Chemical composition

Estuary water is not a simple dilution of seawater. Many subtle changes in composition are involved, varying with local conditions. The relative proportions

Distance upstream in miles

Distance upstream in miles

Figure 8.16 Section of Tyne Estuary showing isohalines, isotherms, percentage oxygen saturation and distribution of dominant planktonts at 2 m depth. Zone A - marine plankton containing Sagitta elegans, Nyctiphanes couchi, Calanus, Paracalanus, Pseudo-calanus, nauplii of Semibalanus balanoides, polychaete larvae, etc. Zone B - very sparse plankton; water contains many fragments of organic debris. Zone C - abundant estuarine copepod, Eurytemora hirundoides, with elvers of Anguilla. Zone D - filamentous freshwater algae, mainly Ulothrix.

(Data obtained during a one-day students' field trip 18 March 1965.)

Figure 8.16 Section of Tyne Estuary showing isohalines, isotherms, percentage oxygen saturation and distribution of dominant planktonts at 2 m depth. Zone A - marine plankton containing Sagitta elegans, Nyctiphanes couchi, Calanus, Paracalanus, Pseudo-calanus, nauplii of Semibalanus balanoides, polychaete larvae, etc. Zone B - very sparse plankton; water contains many fragments of organic debris. Zone C - abundant estuarine copepod, Eurytemora hirundoides, with elvers of Anguilla. Zone D - filamentous freshwater algae, mainly Ulothrix.

(Data obtained during a one-day students' field trip 18 March 1965.)

of the various constituents in river water are always widely different from those of seawater. Some minor constituents of seawater, e.g. Fe, Si, PO4, are often present in river water in much greater concentration than in the sea. Depending largely on the geology of the drainage area, some rivers are relatively rich in Mn, Cu or Zn. Rivers may also transport large amounts of solid matter, and both suspended particles and sediments may absorb or release solutes and colloidal micelles. Between river and sea there are sometimes irregular changes of pH due to the effects of varying salinity on the dissociation of bicarbonate. Consequently, estuary water differs from seawater not only in concentration but also in the relative quantities of many constituents.

8.11.3 Substrata

River water often carries large amounts of suspended silt which become deposited in the estuary where tidal and river currents slow down. Settlement of the fine particles carried down by rivers is enhanced in the estuary by salt flocculation. The influx of salt water results in an increase of electrolyte concentration, which in turn causes fine clay particles to stick together. These clumped particles settle out more quickly. The inflow of seawater along the floor of an estuary also tends to carry sediments into the estuary mouth, adding to the material deposited by the river. This continual trapping and settlement of sediments means that most estuaries are muddy with shores of fine unconsolidated mud. However, if the inflowing river drains mainly hard ground and if the offshore areas are also rock, sand or shell, a predominantly sandy estuary will result (e.g. many of those in north Devon). In narrow parts of an estuary, tidal ebb and flow of great volumes of water can produce powerful currents and heavy scouring of the bottom, particularly when the water carries much silt.

The prevalence of fine silts in estuarine deposits leads to severe deoxygenation of the mud, and silting conditions often change seasonally or even daily with resultant effects on the surface of the substrate.

The intensity of wave action, and the speed and direction of water movement, vary continuously in estuaries through each tidal cycle. At the seaward end of some estuaries, the interaction of waves and currents piles up the bottom sediments forming a zone of shallow water across the river mouth, termed a bar. This prevents large waves penetrating up-river, except at the seaward end at high water when there is sufficient depth over the bar to allow waves to cross without breaking. Throughout the rest of the tidal cycle the water is calm. In these circumstances only the higher levels of the shore receive appreciable wave action and may in consequence be rocky in places. Lower levels of the shore do not experience waves and are therefore likely to be covered with soft sediments, the grade varying at each level depending on the flow rate of the water. The fastest rates of tidal flow occur around the mid-tide periods. Therefore, only the estuary bottom and the shore below mid-tide level encounter the full scouring of fast flow. Consequently an estuary shore often displays considerable changes in substrate between upper and lower tidal levels. At the seaward end there may be rocks or stones at high-water level where wavebreak occurs; muddy sands or mudflats on the upper parts of the middle shore; coarse, unstable sands and often shifting sandbanks on the lower parts of the middle shore; and gravelly deposits or even exposed rocks on the estuary bottom. Depending on the geography of the estuary, tidal flow rates usually reduce upstream, and fine deposits correspondingly predominate until, towards the landward end, the substrate changes to reflect mainly the flow rate and silt burden of the river.

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