State of the Bay Report 2010-Final - Anchor Environmental

Anchor Environmental
include the benthic macrofauna communities, dissolved oxygen levels in the near bottom waters in
the immediate vicinity, trace metals and tainting substances in the RO plant effluent, toxicity of the
effluent, and temperature, salinity and suspended solids in the near-field. Monitoring activities
commenced during the second half of 2010 in order to establish a baseline prior to the RO plant
coming into operation.
4.3.6 Sewage and associated waste waters
Sewage is by far the most dominant (by volume) waste product discharged into rivers,
estuaries and coastal waters worldwide. However, sewage is not the only organic constituent of
waste water, received by sewage treatment plants, other degradable organic wastes, which can
result in nutrient loading, include:
Ulva spp that thrive in high
nutrient
conditions
Agricultural waste
Food processing wastes (e.g. from fish factories and
slaughter houses)
Brewing and distillery wastes
Paper pulp mill wastes
Chemical industry wastes
Oil spillages
Our present knowledge, of the impacts of waste waters on water systems, has until recently
largely been based on lake-river eutrophication studies. However, recent focus on how
anthropogenic nutrient enrichment is affecting near-shore coastal ecosystems is emerging (for a
review see Cloern 2001; Howarth et al 2011). In general, the primarily organic discharge in waste
water effluents contains high concentrations of nutrients such as nitrates and phosphates
(essentially the ingredients in fertilizers). Existing records provide compelling evidence of a rapid
increase in the availability of Nitrogen and Phosphorus to coastal ecosystems since the mid-1950’s
(Cloern 2001). These will stimulate the growth and primary production of fast-growing algae such as
phytoplankton and ephemeral macroalgae, at the expense of slower-growing vascular plants and
perennial macroalgae (seagrasses) which are better adapted to low-nutrient environments. This
process requires oxygen, and with high nutrient input oxygen concentrations in the water can
become reduced which would lead to deoxygenation or hypoxia in the receiving water (Cloern
2001). When the phytoplankton die and settle to the bottom, aerobic and anaerobic bacteria
continue the process of degradation. However, if the supply rate of organic material continues for
an extended period, sediments can become depleted of oxygen leaving only anaerobic bacteria to
process the organic matter, This then generates chemical by-products such as hydrogen sulphide
and methane which are toxic to most marine organisms (Clark, 1986). The sediments and the
benthic communities they support are thus amongst the most sensitive components of coastal
ecosystems to hypoxia and eutrophication (Cloern 2001). The ecological responses associated with
decreasing oxygen saturation in shallow coastal systems include the initial escape of sensitive
demersal fish, followed by mortality of bivalves and crustaceans, and finally mortality of molluscs,
with extreme loss of benthic diversity (Vaquer-Sunyer and Duarte 2008; Howarth et al 2011).
Vaquer-Sunyer and Duarte (2008) propose a precautionary limit for oxygen concentrations at 4.6 mg
O2/liter equivalent to the 90th percentile of mean lethal concentrations, to avoid catastrophic
mortality events, except for the most sensitive crab species, and effectively conserve marine
biodiversity.
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 67