State of the Bay Report 2011-Final.pdf - Anchor Environmental

State of Saldanha Bay & Langebaan Lagoon 2011
Activities & discharges
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
O 2 /litre 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.
Some of the indirect consequences of an increase in phytoplankton biomass and high levels
of nutrient loading are a decrease in water transparency and an increase in epiphyte grown, both of
which have been shown to limit the habitat of benthic plants such as seagrasses (Orth and Moore
1983). Furthermore, there are several studies documenting the effects that shifts in natural marine
concentrations, and ratios of nitrates, phosphates and elements such ammonia and silica, have on
marine organisms (Herman et al 1996; van Katwijk et al 1997; Hodgkiss and Ho 1997; Howarth et al
2011). For instance, the depletion of dissolved Silica in coastal systems, as a result of nutrient
enrichment, water management and the building of dams, is believed to be linked to worldwide
increases in flagellate/dinoflagellate species which are associated with harmful algal blooms, and are
toxic to other biota (Hodgkiss and Ho 1997; Howarth et al 2011). The toxic effect that elevated
concentrations of ammonia have on plants has been documented for Zostera marina, and shows
that plants held for two weeks in concentrations as low as 125 µmol start to become necrotic and
die (van Katwijk et al 1997).
The effects of organic enrichment, on benthic macrofauna in Saldanha Bay, have been well
documented (Jackson and McGibbon 1991, Kruger 2002, Kruger et al. 2005, Stenton-Dozey 2001).
Tourism and mariculture are both important growth industries in and around Saldanha Bay, and
both are dependent on good water quality (Jackson and Gibbon 1991). The growth of attached
algae such as Ulva sp. and Enteromorpha sp. on beaches is a common sign of sewage pollution (Clark
1986). Nitrogen loading in Langebaan Lagoon associated with leakage of conservancy/septic tanks
and storm water runoff has resulted in localised blooms of Ulva sp. in the past. In the summer 1993-
94, a bloom of Ulva lactuca in Saldanha Bay was linked to discharge of nitrogen from pelagic fish
processing plants (Monteiro et al. 1997). Dense patches of Ulva sp. are also occasionally found in
the shallow embayment of Oudepos (CSIR 2002). Organic loading is a particular problem in Small
Bay due to reduced wave action and water movement in this part of the Bay caused by harbour
structures such as the Ore Terminal and the Causeway, as well as the multitude of organic pollution
sources within this area (e.g. fish factories, mariculture farms, sewage outfalls, sewage overflow
from pump stations, and storm water runoff). Langebaan Lagoon is also sheltered from wave action
but strong tidal action and the shallow nature of the lagoon make it less susceptible to the long term
deposition of pollutants and organic matter (Monteiro 1999 in CSIR 2002).
There is one waste water treatment works (WWTW) in Saldanha and one in Langebaan. The
WWTW in Saldanha disposes of treated effluent into the Bok River where it drains into Small Bay
adjacent to the Blouwaterbaai Resort. In addition to sewage waste, the WWTW in Saldanha also
receives and treats industrial waste water from a range of industries in Saldanha:
Sea Harvest
Hoedtjiesbaai Hotel
Protea Hotel
Southern Seas Fishing (not currently operation)
Bongolethu Fishing Enterprises
SA Lobster
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