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 par-
ticulate 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 18th 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 dis-
appeared, 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 treat-
ment system became overloaded and environmental conditions in the estuary dete-
riorated 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 fring-
ing 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 resul-
tant 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:eqn. 6.1Over the last few hundred years there has been large-scale loss of wetland envi-
ronments 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-54 24CH O 232322 ()s+Æ+ ++NO-()aq N()g HCO-( )aq CO()g 3 H O() 1188 Chapter Six