Box 61 Salinity

Salinity is defined as the weight in grams of inorganic ions dissolved in 1 kg of water. Seven ions constitute more than 99% of the ions in seawater and the ratios of these ions are constant throughout the world oceans. Consequently, the analysis of one ion can, by proportion, give the concentration of all the others and the salinity. The density of seawater and light and sound transmission all vary with salinity.

Salinity is measured by the conductance of electrical currents through the water

(conductivity). Measured values are reported relative to that of a known standard; thus salinity has no units—although, in many older texts, salinities are reported in units of parts per thousand (ppt or %o) or grams per litre.

Open-ocean waters have a narrow range of salinities (32-37) and most are near 35. In estuaries, values fall to less than 1 approaching the freshwater end-member. In hypersaline environments salinities can exceed those of seawater, reaching values greater than 300.

t

_ Addition to solution ___

n o

at 1 C

n o u

Removal from solution

dilution line (straight)

Salinity

Fig. 6.3 Idealized plots of estuarine mixing illustrating conservative and non-conservative mixing. CR and CS are the concentrations of the ions in river and seawater respectively. After Burton and Liss (1976), with permission from Elsevier Science.

If the concentration of the measured component is, like salinity, controlled by simple physical mixing, the relationship will be linear (Fig. 6.3). This is called conservative behaviour and may occur with riverine concentrations higher than, or lower than, those in seawater (Fig. 6.3). By contrast, if there is addition of the component, unrelated to salinity change, the data will plot above the conservative mixing line (Fig. 6.3). Similarly, if there is removal of the component, the data will plot below the conservative mixing line (Fig. 6.3). In most cases, removal or input of a component will occur at low salinities and the data will approach the conservative line at higher salinity (Fig. 6.4). Extrapolation of such a 'quasi-conservative line' back to zero salinity can provide, by comparison with the measured zero salinity concentration, an estimate of the extent of removal (Fig. 6.4a) or release (Fig. 6.4b) of the component.

10 15 20 25 30 Salinity

Fig. 6.4 (a) Dissolved iron versus salinity in the Merrimack Estuary (eastern USA), illustrating non-conservative behaviour. Linear extrapolation (LE) of high-salinity iron data to zero salinity gives an estimate of 60% low-salinity removal of iron (after Boyle et al. 1974). (b) Dissolved barium versus salinity in the Chesapeake Bay (eastern USA). In this case linear extrapolation (LE) of high-salinity barium data to zero salinity indicates low-salinity release of barium (after Coffey et al. 1997), with permission from Elsevier Science.

10 15 20 25 30 Salinity

10 15 20 25 30 Salinity

Fig. 6.4 (a) Dissolved iron versus salinity in the Merrimack Estuary (eastern USA), illustrating non-conservative behaviour. Linear extrapolation (LE) of high-salinity iron data to zero salinity gives an estimate of 60% low-salinity removal of iron (after Boyle et al. 1974). (b) Dissolved barium versus salinity in the Chesapeake Bay (eastern USA). In this case linear extrapolation (LE) of high-salinity barium data to zero salinity indicates low-salinity release of barium (after Coffey et al. 1997), with permission from Elsevier Science.

6.2.3 Halmyrolysis and ion exchange in estuaries

The electrochemical reactions that impinge on soil-derived river-borne clay minerals carried into seawater do not finish with flocculation of particles and sedimentation of aggregates. The capacity for ion exchange in clay minerals (see Section 4.8) means that their transport from low-ionic-strength, Ca2+- and HCO--dominated riverwater, to high-ionic strength, sodium chloride (NaCl)-domi-nated seawater demands reaction with the new solution to regain chemical equilibrium (see Box 3.2). The process by which terrestrial materials adjust to marine conditions has been called 'halmyrolysis', derived from Greek roots hali (sea) and myros (unguent), literally 'to anoint with the sea'. Halmyrolysis is imprecisely defined but we will consider it to encompass all those reactions that affect a particle in seawater before burial in sediment.

Various measurements of cation exchange on river clays in seawater have shown that clay minerals exchange adsorbed Ca2+ for Na+, potassium ions (K+) and magnesium ions (Mg2+) from seawater (see Section 4.8), consistent with the differences in ionic composition between river and seawater. In general, components with a high affinity for solid phases, such as dissolved phosphorus (P) or iron (Fe) (Fig. 6.4a), are removed from solution. Thus the rules of ionic behaviour arising from consideration of charge/ionic radius (z/r) ratios (see Section 5.3) are helpful in understanding chemical behaviour in estuarine environments, as well as in weathering.

6.2.4 Microbiological activity in estuaries

As in most environments, biological, particularly microbial, processes are important in estuaries; these can include both primary production by phytoplankton and organic matter decomposition by heterotrophic bacteria. In many estuaries the high particulate concentrations make waters too turbid to allow phytoplank-ton growth. However, in shallow or low turbidity estuaries, or at the seaward end of estuaries where suspended solid concentrations are low due to sedimentation of flocculated particles, sunlight levels may be sufficient to sustain phytoplank-ton growth. Estuaries frequently provide safe, sheltered harbours, often centres of trade and commerce. As a result, in developed and developing countries estuary coasts are often sites of large cities. Discharge of waste, particularly sewage, from the population of these cities increases nutrient concentrations and, where light is available, large amounts of primary production occur (see Section 5.5). In the dynamic environment of an estuary, dilution of phytoplankton-rich estuarine water with offshore low-phytoplankton waters occurs at a faster rate than cells can grow (phytoplankton populations under optimum conditions can double on timescales of a day or so). Thus, phytoplankton populations are often limited by this dilution process, rather than by nutrient or light availability.

The extent of nutrient removal that can occur in estuaries is illustrated for silicate (SiO2) and phosphorus (P) in the estuary of the River Great Ouse in eastern England (Fig. 6.5). In this example there are a number of parameters whose values point to the role of phytoplankton in nutrient removal. Most importantly, silicate removal is related to high particulate chlorophyll concentrations, oxygen (O) supersaturation (arising from photosynthesis—see eqn. 5.19) and removal of other nutrients. During winter, silicate removal ceases when chlorophyll levels are low, allowing oxygen concentrations to fall to lower levels.

In Chesapeake Bay, a large estuary on the east coast of the USA, phytoplank-ton blooms in the high-salinity part of the estuary generate large amounts of organic matter which sink into the deep waters. The deep waters are isolated

(a)

(b)

70

25

•N

60

- V •

N Conservative

X

Conservative

50

N line

.1

line

o E

40

•s / N

ol

15

- •

s

S

E

30

_

s.

10

• \

u

20

_

•. X

O

•• . \

on

10

1 1 1 •

i i i •

0

10 20 30

0

10 20 30

Salinity

Salinity

Fig. 6.5 (a) Dissolved silicate and (b) dissolved inorganic phosphorus (DIP) plotted against salinity in the Great Ouse Estuary (eastern England), illustrating non-conservative removal.

Fig. 6.5 (a) Dissolved silicate and (b) dissolved inorganic phosphorus (DIP) plotted against salinity in the Great Ouse Estuary (eastern England), illustrating non-conservative removal.

from surface waters by thermal stratification and the breakdown of the phyto-plankton debris results in occasional, seasonal, dissolved oxygen depletion and consequent death of fish and invertebrates.

The processes that concentrate sediments in estuaries also concentrate particulate organic matter. If large amounts of organic matter are present in an estuary, oxygen consumption rates, resulting from aerobic bacterial consumption of organic matter, can exceed the rate at which oxygen is supplied. This results in decreasing dissolved oxygen concentrations.

The discharge of sewage from cities often causes low or zero dissolved oxygen concentrations. A particularly well-documented example is the River Thames in southern England. London is built around the Thames estuary and throughout the 18 th century, the wastes of the population were dumped in streets and local streams. During the early 19th century, public health improvements led to the development of sewers and the discharge of sewage directly into the Thames. The result was an improvement in local sanitation but massive pollution of the Thames. Although there was no systematic environmental monitoring at that time, historic evidence reveals the scale of the problem. Salmon and almost all other animals disappeared, the river was abandoned as a water supply and the literature of the time refers to the foul smells. Public concern prompted the development of sewage treatment works and also routine monitoring of environmental conditions in the estuary. The sewage treatment allowed dissolved oxygen concentrations to rise and fish returned to the estuary. However, as the population of London grew, the treatment system became overloaded and environmental conditions in the estuary deteriorated once more. The decrease of dissolved oxygen concentrations, and their subsequent increase, arising from improved sewage treatment in the 1950s, are illustrated in Fig. 6.6. The story of the Thames indicates the interaction between public health improvements and pollution; it also shows that some environmental problems are at least in part reversible, given political will and economic resources.

Most of the discussion above has centred on processes occurring in the water column. We should not forget, however, that estuarine sediments and their fringing marshes and wetlands also play a role in trapping sediment, storing organic matter and promoting microbiological reactions. These often nutrient-rich marsh and wetland environments allow large amounts of plant growth and resultant accumulation of organic matter along with nitrogen and phosphorus. The breakdown of this organic matter in water-logged, low-oxygen sediments in turn promotes denitrification which can convert nitrate into nitrogen gases:

5CH2O(S) + 4NO-(aq) ^ 2N2(g) + 4HCO-(aq) + CO2(g) + 3H2O(D eqn. 6.1

Over the last few hundred years there has been large-scale loss of wetland environments in rivers and estuaries throughout the world due to development pressures and flood control measures. The continued loss of wetlands removes valuable ecological habitats and also reduces the capacity for both carbon storage and nutrient removal by the processes described above. In the Humber estuary (UK), more than 90% of the intertidal marshes and supratidal wetlands have been lost to land reclamation over the last 300 years or so, resulting in a 99% reduc-

1900 1950

Year

Fig. 6.6 Average autumn dissolved oxygen concentrations in the tidal River Thames. After Wood (1982).

tion of carbon burial. The situation for nitrogen and phosphorus is complicated by the increase in fluxes from human activity that has paralleled the loss of wetlands. However, it is estimated that restoration of all the wetlands in the Humber system could remove 25% of the phosphorus and 60% of the nitrogen currently flowing through the estuary, and preventing it reaching the North Sea where it can cause eutrophication.

0 0

Post a comment