An Introduction to Environmental Chemistry

The Oceans 187

6.2.4 Microbiological activity in estuaries

As in most environments, biological, particularly microbial, processes are impor-

tant 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 (SiO 2 ) 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 impor-

tantly, 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 chloro-

phyll 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

70

60

50

40

30

20

10

0102030

Silicate (

mmol l

–1

) Conservative

line

Salinity

0102030

Salinity

25

20

15

10

5

DIP (

mmol l

–1

)

Conservative

line

(a) (b)

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.