SCOPE 42 - Biogeochemistry of Major World Rivers

13.1 INTRODUCTION

Particles are supplied to estuaries from a variety of sources. Rivers bring annually c.13.5 X 10⁹ t of suspended matter into river mouths, together with 1-2 x 10⁹ t of bedload (sand and gravel; Milliman and Meade 1983). Considerable amounts of sediment can also come from the coastal sea, from local runoff, from erosion of older deposits, from the atmosphere (mostly dust), from glaciers, from waste dumping and from organisms. In most rivers, however, the bedload does not reach the estuary but is deposited in the lower reaches of the river. The flux of sediment through estuaries, in suspension or as bedload, varies greatly. In some estuaries there is a net influx of sediment, in others a net outflow, but in many estuaries the natural balance of in- and outflow is strongly influenced by man through dredging, land reclamation, construction of harbour works, dikes, bridges, etc.

Reworking (recycling) of particulate matter deposited in an estuary can take place almost immediately after deposition, or can take hundreds, thousands, millions or even many millions of years: The minerals left over from weathering processes (quartz, feldspars, clay minerals) are very resistant to further breakdown. The particles of organic origin (carbonate, opal, organic matter) are usually less stable and changes in chemical and isotopic composition of the particulate organic matter occur in virtually all estuaries, as far as known. Important changes in size may occur because of flocculation and deflocculation processes in the estuary. The various sources of particulate matter in estuaries and the principal processes affecting them are summarized in Figure 13.1.

The fine-size fractions, which are transported in suspension, have a relatively large specific surface, contain clay minerals, have a relatively high content of organic matter and because of this are far more surface-active than sand or gravel, although these usually also have surface-active coatings of organic matter, iron hydroxides or carbonate. Because of their high organic content the fine-size fractions are a food source for planktonic and benthic organisms. Through the interaction with biological and chemical cycles the fine-size fractions are a factor in other geochemical cycles as well, which is much less so for the sand- and gravel-sized bedload material.

Figure 13.1 General scheme of the sources of suspended matter in estuaries and the principal processes affecting the suspended particulate material. The arrows indicate the directions of transport. The supply from, or the loss to, the atmosphere is relatively small compared to the other sources and sinks

Suspended particles move approximately following the water transport and their transport velocity is several orders of magnitude higher than the transport velocity of the bedload. Sand moving along the bottom is temporarily stored in ripples and banks of varying size, while gravel rolling over the bottom moves even more slowly. The fine fractions are therefore transported more quickly and dispersed more widely than the bedload. The transport and deposition of particulate matter in estuaries of different types is conveniently described following the distinction made on the basis of the relative influence of the tides and the river flow, stratified where the tidal influence is very low, partially mixed where both river discharge and tidal flow are important and fully mixed where tidal mixing dominates. In arid areas, where evaporation is high, a reverse circulation can develop when water heavier than sea water is formed in the river or the estuary, so that the surface flow is inward and the bottom flow is outward (anti-estuarine circulation, e.g. the Salum River in southern Senegal during part of the year). In many estuaries one type of particle supply dominates: river supply (at many very large rivers such as the Amazon, Changjiang, Huanghe), primary production (during plankton blooms in estuaries with little supply from other sources), marine supply (Wadden Sea, many small estuaries), waste discharges (near dense population centres and industrial conglomerations, e.g. Long Island Sound).

Estuaries are not clearly defined. The inward limit can be taken at the salt water contact or at the inward limit of tidal influence. The outward limit may be inward of the coastline or far out in the coastal sea. At some large rivers the mixing of fresh water and sea water takes place almost entirely outside the river mouth, covering a large area of the continental shelf (Amazon, Zaire). In this chapter the mixing zone of fresh water and salt water including, where necessary, the inward fresh water zone of tidal influence, is regarded as 'estuarine'.

13.2 INORGANIC PARTICLE FORMATION

Particles can be formed in estuaries in several ways: by organisms producing them; by inorganic processes (precipitation, flocculation); and by break-up (erosion, resuspension) of sediments and rocks in the estuary. The organic production is discussed in Section 13.3. Erosion of older deposits occurs regularly in estuaries. Repeated cycles of deposition and erosion, tidal and seasonal, are a normal feature. In many estuaries where older sedimentary rocks are present, erosion of such rocks may contribute considerably to the particulate matter in the estuary (e.g. in the Bay of Fundy). Erosion and resuspension occur primarily when currents and waves are strong; the material is stirred up in clouds and mixing may take some time, so that in such areas the suspended matter temporarily may retain its original composition.

Particles formed by flocculation of dissolved material, like river supplied iron oxides and hydroxides and organic compounds, are small, ranging from colloids to particles larger than 0.4 µm, which is, by general agreement, the lower size limit of particulate matter in suspension (Sholkovitz 1976). Also at the salt water contact gypsum particles of c.2 µrn, as well as larger ones, are regularly found (Eisma et al. 1980), but the mechanism of their formation is not clear. The particles are not pure gypsum but have a variable Ca : S ratio and contain many impurities. They can be found regularly in the suspended matter but, in estuaries with high mineral particle supply, the gypsum particles are mixed with large numbers of other fine particles and have to be sought very carefully. Their presence often may be the result of washing the filters with distilled water to remove sea salts. Mixing experiments have shown that such gypsum particles are formed when fresh water and salt water are mixed with a large excess of fresh water (Eisma et al. 1980). This does not exclude the possibility that they are also formed in nature during estuarine mixing but this cannot be distinguished at present from an artificial origin.

Other particles found in estuaries and formed by inorganic physicochemical reactions include sulphides, particularly pyrite, that can be formed in bottom deposits under reducing conditions. Resuspension of the sediment brings the sulphide particles into suspension. On the whole, however, the inorganic particles formed in estuaries are quantitatively negligible compared to the particles of other origin: detrital mineral particles and particles of organic origin.

13.3 ORGANIC PARTICLE COMPOSITION

In estuaries, particles formed by organisms largely consist of organic matter. Biogenic production of calcium carbonate and opal (diatom frustules) also occurs, but is far less studied and quantitative data are hardly available (Milliman and Boyle 1975; Beukema 1980). Therefore we will concentrate on organic matter production in estuaries.

Particulate organic matter (POM) in the water column of estuaries is composed of living organisms (mainly phytoplankton, as well as bacteria and animals) and non-living particles (detritus, i.e. all types of biogenic material in various stages of decomposition, which represent potential energy sources for consumer species). A continuous spectrum exists from small dissolved organic molecules via colloids to large particles (floating trees, whales). The separation made between dissolved organic matter (DOM) and POM by filtration over 0.45 µm filters is an artificial one. Colloids, ranging in size from 0.001 to 1 Jim, are partly included in DOM, partly in POM; small organisms like viruses, bacteria, small protozoa and algae may be included in DOM.

Sources of POM are in situ production by autotrophs and allochthonous , inputs from river, sea or air. Moreover, several processes have been described for estuaries in which DOM is changed into POM. The main process is bacterial uptake of DOM (Sepers 1977), but also DOM uptake by algae and a large number of animal groups has been observed (see review by Sepers 1977). Preferential uptake of some dissolved organic molecules by clay minerals has been described (Hedges 1977) and DOM compounds play a role in flocculation (this chapter).

The processes by which DOM is changed into POM, however, are probably not of major importance for the DOM and POM pools in estuaries, as DOC is usually found to behave conservatively in estuaries (see Cadée 1984; Mayer 1985, and the reference therein). Conservative behaviour can only be apparent when processes by which DOM is changed into POM are counterbalanced by processes by which POM is changed into DOM. Such processes are, e.g. leaching of DOC from fresh water algae entering the estuary, production of exudates by phytoplankton ( estimated as c. 5% of the primary production; Sharp 1977) and macrophytes, and release of DOM by invertebrates during feeding on algae or other invertebrates (Johannes and Webb 1970). Deviation of conservative behaviour of DOC, if found in estuaries, occurs either at the low or high salinity end member. At low salinities it was ascribed to plasmolytic release from fresh water algae (Morris et al. 1978) or upstream effects of the turbidity maximum, diffusion of particles too small to be collected as POC on filters or a shift in the POC-DOC equilibrium (Cadée and Laane 1983). Willey (1984) mentioned the possible interaction of DOC and Mg ions in sea water. Near the high salinity end member deviations occur due to DOC production by enhanced phytoplankton growth reated to river-induced eutrophication (see Szekielda 1982; Cadée 1984, for references).

Whereas different methods have been used to separate POC in its different fractions (living/non-living, phytoplankton, bacteria, zooplankton) they are usually time consuming because they involve microscopic inspection. Pissierssens et al. (1985) proposed a staining method using Trypan Blue which stains only proteins in dead organic matter. Wienke and Cloern (1987) give a useful method to separate phytoplankton POC from non-phytoplankton POC. For San Francisco Bay they found that phytoplankton-C constitutes about one-third of (suspended) POC ranging from 20% during spring to 95% during phytoplankton blooms. POC ranged from 2.5% to 12.2% of TSM (POM from 5% to 25%). Using the same chlorophyll/C ratio of 51 of Wienke and Cloern (1987) we get about the same figures for the western Wadden Sea; an average 30% of POC is phytoplankton, ranging from 10% in February to 90% during blooms. POC amounted to 2% to 20% of TSM (average 3%). One should be aware, however, that such average data have little practical value. Usually spatial, tidal and seasonal variations are high in estuaries (Postma 1967). Wellershaus (1981) observed a 14-day neap-spring tidal cycle and Postma (1980) measured considerable effects of a storm on TSM.

13.4 ORGANIC PARTICLE PRODUCTION

Phytoplankton, benthic algae and vascular plants are the predominant groups of autotrophs, supplying most of the in situ primary production in estuaries. The contribution of each differs from estuary to estuary. Knox (1986) summarized part of the available data and indicated that macrophytes (mangroves, seagrasses, Spartina, macroalgae) contributed 30% to 90%, microphytobenthos 5% to 40% and phytoplankton 2% to 45% of the primary production. For the Dutch Wadden Sea Cadée (1980) estimated phytoplankton and microphytobenthos to contribute equal amounts to primary production and a low contribution by macrophytes (c. 5%).

Phytoplankton produces organic particles in the water column, microphytobenthos production takes place on the bottom, particularly on tidal flats.

Microbenthic algae will only occasionally be brought into suspension, e.g. during storms or during maxima of tidal currents (Baillie and Welsh 1980). Macrophytes will mainly occur as detritus fragments in the water column, macroalgae may occur free-floating and still growing in the water column (Ulva in the Wadden Sea).

A large number of publications on phytoplankton primary production in estuaries have recently been reviewed by e.g. Boynton et al. (1982), Kennish (1986), Knox (1986), Cole and Cloern (1987) and Cloern (1987). Annual phytoplankton primary production in estuaries averages about 100 g C/m² and published data range from 6.8 to 530 g C/m²/year (Kennish 1986). Boynton et al. (1982) found a significant positive relation between phytoplankton production and nitrogen loading rate (but not with phosphorus) in a comparison of 14 estuarine systems. Cloern (1987), in comparing 26 estuarine systems, stresses the importance of turbidity and the ratio of photic depth to mixed depth.

Whereas in such comparisons it is easy to have one figure for primary production for each estuary, we have to keep in mind that variation in primary production along the length of an estuary (Cloern 1987) makes such comparisons complicated. Usually a gradient is observed in estuaries with high turbidity levels and high nutrient levels on the riverine side and clear , low nutrient waters on the sea side. A model was proposed in which phytoplankton is light limited on the riverine side and nutrient limited on the other side, leading to maximum production in between (Cadée 1978). Moreover, in the few cases time series are available, they show year-to-year variations of a factor 2 to 3. In Chesapeake Bay this was related to the effect of a hurricane passing through the watershed region in one year providing an extra input of nutrients to Chesapeake Bay and consequently high production for two years (Boynton et al. 1982). In the Dutch Wadden Sea primary production showed an increase related to eutrophication (Cadée 1986).

POM forms the most important food source for heterotrophs in estuaries. Stable isotopes (C¹²/C¹³ ratio) have proved an important tool in discriminating between the role of the different fractions of POM as food sources ( e.g. Simenstad and Wissmar 1985). By this method Petersen and Howarth (1987) were able to settle a long-standing debate about the relative importance of Spartina detritus and phytoplankton in supporting marsh and estuarine secondary production; both sources appeared to be equally important.

Estuaries act as a trap for organic and inorganic particles from the river and the sea (Postma 1967; Meade 1972). Eisma et al. (1985) estimate that up to 70-90% of river inputs of POC are lost in estuaries. A research-stimulating debate is still going on whether estuaries import or export POC to the sea. As so often in such debates both situations occur. Kennish (1986) discriminates between' American type estuaries', bordered by extensive wetlands and exporting, and 'European type estuaries' with broad, relatively bare intertidal flats that import POC. In an interesting 'model experiment' for the Ems-Dollart Estuary, Baretta and Ruardij (1987) showed that import or export of POC may also be dependent on TSM concentration at the seaward boundary of the estuary .Lowering this concentration by 50% resulted in a lowering of turbidity throughout the estuary and an increased phytoplankton primary production changing the system from importing POC to exporting POC.

13.5 FLOCCULATION AND PARTICLE SIZE

The suspended matter in rivers, estuaries and the coastal sea is usually at least partly flocculated. Flocs of different sizes and shapes can be seen under a microscope but in situ much larger flocs (marine snow, macroflocs) have been observed in those estuarine waters where up to now observations have been made (Eisma et al. 1983; Kranck 1984; Eisma 1986; Wells and Shanks 1987). Although stable in the water at current velocities up to 1.5 m/s, the in situ flocs easily break up during sampling and size analysis, so that size analysis, as it is usually carried out by Coulter counter, pipette or microscope, etc. , involves broken-up macroflocs. Flocs are usually a mixture of inorganic particles and organic matter, and often contain also diatom frustules and other organogene particles. Flocs have been produced in the laboratory without organic matter (e.g. from clay suspensions) but in nature probably everywhere organic material is involved, if only in the form of coatings.

Three kinds of size distribution have been determined of suspended material:

1. The size distribution of deflocculated particles, here referred to as grain size. This is the particle size usually determined in bottom deposits, and requires removal of the organic matter and disaggregation prior to analysis.
2. The size distribution of flocs and single particles as they occur in situ in suspension.
3. The size distribution of broken-up macroflocs (microflocs) and single particles as they are measured with a Coulter counter or pipette analysis after sampling.

Natural flocs (macroflocs) in estuaries and coastal waters have been studied up to now only by direct observation in the water, by in situ photography and by sampling them intact with a special sampling device. In situ photography has yielded some information on size and shape of the flocs but reliable size distributions have not yet been obtained, mainly because the techniques of size analysis of flocs in situ have not yet given quantitatively reliable results for the whole size range (Eisma et al. 1983; Bale and Morris 1987; Wells and Shanks 1987). Sampling of individual flocs of c. 100 µm to several millimetres diameter and observation by SEM indicated that the large particles seen in the photographs were indeed large flocs consisting of a mass of mineral grains and organic matter. The maximum size was found to be 3-4 mm in the Ems Estuary (Eisma et al. 1983) and 4.4 mm (with an average of 0.29-0.45 mm) in Cape Lookout Bight (Wells and Kim 1989). Since the flocs break up easily during sampling and handling of the samples, all size distributions of suspended matter, obtained after sampling without removing the organic matter and disaggregation, are size distributions of mixed populations of (macro) floc fragments (microflocs) and single grains. The fragments (microflocs) are usually smaller than 125 µm, but careful sampling may leave larger fragments intact. With the usual techniques of sampling and size analysis (Niskin bottles or 11 glass bottles for sampling, Coulter counter or pipette for size analysis), it was found in the Dutch coastal waters that only those microflocs remain that are difficult to break further apart (Eisma and Kalf 1979; Eisma et al. 1980); even ultrasonic treatment of 10-15 min does not result in complete break-up of flocs and all organic matter has to be removed by oxidation to obtain a population of single particles. The size distributions are therefore reproducible and storage of the samples will only result in changes after c. 24 hours or longer. It is assumed that bacterial degradation of the organic matter in the samples leads to further weakening of the larger flocs and to aggregation of the fine particles so that the size of the (micro) flocs becomes more uniform (Eisma and Kalf 1979).

Up to a few years ago it was hardly realized that macroflocs are commonly present in rivers, estuaries and coastal waters, but they were found in all those areas where observations have been made with a camera or by diving (Western Europe, Canada, US). Since it is rather difficult to study the size and nature of the flocs as they are in situ, our knowledge of macroflocs and the flocculation process is still largely based on indirect observations, theoretical considerations and laboratory experiments. These have indicated the importance of turbulence, constituent grain size, differential settling, Brownian motion, salt flocculation, particle concentration, and organisms for the flocculation of suspended particles (for summaries see McCave 1984; Eisma 1986). Observations in nature led to the conclusion that salt flocculation (often regarded as the main flocculation mechanisms) is probably of little importance, except when very small particles (colloids) are involved. Comparison of the maximum size of macroflocs, as seen with in situ photography with data on turbulence, indicated that the maximum size is likely to be controlled by the smallest turbulent whirls on the Kolmogorov microscale. Observations on the feeding behaviour of estuarine and coastal organisms have shown that a large part of the suspended matter, if not all, is involved in biodeposition as pellets or faeces and resuspended again (Verwey 1952; Biggs and Howell 1984).

An indication of how macroflocs may be held together, could be obtained by comparing microfloc size distributions with the concentration of certain organic compounds, using the standard techniques for sampling and size analysis; the microfloc size indicates the degree of break-up of the flocs, or the strength by which the (macro) flocs are held together. In a number of estuaries¾of the Miramichi (Kranck 1981), the Ems, Rhine, Elbe, Gironde, Rhone, probably also the Surinam and the Amazon (all listed in Eisma 1986), and the Rapahannock (Pierce and Nichols 1986)¾a decrease in microfloc size occurred at the salt water contact at salinities below 1 per 1000, which was in most estuaries ascertained by microscopic observation (light microscope and -SEM}. In the Schelde Estuary the same decrease was found in the bottom water as in the surface water, but related to the salt water contact, which in the bottom water is located several kilometers more inward than in the surface water. The macrofloc size, in those estuaries where this has been observed by in situ photography (Ems, Rhine, Gironde), did not show such a decrease at low salinities; the same large flocs were seen in the river water as in the low salinity water.

In the Ems and the Rhine Estuaries the microfloc size decrease was found to coincide with the presence of a peak in the concentration of dissolved carbohydrates. This was found in winter, when biological activity was low, and it could be shown that this peak was not related to the presence of algae decaying at the salt water contact. This indicates that the carbohydrates came from the suspended particles, where they may have been present as polysaccharides or as fulvic acids. Polysaccharides are produced by bacteria and plankton organisms, which are generally present in rivers and estuaries. The borate in the sea water mixing with the fresh water is present in sufficiently high concentrations to mobilize carbohydrates in concentrations of several milligrams per litre at salinities below 1 per thousand. Another, and more elegant explanation of the carbohydrate peak may be the dissociation of fulvic acids. It was found by Leenheer (pers. comm.) that fulvic acids break up into an aromatic fraction and a carbohydrate fraction when the ion strength of the solution increases. The higher the ion strength, the more fulvic acids are dissociated. The dissolved carbohydrates at the salt water contact may there- fore come from fulvic acids that hold particles together in flocs. Both processes, mobilization of polysaccharides and dissociation of fulvic acids, would result in weakening the forces that hold particles together in flocs, causing a reduction in microfloc size. 

Another feature in estuaries is the low organic content of the suspended matter as compared with the river, indicating that 50% to more than 90% of the particulate organic matter supplied by the river is lost in the Ems and Gironde Estuaries and, e.g. the Amazon mouth (Eisma et al. 1985). The isotopic composition of the particulate organic matter also changes from the low values usually found in rivers towards the higher values found normally in marine conditions. A similar loss of organic matter, but without the change in isotopic composition, occurs in the coastal sea. In the North Sea the organic content of the bottom sediment is much lower than that of the suspended matter, even in winter when there is no primary production adding organic particles to the suspended matter (Eisma and Kalf 1987a), so that deposition results in loss of organic matter. This is most probably caused by consumption of the organic matter by bacteria and bottom fauna, which implies that a large part of the particulate organic matter is transformed into energy. Tidal and seasonal cycles of deposition and resuspension are a common feature of estuaries, while primary production is usually restricted because of the high turbidity, resulting in a low organic content of the suspended matter. In the North Sea smaller microflocs are associated with lower organic matter content of the suspended matter, high suspended matter concentrations, shallow water depths, more exchange of the suspended matter with bottom sediments and relatively short residence times in the water (in the order of days or weeks) .Conversely, large microflocs are associated with higher organic matter contents, deeper water, low suspended matter concentrations, virtually no exchange with the bottom sediments and relatively long residence times in the water (in the order of a year or more; Eisma and Kalf 1987a). This, together with the decrease in micro floc size at the salt water contact in estuaries, suggests a relation between floc strength and the organic matter content of the flocs. Recent results, however, from the estuary and plume of the Mahakam River (Kalimantan, Indonesia) indicate no relation between the presence and size of microflocs and the organic content of the suspended matter, but a clear relation with the particle concentration and the biological activity in the surface water. This agrees with the results obtained from the Ems and the Rhine that not the bulk organic matter but specific organic compounds (polysaccharides, fulvic acids) are important for flocculation.

Flocculation strongly influences the transport and deposition of fine-grained particles. Kranck (1980) has shown that, at least in theory, flocs in suspension are composed of all grain sizes of lower settling rates than its own. Grains of all sizes, as part of flocs, settle at the same rate and in proportion to their total concentration. By floc settling a sediment is deposited that has the same unsorted character as the original parent material. The larger particles settle as single grains. Bottom deposits therefore usually show a grain size distribution that consists of a fine part with a horizontal slope (log concentration versus log grain size) and a coarser part showing a peak, suggesting that the fine part has been deposited as flocs and the coarse part as single grains. Nearly flat grain size spectra prevail also in the suspended matter of rivers, particularly of large rivers (Kranck and Milligan 1983), suggesting the general occurrence of flocs as the mode of transport of fine-grained material.

Flocculation enhances the deposition rate, as has been demonstrated for marine snow, which has a high settling rate compared to the settling rate of the constituent particles (Shanks and Trent 1980). In laboratory experiments it was found that flocculated mixtures of organic matter and mineral particles have a much higher settling rate than either the separate mineral particles or the organic matter (Kranck 1984). This also applies to the macroflocs of near- and inshore waters (Wells and Shanks 1987) and thus may explain the rapid settling of suspended particulate matter in coastal waters, particularly during periods of low wave activity. Rapid settling into .areas from where it is not easily resuspended may contribute to the high deposition rates (1-10 cm/ year) even on open coasts, where the particles settle from low suspended matter concentrations (Cape Lookout, southern North Sea; see Wells and Shanks 1987; Eisma and Kalf 1987b).

Flocculation also creates microenvironments in the water, forming miniscale 'nutrient patches and providing food for marine micro-organisms. This has been studied in more offshore oceanic waters with much lower suspended matter concentrations than found nearshore, in estuaries or in fresh waters, but the presence of bacteria and other micro-organisms on the flocs (Lake Tahoe, Pearl 1973; Rhine River, Uiterwijk Winkel 1975; Chesapeake Bay, Zabawa 1978; Dutch Waddensea, Bos, pers. comm.) suggests that such microenvironments are also formed at high suspended matter concentrations and/or low salinities and in fresh water.

REFERENCES

Baillie, P. W. and Welsh, B. L. (1980) The effect of tidal resuspension on the distribution of intertidal epipelic algae in an estuary. Estuar. Coast. Mar. Sci. 10, 165-80.

Bale, A. J. and Morris, A. W. (1987) In situ measurements of particle size in estuarine waters. Estuar. Coast. Shelf Sci. 24, 253-63.

Baretta, J. W. and Ruardij, P. (1987) Evaluation of the Ems Estuary ecosystem model. Cont. Shelf Res. 7, 1471-76.

Beukema, J. J. (1980) Calcimass and carbonate production by molluscs in the tidal flats in the Dutch Wadden Sea; I. The tellinid bivalve Macoma baltica. Neth.J. Sea Res. 14, 323-38.

Biggs, R. B. and Howell, B. A. (1984) The estuary as a sediment trap: alternate approaches to estimating its filtering efficiency. In: Kennedy, V. S. (Ed.) The Estuaryas a Filter, Academic Press, Orlando, pp. 107-29.

Boynton, W. R., Kemp, W. M. and Keefe, C. W. (1982) A comparative analysis of nutrients and other factors influencing estuarine phytoplankton production. In: Kennedy, V. S. (Ed.) Estuarine Comparison, Academic Press, New York, pp. 69- 90.

Cadée, G. C. (1978) Primary production and chlorophyll a in the Zaire River, estuary and plume. Neth. J. Sea Res. 12, 368-81.

Cadée G. C. (1980) Reappraisal of the production and import of organic carbon in the western Wadden Sea. Neth. J. Sea Res. 14, 305-22.

Cadée, G. C. (1984) Particulate and dissolved organic carbon and chlorophyll a in the Zaire River, estuary and plume. Neth. J. Sea Res. 17, 426-40.

Cadée, G. C. (1986) Increased phytoplankton primary production in the Marsdiep area (western Dutch Wadden Sea). Neth. J. Sea Res. 20, 285-90.

Cadée, G. C. and Laane, R. W. P. M. (1983) Behaviour of POC, DOC and fluorescence in the freshwater tidal compartment of the River Ems. In: Degens, E. T., Kempe, S. and Soliman, H. (Eds) Transport of Carbon and minerals in Major World Rivers, Pt. 2. Mitt. Geol.-Paläont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd. 55, 331-342.

Cloern, J. E. (1987) Turbidity as a control on phytoplankton biomass and productivity in estuaries. Cont. Shelf Res. 7, 1367-81.

Cole, B. E. and Cloern, J. E. (1987) An empirical model for estimating phytoplankton productivity in estuaries. Mar. Ecol. Progr. Ser. 36, 299-305.

Eisma, D. (1986) Flocculation and deflocculation of suspended matter in estuaries. Neth. J. Sea Res. 20, 183-99.

Eisma, D. and Kalf, J. (1979) Distribution and particle size of suspended matter in the Southern Bight of the North Sea and the eastern Channel. Neth. J. Sea Res. 13, 298- 324.

Eisma, D. and Kalf, J. (1987a) Distribution, organic content and particle size of suspended matter in the North Sea. Neth. J. Sea Res. 21, 265-85.

Eisma, D. and Kalf, J. (1987b) Dispersal, concentration and deposition of suspended matter in the North Sea. J. Geol. Soc. (London) 144, 161-78.

Eisma, D., Kalf, J. and Veenhuis, M. (1980) The formation of small particles and aggregates in the Rhine Estuary. Neth. J. Sea Res. 14, 172-91.

Eisma, D., Boon, J. J., Groenewegen, R., Ittekkot, V., Kalf, J. and Mook, W. G. (1983) Observations on macro-aggregates, particle size and organic composition of suspended matter in the Ems estuary. In: Degens, E. T., Kempe, S. and Soliman, H. (Eds) Transport of Carbon and Minerals in Major World Rivers, Pt. 2. Mitt. Geol.-Paläont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd. 55, pp. 295-314.

Eisma, D., Bernard, P., Boon, J. J., van Grieken, R., Kalf, J. and Mook, W. G. (1985) Loss of particulate organic matter in estuaries as exemplified by the Ems and the Gironde estuaries. In: Degens, E. T., Kempe, S. and Herrera, R. (Eds) Transport of Carbon and Minerals in Major World Rivers, Pt. 3. Mitt. Geol.- Paläont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd. 58, 397-412.

Hedges, J. I. (1977) The association of organic molecules with clay minerals in aqueous solutions. Geochim. Cosmochim. Acta 41, 1119-23.

Iohannes, R. E. and Webb, K. L. (1970) Release of dissolved organic compounds by marine and freshwater invertebrates. In: Hood, D. W. (Ed.) Organic Matter in Natural Waters. Occ. Publ. Inst. Mar. Sci. Univ. Alaska, Vol. 1, pp. 257-73.

Kennish, M. J. (1986) Ecology of Estuaries. Vol. 1. Physical and Chemical Aspects. CRC Press Inc. Boca Raton, Florida, 254 pp.

Knox, G. A. (1986) Estuarine Ecosystems: A Systems Approach. CRC Press Inc. Boca Raton, Florida, Vol. 1: 289 pp.; Vol. 2: 230 pp.

Kranck, K. (1980) Experiments on the significance of flocculation in the settling of fine-grained sediment in still water. Can. J. Earth Sci. 17, 1517-26.

Kranck, K. (1981) Particulate matter grain-size characteristics and flocculation in a partially mixed estuary. Sedimentology 28, 107-14.

Kranck, K. (1984) The role of flocculation in the filtering of particulate matter in estuaries. In: Kennedy, V. S. (Ed.) The Estuary as a Filter. Academic Press, Orlando, pp. 159-75.

Kranck, K. and Milligan, T. G. (1983) Grain size disiributions of inorganic suspended .river sediment. In: Degens, E. T., Kempe, S. and Soliman, H. (Eds) Transport of Carbon and Minerals in Major World Rivers, Pt. 2. Mitt. Geol.-Paläont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd. 55, pp. 525-34.

Mayer, L. M. (1985) Geochemistry of humic substances in estuarine environments. In: Aiken, G. R., McKnight, D. M., Wershaw, R. L. and MacCarthy, P. (Eds) Humic Substances in Soil, Sediment, and Water: Geochemistry, Isolation, and Characterization, Wiley, New York, pp. 211-32.

McCave, I. N. (1984) Size spectra and aggregation of suspended particles in the deep ocean. Deep-Sea Res. 31(4), 329-52.

Meade, R. H. (1972) Transport and deposition of sediments in estuaries. Geol. Soc. Amer. Mem. 133, 91-120.

Milliman, J. D. and Boyle, E. (1975) Biological uptake of dissolved silica in the Amazon river estuary. Science 189, 995-7.

Milliman, J. D. and Meade, R. H. ( 1983) World-wide delivery of river sediment to the oceans. J. Geol. 91, 1-21.

Morris, A. W., Mantoura, R. F. C., Bale, A. J. and Howland, R. J. M. (1978) Very low salinity regions of estuaries: important sites for chemical and biological reactions. Nature 274, 678-80.

Pearl, H. W. (1973) Detritus in Lake Tahoe: structural modification by attached microflora. Science 180, 496-8.

Petersen, B. J. and Howarth, R. W. (1987) Sulfur, carbon, and nitrogen isotopes used to trace organic matter flow in the salt-marsh estuaries of Sapelo Island, Georgia. Limnol. Oceanogr. 32, 1195-213.

Pierce, J. W. and Nichols, M. M. (1986) Change of particle composition from fluvial into an estuarine environment: Rappahannock River, Virginia. J. Coast. Res. 2(4), 419-25.

Pissierssens, P. , Bergmans, M. and Polk, Ph. (1985) On the need for independently estimating organic tripton carbon content. Mar. Ecol. Progr. Ser. 25, 107-9.

Postma, H. (1967) Sediment transport and sedimentation in the estuarine environment. In: Lauff, G. H. (Ed.) Estuaries. A.A.A.S. Publ. 83, 158-79.

Postma, H. (1980) Sediment transport and sedimentation. In: Olausson, E. and Cato, I. (Eds) Chemistry and Biochemistry of Estuaries, Wiley, New York, pp. 153-85.

Sepers, A. B. J. (1977) The utilization of dissolved organic compounds in aquatic environments. Hydrobiologia 52, 39-54.

Shanks, A. L. and Trent, J. D. (1980) Marine snow: sinking rates and potential role in vertical flux. Deep-Sea Res. Pt. A 27, 137-43.

Sharp, J. H. (1977) Excretion of organic matter by marine phytoplankton: Do healthy cells do it? Limnol. Oceanogr. 22, 381-99.

Sholkovitz, E. R. (1976) Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater. Geochim. Cosmochim. Acta 40, 831-45.

Simenstad, C. A. and Wissmar, R. C. (1985). d¹³C evidence of the origins and fates of organic carbon in estuarine and nearshore food webs. Mar. Ecol. Progr. Ser. 22, 141-52.

Szekielda, K.-H. (1982) Investigations with satellites on eutrophication of coastal regions. In: Degens, E. T. (Ed.) Transport of Carbon and Minerals in Major World Rivers, Pt. 1. Mitt. Geol.-Paläont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd. 52, pp. 13-37.

Uiterwijk Winkel, A. P. B. (1975) Microbiologische aspecten en het sedimentatiegedrag van rivierslib. Rijkswaterstaat, Dir. Wawa, District ZW, Report 44.006.01, 60 pp.

Verwey, J. (1952) On the ecology of distribution of cockle and mussel in the Dutch Wadden Sea, their role in sedimentation and the source of their food supply. Arch. néerl. Zool. 10, 171-239.

Wellershaus, S. (1981) Turbidity maximum and mud shoaling in the Weser estuary. Arch. Hydrobiol. 92, 161-98.

Wells, J. T. and Kim, S.- Y .(1989) Variability of large aggregates and their effect on the attenuation of light: field observations (in press).

Wells, J. T. and Shanks, A. L. (1987) Observations and geologic significance of marine snow in a shallow-water, partially enclosed marine embayment. J. Geophys. Res. 92(C12), 13 185-90.

Wienke, S. M. and Cloern, J. E. (1987) The phytoplankton component of seston in San Francisco Bay. Neth. J. Sea Res. 21, 25-33.

Willey, J. D. (1984) The effect of seawater magnesium on natural fluorescence during estuarine mixing, and implications for tracer applications. Mar. Chem. 15, 19-45.

Zabawa, Chr. F. (1978) Microstructure of agglomerated suspended sediments in northern Chesapeake Bay estuary. Science 202, 49-51.