Biodiversity Strategy - Gosford City Council - NSW Government

• Structural flood mitigation works - When a barrier, such as a weir or floodgate, is
built across a tidal waterway, the upstream habitat changes from brackish to
freshwater. This means that estuarine fish habitats such as mangroves and
seagrasses above the barrier are lost.
• Pollution - Discharge of nutrients into the water from sewage outfalls, factories,
abattoirs, or from general agricultural run-off, may cause algal blooms and excess
epiphyte growth on seagrasses, both of which restrict the light to the plants.
• Poor land management - The clearing of land vegetation in steep areas, particularly
of trees along river banks, often leads to erosion and the washing of sediment into
waterways. Rapid siltation smothers and destroys seagrass beds, and causes
excess turbidity.
• Recreation - People boating around seagrass meadows should be careful not to
damage them. Swing moorings with their chains dragging across the bottom, boat
anchors and propellers, and bait digging all damage seagrass beds.
Many of these activities impact on saltmarshes in similar ways. Urban runoff, sewage overflows
and septic seepage threaten seagrass habitat by the continuous addition of suspended
sediments and nutrients to estuaries. These inputs can cause higher than natural turbidity and
lower light penetration, both directly and through excessive phytoplankton growth. Such turbidity
is most evident where flushing is poor and inputs are great. Increased turbidity reduces the light
available to seagrass, with the deeper parts of a bed being particularly vulnerable to consequent
damage. Whilst extra nutrients can actually enhance seagrass growth, very high levels are likely
to cause heavy epiphytic growth that can smother and shade seagrass, and eventually lead to
its decline. Nutrient enrichment may also promote the competitive replacement of seagrass by
Caulerpa spp. (green macroalgae morphologically similar to seagrass) or other algae. The
coastal lagoons of Gosford, particularly Avoca and Cockrone Lagoons, experience an annual
spring outbreak of filamentous algae that form large mats, anchored to the shallow substratum.
The algal mats are made up of a few species of filamentous algae, mostly Enteromorpha
intestinalis and Chaetomorpha linum.
Sedimentation gradually makes areas too shallow for seagrass, particularly in bays receiving
urban runoff. Heavy loads of fine sediment washed down after heavy rain can coat seagrass
leaves, reducing photosynthetic efficiency and therefore vigor (Freewater 2004). This can be a
problem in sheltered bays where waves and currents are slight. Sedimentation also alters the
nature (particularly with respect to parent material and grain size composition) of substrates
supporting seagrass; this can cause changes both in the seagrass itself, and in the invertebrate
community associated with the substrate.
The marine vegetation of Gosford has been mapped but aside from work undertaken for the
Brisbane Water Estuary Processes Study, the accuracy of map features has not been tested.
Council does not monitor riparian or marine habitats so little information is available on long
term trends. Council is supporting a Community Environment Network (CEN) initiative to
encourage community monitoring of seagrasses on the Central Coast. The program would be
similar to Waterwatch/Streamwatch, which monitors waterways for a selection of water quality
parameters.
Council's water quality monitoring program involves monthly sampling at a variety of locations in
Brisbane Water and the Coastal Lagoons. The data collected are used in Council's
Sustainability Report. The data provides a snap-shot of selected water quality parameters for a
discrete volume of surface water at a discrete moment in time. However, the data provides little
information on water quality trends or the impact that water quality is having on the biota of the
waterways. A more integrated program would include sediment quality, which provides a timeintegrated
measure of water quality, and a measure of some biological endpoint, such as
Biodiversity - Technical Report Page 164