Biodiversity Strategy - Gosford City Council - NSW Government

A - 7.10 Degradation of Aquatic and Riparian Habitat and Water Quality
Water quality may be impacted by a number of human activities including the following:
• increased sedimentation due to erosion as a result of vegetation clearance,
• runoff of nutrients from adjoining areas (dog faeces, road surfaces, fertilisers used
for lawns and in landscaped areas), and
• pollutants such as heavy metals, pesticides, organochlorides, paints, solvents and
fertilizers entering the stormwater system.
Metals and organic chemicals behave in similar ways in estuarine environments and although
precise chemical pathways may differ, physical and biological pathways are essentially the
same. Organic chemicals also undergo a variety of physical and chemical processes, as well as
processing by the biological community. These processes include hydraulic transport,
partitioning between dissolved and particulate matter, bioaccumulation, photo-degradation,
biodegradation, sedimentation and recycling. For example, chemicals and metals present in the
dissolved phase in the water column are subject to transport based on the hydraulic movement
of the water itself. Therefore, their resident time in the dissolved phase is equal to the residence
time of the water body. This suggests that dissolved pollutants entering areas of an estuary
where tidal (and other) currents are weak are likely to persist in those areas for longer than they
might in areas where currents are stronger. This in an important point as it has significant
implications for potential impact on spatially discrete populations of biota and the
appropriateness of scientific methods used to assess such impact.
Whilst seagrasses are among the most productive and biodiverse habitats, they are also among
the most fragile. Although the leaves of the seagrass grow fast, the rhizome grows relatively
slowly. As a result, once seagrass meadows are damaged, their recolonisation is very slow.
Many major estuaries in NSW have lost as much as two-thirds of their seagrass beds in the past
30 to 40 years. There are numerous activities that physically damage or impact on seagrasses
as described below.
• Changed hydrology - Seagrasses generally occur where there are slow currents.
Where the speed of flow has increased due to removal of sand, rock or other
seagrass beds, seagrasses often cannot re-establish from seed.
• Sedimentation - A large amount of sediment in the water column can cause excess
turbidity (muddiness) in estuarine waters; this blocks out the sunlight needed for
growth of seagrasses.
• Eutrophication - Excessive nutrient levels in the water can cause high algal growth.
Blooms of algae or excessive growth of algal epiphytes can restrict light to
seagrasses.
• Reclamation - Shallow estuarine areas occupied by seagrasses, mangroves, and
saltmarshes, are often seen as ideal sites for waterfront developments. Their
reclamation, by the dumping of fill, totally destroys valuable fish habitat.
• Dredging - Dredging has a number of adverse (degrading) effects on estuaries. The
removal of underlying sediments may destroy seagrass beds. Dredging often
increases the depth, so that there is not enough light for seagrasses to grow. These
deep holes may also become stagnant and without enough oxygen in the substrate
(bottom) for plants and animals to survive. Dredging can release large amounts of
sediment into the water, which may smother seagrass beds or other aquatic life.
Also, the resulting high turbidity inhibits the growth of seagrasses.
Biodiversity - Technical Report Page 163