A significant area of seagrass lies adjacent to the Towra wetland. The mangroves and seagrass provide protection and food for juvenile fish species, and the release of crab larvae from saltmarsh areas during spring ebb tides provides a reliable source of food for a variety of fish species and a critical link in the estuary’s food web.
Dredging in Botany Bay in the 1960s and 1970s caused ongoing detrimental effects to Towra Point by altering wave patterns. This increased the erosion rate at Towra Beach and changed sedimentation patterns. Future developments on Kurnell Peninsula have proposed dredging in Botany Bay which will again alter the bay’s hydrology. There are two projects that involved dredging of Botany Bay recently; the first is the desalinated water distribution pipeline for Sydney Water and the second is two underground electricity cables for Energy Australia. Dredging not only affects wave patterns, it changes the way in which water moves throughout the system, therefore changing the physicochemical processes and sedimentation patterns.
The seagrass meadows in Botany Bay are relatively large for the Sydney area and are sensitive to turbidity. Turbidity is particularly relevant to Towra Point and surrounding seagrass meadows as it determines the conditions for seagrass growth and is a major factor in aquatic biodiversity. Increased turbidity reduces light available to seagrasses on the seabed or river bed. Seagrass form a critical component of the Towra Point ecosystem as they protect the shoreline from erosion, provide shelter for juvenile fish and crustaceans and assist in nutrient and energy transfer. Seagrasses stabilise the seabed and reduce the effects of waves, and a reduction in seagrass area could have a deleterious effect as the effects of waves on the area increase (Larkum 1976). Therefore, dredging may cause loss or change of habitats and may lead to loss of species (MS 2007).
Towra Point supports a large number of interactions between organisms and the transfer of nutrients and energy, and the biodiversity of the reserve is a consequence of this. Seagrass meadows adjacent to Towra Point Nature Reserve and mangrove and saltmarsh communities are critical links in the food chain due to the large amounts of organic matter, or detritus, they produce (Connolly et al. 2005a, 2005b), which is a reliable food source for invertebrates. The hydrological cycle in Botany Bay is dynamic and continues to change with anthropogenic influence. Most of the plants and animals of Towra Point have adapted to the current hydrological regime. However, some species such as the green and golden bell frog have been lost in certain areas due to hydrological changes.
Maintenance and monitoring of a hydrological regime within natural variation is critical in maintaining the biodiversity and sustainability of the wetland. Anthropogenic alterations to Botany Bay and Kurnell Peninsula have changed how water moves throughout the system which has resulted in alteration of the ecological character of the Ramsar site since it was listed in 1984. Pollutants and nutrients suspended in the water affect the growth of seagrass by inhibiting light for growth. These seagrass meadows are largely outside the Ramsar site boundaries (except for in Weeney Bay). However interaction between the seagrass beds and mangrove/saltmarsh communities within the Ramsar site boundaries appears to contribute to the site’s value for fisheries. Once found along the northern and southern shores of Botany Bay, seagrass areas have varied due to natural and anthropogenic activity. The species of Towra Point Nature Reserve Ramsar site are Posidonia australis, Zostera capricorni and Halophila ovalis.
Within Botany Bay, seagrass is found to colonise seabeds to three metres in depth, which is shallow in comparison to seagrass beds in similar environments, such as Pittwater (seven metres), Port Hacking (eight metres) and Jervis Bay (nine metres). Pollution may be the limiting factor in seagrass depth in Botany Bay as the suspended sediments increase turbidity in the water. The presence of seagrass meadows adjacent to the mangrove and saltmarsh communities at Towra Point is a critical link in the food chain and helps to maintain the biodiversity of the wetland. In addition, they stabilise the seabed, act as a buffer against wave energy and improve water quality through nutrient uptake (Larkum 1976; SPCC 1978a; West 1983). Smaller areas of seagrass meadows are less able to withstand wave and tidal energy. Therefore loss of sections of seagrass may have a negative effect on the whole community (Larkum 1976).
Seagrass meadows can change in response to chemical (e.g. salinity and pH and changes in water movement due to coastal developments (including ports and marinas). Seagrass meadows may also contract when mangrove areas expand in response to increased suspended sediment loads. Extraction, filling, dredging and sand mining activities also cause direct damage to seagrass areas. Four species of seagrass are to be found on the seabed, the two most important being strapweed (Posidonia Australia) and eelgrass (Zostera capricom). The entire seabed in the aquatic reserve was carpeted with sea grasses, but more than one-third has been lost due to erosion.
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Butler, A.J. 1999. Seagrass in Australia: Strategic Review and Development of an R&D Plan, FRDC Project 98/223
Connolly, R.M., Hindell, J.S. and Gorman, D. (2005b), Seagrass and epiphytic algaesupport the nutrition of a fisheries species, Sillago schomburgkii, in adjacentintertidal habitats, Marine Ecology Progress Series 286: 69–79
DECCW NSW GOV ‘Towra Point Nature Reserve Ramsar Site: Ecological character description’, Chap 6, pg106 http://www.environment.nsw.gov.au/wetlands/10510towrapointecd.htm
Larkum, A.W.D. (1976), Ecology of Botany Bay. I. Growth of Posidonia australis(Brown) Hook. f. in Botany Bay and other bays of the Sydney basin, Marine andFreshwater Research 27(1): 117–127
Loneragan, N.R., Kenyon, R.A., Haywood, M.D.E., and Staples, D.J. 1994. Population dynamics of juvenile tiger prawns (Penaeus esculentus and P. semisulcatus) in seagrass habitats of the western Gulf of Carpentaria, Australia. Marine Biology 119, 133-143}.
MS (2007), Botany Bay Cable Project: environmental assessment, Molino StewartPty Ltd, report prepared for Energy Australia
OzCoasts Coastal Indicators; Changes in seagrass areashttp://www.ozcoasts.org.au/indicators/changes_seagrass_area.jsp
SPCC (1978a), Environmental control study of Botany Bay: Seagrasses of Botany
Bay, BBS 3, State Pollution Control Commission, Sydney
The Wildlife Preservation Society of Australia 2010, ‘The Saving of Towra Point’ http://www.wpsa.org.au/pdf/towra/Towra_Point_the_WPSA.pdf
West, R.J. (1983) The seagrasses of New South Wales estuaries and embayments,Wetlands (Australia) 3(1): 34–44
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