cope with such a challenge. Along the North Sea coast of the Netherlands and Germany dykes have been raised to 6-8 m above mean sea level. Danger does probably not arise form an increase in sea level alone as only severe storm surges can break these battlements. Furthermore, changes in tide levels have been noticed along the German coast in a magnitude much larger than the global sea level changes. Since the last century the mean low water is falling (Siefert, 1982) but the amplitude of the tide and the frequency of storm surges is increasing in the estuaries (Plate, 1983). For the station St. Pauli, Hamburg, these two trends amount to roughly 80 cm and 140 cm since 1900, respectively. These trends are, in part, man- made. The deepening of the estuary for shipping, the dyking of marshes and other measures have contributed to these trends. However, climatic and stochastic causes must be assumed in addition because the trends are the same for all German rivers regardless of their development (Siefert, 1982). Numerical modelling shows that storm surges depend on direction and force of wind much stronger than on morphometric factors (Sündermann and Zielke, 1983).

Sea level changes will cause much higher damage in developing nations. Millions of people live in delta regions practically at sea level (Nile delta, Indus delta, Bangladesh, Thailand, Mekong delta, Changjiang delta). Building effective dykes will, in many cases, be impossible due to lack of capital. However, even if the capital could be found, it might not be economically feasible to build the dykes at all. The advance of brackish groundwater inland may cause salinization of the ground, thus preventing effective agricultural use of the saved land. Coastal settlements will also suffer, especially in those areas, where no tides occur and people are not used to defending themselves against storm surges and inundations and where half a metre of sea level difference would cause the loss of protective beaches, mangroves, or lagoon barriers. However, if the increase occurs slowly enough, settlements can be rearranged within the normal life time of such structures.

Changes in the riverine input to the estuary

There is hardly any river whose water shed is not severely affected by man's activity. Activities altering the sediment load of a river also alter the natural state of its estuary. Man has either increased the load of sediment or decreased it. Deforestation and the advance of agriculture are usually connected with large increases in the erosion rate: in the Black Sea sedimentation rate increased significantly starting 1500 years ago, when more and more of the Russian plains became farmlands (Degens et al. , 1980). Meade (1982) estimates, that the erosion in the east coast area of the United States increased by a factor of 10 at the arrival of European farmers. These increases lead to the advanced siltation of channels and to the accelerated growth of deltas.

The opposite effect is caused by building dams on major streams. The diversion of water for irrigation and industrial purposes not only decreases the discharge (the Columbia, for example, lost 20% of its runoff since 1874; Kempe, 1982a), but the reservoirs behind the dams also serve as sediment traps. The most spectacular example is the Assuan High Dam on the Nile (Figure 13.8). Since its closure in 1965 the ecology, hydrochemistry and SM loads of the Nile have dramatically altered (Kempe, 1983). Before the dam closure more than 50 X 10⁶ t/yr of SM passed the Delta Barrage, while today only 4 X 10⁶ t/yr are monitored (Schamp, 1983). Consequences for the Nile delta are grave: fishery declined, coastal erosion set in, inundations threatened, and fields were ruined by salts and changing drainage.

In most rivers, the hydrochemistry is similarly out of balance. Rivers which are chemically unaffected by civilization are rare today as can be seen from the three volumes of documentation on river chemistry produced by the SCOPE/UNEP Project 'Transport of Carbon and Minerals in Major World Rivers' (Degens, 1982; Degens et al., 1983, 1985; Kempe, 1984). Spectacular is the increase in nutrient loads with the advance of intensive agricultural techniques in recent years. Figure 13.9 relates average phosphate and nitrate concentrations of major world rivers. Rivers from industrial regions display concentrations two orders of magnitudes higher than rivers from pristine tropical and arctic areas. It is interesting to note that, on average, pollution of nitrate and phosphate occurs at a mole ratio close to 15 : 1. This is the rate with which marine plankton uses these two elements. Thus rivers can fertilize the estuaries and coastal seas. The Mississippi has become the largest single source of nutrients to the ocean and the Rhine carries already half the nutrient load of the Amazon with only 1% of its discharge (Meybeck, 1982; Kempe, 1982a, 1984). The increase in nutrient load is by no means under control. Figure 13.10 shows the steep increase in the nitrate concentration of the Mississippi from 1963 to 1979. Similar steep increases are recorded for the Rhine where values of 20 mgNO₃/1 and 1 mgPO₄/1 are common.

Figure 13.8 Seasonal variation in suspended matter concentration of the Nile at Gaafra (35 km below Assuan) before and after the closure of the Assuan High Dam in 1965 (Schamp, 1983)

Similar to nutrients, the load of organic matter has increased in rivers. Forced erosion delivers particulate humic matter into the rivers and industrial, agricultural, and municipal waste inputs add labile dissolved and particulate organic matter. Parameters such as BOD (biological oxygen demand), COD (chemical oxygen demand), TOC (total organic carbon) and others increased. Added load of organics fuels excess respiration in rivers, pushing their internal CO₂ pressure into ranges ten or twenty times that of the atmospheric pressure. Figure 13.11 displays the long-term longitudinal profile of the PCO₂ of the Rhine. One can note several features on the graph: The upstream source of the turbulent Rhine, Lake Constance, is of low PCO₂ because here phytoplankton consumes nutrients and hence CO₂ in the limnic environment. In the river itself, respiration takes over as phytoplankton cannot grow in the turbid and turbulent Rhine water. Consecutive input of organic matter fuels the respiration further and CO₂ is given off to the atmosphere. In fact, respiration runs at such a high rate, that all oxygen should be consumed were it not for the high nitrate concentration of the Rhine which serves as a second source for oxygen by means of the denitrification reaction (Kempe, 1982a). Thus estuaries of polluted rivers receive waters high in organic substance, high in PCO₂, High in nutrients, and low in oxygen not to talk about heavy metals, chlorinated hydrocarbons, radioactive substances, heat, or other wastes of human productivity.

Figure 13.9 Plot of mean nitrate versus mean phosphate concentrations for various world rivers. Exaplanation of signs: 76, average of 1976; (10)81, single sample October 1981; t.w., time weighted mean; d.w., discharge weighted mean; TDP, total dissolved phosphate; TDN, total dissolved nitrogen (inorganic); (Kempe, 1984)

Figure 13.10 Detailed record of the nitrate concentration increase of the Mississippi of two stations shortly above its estuary (1963-1979) (Kempe, 1982a)

Figure 13.11 Longitudinal PCO₂ profile of the Rhine as calculated from long-term hydrochemical monitoring at 13 stations. Times for which means apply are given. River flow is from right (outflow of Lake Constance) to left (Rhine delta in the Netherlands) (KempeJ.1982a) 

Not only upstream alterations change the physiography of an estuary, but most estuaries are directly affected by civil engineering. Harbours necessitate the dredging of the main channel down to a depth which is not in equilibrium with sedimentation/erosion. Adjacent marshes are dyked to obtain land for cattle raising, agriculture, or industry and the river loses its storm flood plain. Consequently, tides travel further upstream and intensify. More dykes need to be built and more dredging is necessary to keep moving sediment at bay. Figure 13.12 displays the development of the Seine Estuary within the last 150 years from an open tidal flat estuary to a dyked channel (Avoine et al. , 1981 ). In the mouth of the Rhine annually 10.5 x 10⁶ t of material are dredged out (Eisma, 1988) which is a far larger amount than the total load of the Rhine across the German/Dutch border (3.4 x 10⁶ t SM/yr, average 1966-1973; Kempe et al. , 1981). In effect, the dredging battles mainly the net inflow of SM from the North Sea estimated to amount to 13 x 106 t/yr (Eisma, 1988). Such figures illustrate the great magnitude of man's efforts to effectively manage the Rhine mouth, a statement which certainly applies to many estuaries allover the world.

Figure 13.12 Change in the physiography of the Seine Estuary between 1834 and 1979 (from Avoine et al., 1981)

The Elbe River and the German Bight, a case study

The Elbe River, which drains highly industrialized regions in Central Europe, is one of the most polluted rivers on Earth (compare Figure 13.9, data since 1978 available from Arbeitsgemeinschaft für die Reinhaltung der Elbe.) Thus it can serve as an extreme case of anthropogenic change. Thames, Rhine, Weser , and Elbe are the four most important riverine inputs to the North Sea. With additional inputs by dumping of waste and dredge material, oil spills, and input from air pollution the North Sea is probably the most polluted coastal sea (Weichart, 1973). Due to its shallowness (50-100 m) pollutants are moved about for a long period because almost no new sediment is formed to trap these substances. Transport of material is along the German and Danish coast to the north and suspended matter is finally deposited in the Skagerrak where water depths exceed 300 m (Jansen et al. , 1979).

Pollution of the Elbe with organic substances and nutrients seems to date back more than 30 years. Data of the Hamburgian Water Works, dating back to the early 1950s, already show high nutrient concentrations. Also the PCO₂, which can be calculated from standard hydrochemical records to give a measure of organic matter respiration if direct measurements of organics are missing, indicates quite high levels of organics throughout the past thirty years (Kempe, 1982a). This behaviour is unlike the Rhine, which shows significant trends of concentrations with time (increases for nutrients throughout the period, increase for organic matter until 1972 and decrease since then).

Figure 13.13 gives a simplified view of the transformations some of the pollutants suffer while passing through the estuarine system. Phosphate, nitrate, and dissolved organic matter seem to pass conservatively through the turbidity maximum zone. Figure 13.14 plots their concentrations with decreasing salinity for measurements made in October 1981. During other times of the year DOC, however, can behave non-conservatively (Seifert, 1985). Also, particulate organic carbon (POC in % of total suspended matter not an absolute content per litre) shows a conservative behaviour (Ittekkot et al. , 1982). This is even true for the particulate carbohydrates, which seem to vary conservatively between the fresh water and the seawater suspended matter composition (Lohse and Michaelis, 1983). Particulate carbohydrates suggest that terrestrial plant matter is only a small part of the organic matter present. Rather, living and dead plankton of riverine or marine provenience and bacterial cell walls seem to determine carbohydrate spectra. Seifert (1985) could show that the POC and particulate carbohydrate composition is influenced by seasons (primary productivity) and headwater volume of the river and may also-especially in spring-deviate from conservative mixing. Figure 13.15 (distribution of DOC in the Elbe-Weser Plume, Seifert, 1985) shows that the influence of the Elbe River reaches far into the inner German Bight and is directed towards the North (which is also known from satellite pictures). Distribution of salinity is very similar to this DOC pattern. This suggests, that most of the DOC is conservatively mixed into the North Sea water which one can see intruding from the west in the form of water masses low in DOC. How much of the DOC in the plume is derived from nutrient induced in-situ phytoplankton blooms still needs to be shown.

Figure 13.13 Scheme of fluxes and geobiochemical processes for the elements  C, N, P in polluted estuaries (Kempe, 1982b)

More complicated is the fate of the high PCO₂ of the Elbe water. In a numeric thermodynamic mixing experiment (using the WATMIX model of Wigley and Plummer, 1976) one can calculate how the PCO₂, the pH, and the saturation index of calcite should change if the mixing would be mass- conservative between the river water and the North Sea end members. Figure 13.16 gives the result: due to the high PCO₂ of the Elbe water the river is undersaturated with respect to calcite (the same can be calculated to be true for aragonite and dolomite) but the North Sea water is-as any seawater-highly supersaturated. Undersaturation should persist up to a point whert more than 60% seawater has been admixed to the fresh water. This long persistence of an undersaturation derives from non-linear redistribution among species of the carbonate system (Wigley and Plummer, 1976) and lead: even to an increase in undersaturation in the innermost estuary, at least in the numeric calculation. In effect, additional CO₂ is liberated during the mixing process formerly locked up in ionic form. Nature, however, does not necessarily follow mass-conservation during the mixing process: Figure 13.17 gives the plots of the carbonate alkalinity and pH versus the salinity for October 1981 in the Elbe estuary. One can see that both parameters (which are independent measurements) display steep jumps very early in the mixing process. The increase in pH signals the loss of free CO₂ from the water, and the increase in alkalinity indicates dissolution of carbonates. What happens is that marine carbonates become dissolved and saturate the high undersaturation of the Elbe with respect to calcite very early in the mixing (Kempe, 1982b, 1984). Thus the high CO₂ pressure of the Elbe is not degassed to the atmosphere nor is it mixed mass conservatively into the North Sea water , rather it is titrated by marine carbonates to form excess alkalinity. In fact, the maximum of the undersaturation (or corrosion) potential in Figure 13.16 can  be calculated to be equivalent to about 100 µmol CaCO₃/1 which would result in an increase in the alkalinity of about 200 µmol/1 which is the height of the observed jump. Measurements during other seasons show that this alkalinity jump occurs at times of low alkalinity in the head waters and is a rather normal feature in the hydrochemistry of the Elbe estuary. It must be concluded that such active carbonate corrosion and therefore a buffering of the anthropogenically increased PCO₂ may occur in other rivers highly polluted with labile organics as well.

Figure 13.14 Plot of concentrations of various parameters versus salinity for the Elbe Estuary. Data of October 1981, R. V. VALDIVIA Cruise (Ittekkot et al., 1982; Kronberg et al., 1982). Note conservative mixing for DOC, POC, phosphate and nitrate and non-conservative mixing for silica

Figure 13.15 Distribution of dissolved organic carbon at the surface of the eastern half of the German Bight in May 1983 (VALDIVIA Cruise 12). Note the northward transport of the plume of the Elbe/Weser estuaries along the coast and bulges of North Sea water lower in DOC intruding into the plume from the west (Seifert, 1985)

Figure 13.16 Thermodynamic model of various mixtures of Elbe fresh water (left) and of North Sea water (right, 30.5% salinity). End-member solutions as of composition as found in October 1981 R. V. VALDIVIA stations Elbe I and Elbe 10 (Kempe, 1982b). SI Cc = saturation index of calcite; pPCO₂ = log CO₂ pressure, calculated by programme WATMIX (Wigley and Plummer, 1976)

Figure 13.17 Actual alteration of alkalinity (top) and pH (bottom) in the Elbe Estuary with increasing salinity as measured in October 1981 R. V. VALDIVIA Cruise (Kempe, 1982b)

Figure 13.18 Long-term record of PO₄-P (top) and NO₃-N (bottom) concentrations of the inner German Bight at Helgoland, 1962-1984. Solid line = linear regression small broken lines = 98% confidence interval; large broken line = overall average; sol line represents low-pass filter, periods greater than 2 years are shown. Equations  linear regressions: y = 0.548 + 0.00005*X (for PO₄-P) and y = 5.541 + 0.001299*X (for NO₃-N). Data by Harms and Mangelsdorf, statistics by Berg and Radach (1985)

Respiration consumes oxygen from the water. However, due to the advance of waste treatment this seems not to be the main threat for the oxygen concentration of the Elbe estuary. Rather, release of ammonia from treatment plants in East and West Germany is important. This ammonia triggers bacterial nitrification reactions if, in early summer, temperatures rise above about 16°C. Thus downstream of the sewage plant outlet of Hamburg (said to be the fourth largest tributary of the Elbe) oxygen is depleted regularly and anoxia can occur (Arbeitsgemeinschaft für die Reinhaltung der Elbe, 1978-1984). Fishes are killed and drift to shore. These paroxysms of a heavily polluted system attract the interest of the news media much more than the overall pollution of the river. Cure for this illness is in sight as Hamburg will soon employ a nitrification unit at its sewage plant. Similar anoxic stretches are reported from other estuaries as well. The Scheldt is an especially well documented example (Wollast, 1983). Cynically, though, one may ask, what does it help to save the life of fish, if these fishes are polluted by heavy metals to such an extent that professional fishery is prohibited since 1984 in the Elbe, estuary and no fish from the Elbe may be sold?

How, if at all, eutrophication by rivers is affecting the coastal seas is a long standing debate. This debate mostly suffers from the fact that no long-term data are available for coastal seas. Ship measurements are infrequent and tend to cluster in times of fine, or at least quite fair, weather conditions. Winter and storm data are missing. In-situ techniques have been developed to provide better data. However, these sets of data are not numerous and they are being utilized only now in a few places. For example, for the first time such long-term record is available only now for discussion: it comprises data collected since 1962 by the Helgoland Biological Institute (Gil1bricht, Treuner, Harms, Mangelsdorf, published in Biologische Anstalt Helgoland, 1962-1984) at their station on the Island of Helgoland in the Inner German Big outside the Weser and Elbe estuary (compare Figure 13.15). At the station up to 50 parameters are measured daily during work days throughout the year. The data have been digitized, screened for outliers and tested for statistic significant trends by Berg and Radach (1985). Salinity at the station is , average around 32% but changes seasonally with discharge of the rivers by much as 4%. Figures 13.18 and 13.19 show four of the most interesting long-term curves: PO₄-P, NO₃-N, diatoms-C, and flagellates-C. For both PO₄-P and NO₃-N positive and highly significant linear trends were found. PO₄-P increased from 0.55 to 0.97 µg-at/1 and NO₃-N from 5.56 to 16.45 µg at/1 in 23 years. Apparently also the seasonal amplitude increased but final tests still have to be run. Even though more and more nutrients seem to become available, phytoplankton in summer still brings concentrations down to values near total depletion. As a consequence one must expect that standing phytoplankton-C must have increased. This is in fact what is found, phytoplankton-C increased exponentially by a factor of four (from 8.9 to 36.6 µgC/1). Most of this increase is due to flagellates (Figure 13.19, bottom). Diatoms, however, did not increase at all (Figure 13.19, top). Again, as one would expect because one of their nutrients, silica is not influenced by pollution and has remained relatively unchanged throughout the period. One must realize that there is no accumulation of nutrients taking place at the station as the residence time of both water and recent sediments in the North Sea is low. Thus the station probably monitors annual pollution events, i.e. those nutrients provided by rivers to the coastal current within the last few weeks, months, or during the last season if one assumes a small sediment pool as an interim buffer. This inference is substantiated, for example, for the nitrate curve, the long-term frequency curve of which has a high correlation with the discharge record of the Elbe (Berg and Radach, 1985). One conclusion of hope can be drawn from this: cutting the nutrient input by rivers to the North Sea may, within a few years, curb eutrophication. Speculation exists as to the final resting place of the exported nutrients: possibly the young sediments in the Skagerrak bury most of them.

Figure 13.19 Long-term record of diatom carbon and flagellate carbon of the inner German Bight at Station Helgoland, 1962-1984. For explanation of regressions Figure 13.18. Equations of linear regressions: log(Y)=0.786+0.000012*X for diatom-C) and log (Y) = 0.393 + 0.00012*X (for flagellate-C). Data by Gillbri, statistics by Berg and Radach (1985)

Another part of the pollutants seems to be mixed into the sediments of tidal flats along the Dutch, German, and Danish coasts. Here lateral migration of tidal channels and heavy waves during storm tides move large quantities of sediments around and thereby mix younger substances with older sediments of longer residence times. Schwedhelm and Irion (1985) have recently shown that heavy metal and phosphorus (Figure 13.20) enrichment is quite evident throughout the German Wadden Sea. Concentrations are especially high in the immediate vicinity of the Elbe and Weser Estuaries. The heavy metal measurements show that in places these pollutants have been mixed dawn up to several metres. In this upper mixing zone no general concentration gradient can be defined, suggesting recent reworking of the column. The case for phosphorus is somewhat different. Its concentration profile does not show a break at the same erosion basis, rather the concentrations decline quickly within the first few centimetres, suggesting that most of the phosphate is remobilized quickly. In fact, most of the phosphorus is bound in easily reducible Fe and Al phosphates (Figure 13.20, inorg. fraction), thus allowing easy remobilization to pore waters. The pore fluids are released during low tides or when the sediment is reworked by benthic fauna or resuspended during storms. Thus phosphorus is kept in the biological cycle for several turnovers before finally enclosed in sediments or exported to the Atlantic Ocean. Also PCBs show large concentration increases in the North Sea and in the Wadden Sea (Boon et al. , 1985). Main input seems to be the River Rhine.

Figure 13.20 Pollution of surface sediments of the German Wadden Sea between the Dollart (south-west) and Sylt (North) for the sediment size fraction < 2 µm. Map shows distribution of Zn, Pb, Cd, and Cu and inset graphs contents of total phosphorus, inorgranic phosphorus, and organic phosphorus (Schwedhelm and Irion, 1985)

So far no geochemical model has been coupled to a circulation model of the North Sea (e.g. Backhaus, 1985) and calculation of fluxes, inputs, and outputs to the system is only at its beginning (e.g. Weichart, 1973; Postma, 1981; Eisma, 1981; Wollast 1983; Eisma and Kalf, 1987; Schwedhelm and Irion, 1985).

OUTLOOK

Even without the anthropogenic interference estuaries are places of complicated hydrodynamic, chemicophysical, geological, and biological processes. The state of the system changes with weather, tides, seasons, sea level, glacials, and tectonic movements over a wide range of frequencies. We are far from understanding all of these interdependencies. The example of the Elbe and the inner German Bight teaches that many conflicting uses alter the estuarine system often in an unwanted and unexpected way. Scientists only now begin to understand what the dredging of channels, the dyking of marshes and the digging of harbour basins means for the tidal behaviour of an estuary and its vulnerability to storm surges. Pollution is another unwanted effect of industrial and intensified agricultural activity. Quietly heavy metalsand nutrients have spread from the river mouths into the coastal sea, its water , sediments, and biota. Adverse effects are now evident: the fauna and flora have changed and-in the case of the Elbe-it is forbidden to sell fish. Pollutants are measurable throughout the North Sea and it may be only a question of time until its fish cannot be sold any more.

Certainly, further research is needed to describe the system more completely and to derive models capable of predicting future changes. Data acquisition is poor, even for relatively well investigated estuaries like the Elbe. In-situ monitoring should be pushed especially to obtain data for storm surges, winter conditions, and extreme tides. Numerical hydrodynamic models need to be coupled to geochemical and ecological models. However, these activities should not obscure the fact that only stringent pollution control will effectively decrease current input to estuaries and that a full 'clean-up' may never be reached. Developing nations have the opportunity to 'learn' from the mistakes of industrialized regions and to organize their industrialization in a less harmful way.

ACKNOWLEDGEMENTS

The author is supported by a grant of the German Minister for Research and Technology (BMFT).

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