COASTALRESEARCH.CSIRO.AU

Estuarine habitat mapping and geomorphic characterisation of the lower Hawkesbury River and Pittwater estuaries

Details
Aims: 
  1. Data consolidation of estuarine habitats (spatial data)
  2. Identification of the values of estuarine habitats & biodiversity.
  3. Risk assessment of key threats to estuarine habitats & biodiversity.
  4. Prioritisation of on-ground works required to protect estuarine habitats & biodiversity.
  5. Management initiatives and recommendations.
  6. Integration into planning instruments, documents and strategies.
Time Scale: 

The project was undertaken in 2 years during which ground proof of estuarine habitats along 570 km of shore line was undertaken in the Lower Hawkesbury Estuary (LHE).

Outputs: 

This study, for the first time, has assessed the risk to estuarine habitats from a range of human activities occurring in the Lower Hawkesbury Estuary (LHE). Previously, ecological risk assessments have only been done for commercial fishing (e.g., NSW Fisheries 2001) or for specific developments (e.g., marina construction). Undertaking an extensive ecological risk assessment for a large range of human activities has brought into focus where there is potential for interactions between these activities and estuarine habitats which may have been previously overlooked.

The intensity of recreational fishing activity along the shoreline and from boats in shallow water in some subcatchments has shown that the potential for this activity to interact with estuarine habitats such as mangroves, seagrass and mudflats is substantial.

A unique feature of this ecological risk assessment was the use of maps of human disturbances within the LHE, such as the location of seawalls, foreshore parks and unsewered housing. These maps enabled the development of measures of the stress from these activities on the estuarine habitats

The ecological risk assessment brought to light:

  1. the substantial knowledge gaps in our understanding of the interactions between human activities and estuarine habitats in the LHE. There was a lack of knowledge in the direct measures of some of the stressors of human activities and in the extent and magnitude of a number of human activities at the sub-catchment or reach scales.
  2.  the lack of consistency in reporting of data for some stressors to local management agencies 

There are many habitats within the LHE that are in relatively good condition and currently at low risk to human activities based on current information. For an estuary in such close proximity to a high population urban area as Sydney this is rare. But this risk assessment should alert all those responsible for this estuarine ecosystem to be vigilant in continuing to keep these habitats in good condition. Diligent attention, via adequate resourcing and holistic management approaches, to the issues identified will substantially aid this task

Executive Summary: 

Hawkesbury Nepean Catchment Management Authority (HNCMA) and the Hornsby Shire Council (HSC) have recently set in place management plans for the lower Hawkesbury estuary (LHE) (Hawkesbury-Nepean Catchment Management Authority, 2008, Haines et al., 2008). To implement these plans it is required to better understand the distribution of estuarine habitats and the potential threats to these habitats from human activity within the LHE. Estuarine habitat mapping and geomorphic characterisation of the lower Hawkesbury River and Pittwater estuaries project was designed to provide some of this information. It consisted of mapping the estuarine habitats and undertaking an ecological risk assessment of human activities on those habitats.

Eight different estuarine habitats were mapped throughout the LHE; seagrass, mangroves, saltmarsh, mudflats, sandflats, rocky reef, foreshore habitat and water column. The macrophyte habitats (seagrass, saltmarsh and mangroves) were mapped comprehensively for the whole of the LHE using a combination of aerial photos and ground surveys. Mudflats and sandflats were mapped from aerial photos but there was not sufficient time to include ground surveys. Rocky reef was mapped using side scan sonar images and aerial photos. Due to the long total shore line length of approximately 570 km and the time consuming mapping process, subtidal reef was not completely mapped in the LHE Foreshore Habitat was mapped for a total of 566 km. This habitat was mapped by a combination of aerial photo interpretation and extensive field validation. Water column was mapped as the total water area within the estuary. This layer was also separated into two main classes of depth less than 5 m and depth greater than 5 m. Areas with a depth of greater than 5 m were considered to represent deep subtidal habitat.

Pittwater had the largest area of seagrass beds including extensive beds of the vulnerable Posidonia australis. Mangrove Creek had the largest area of mangrove forest and saltmarsh community. The fluvial delta and riverine channel also had large areas of mangroves. Mooney Mooney Creek had the most extensive areas of mudflats and sandflats were most extensive in Pittwater and the marine reach. Rocky reef that was mapped had the largest areas in the fluvial delta, but due to sampling limitations it may be under-estimated in other areas. The dominant foreshore habitat was Natural Soft and was mostly found in the upper reaches of the estuary including Mangrove Creek, riverine channel Berowra Creek, Fluvial Delta and Mooney Mooney Creek. Natural Horizontal Hard foreshore habitat was predominantly found in the lower portion of the LHE including Cowan Creek and Berowra Creek.

A qualitative ecological risk assessment method previously developed by Industry and Investment NSW (I&I NSW) was used to evaluate the risks from human activities on the estuarine habitats in the LHE. The purpose of the risk assessment was to determine which habitats were at intolerable levels of risk from which human activities and then to identify the issues that need to be addressed if these risks are to be reduced. The LHE was divided into sub-catchments and reaches and the risk assessment was done on each of these separately. This enabled specific issues to be identified for each area. The sub-catchments were Pittwater, Cowan, Berowra, Mangrove, Mooney, Mullet and Patonga. The reaches of the Hawkesbury were the marine delta, fluvial tidal delta and riverine channel. Seven human activities were assessed – recreational fishing, aquatic recreation, foreshore development, stormwater/catchment run-off, sewage, dredging and sedimentation and commercial vessels. Risk assessments of commercial fishing in the LHE had been done separately by I&I NSW under the environmental impact assessment process for commercial fisheries in NSW (NSW Fisheries, 2001, 2002). The results of the EIS for these fisheries were incorporated into this project.

The risk assessment identified substantial knowledge gaps with regard to the magnitude, frequency and duration of various components of many of the human activities. These knowledge gaps themselves contribute to the risk to habitats because the level of stress from these human activities on habitats could be large but there is insufficient information to appropriately manage them. In particular, knowledge gaps were found in the amount of recreational boating (non-fishing) throughout the LHE, nutrient loads at the sub-catchment and reach scales from stormwater, upper catchment run-off and non-point source pollutants, the proportion of contaminated sediments and subtidal erosion and accretion of sediments around habitat edges.

Habitats that had consistently acceptable levels of risk were saltmarsh, wherever it was present, seagrass in Cowan, Berowra, Mooney and Mullet and mangroves in Cowan and Berowra. However, saltmarsh was identified as a priority habitat in all sub-catchments because it has declined over the last ten years. Similarly, seagrass in Cowan and Berowra was also a priority habitat because of its decline. Seagrass in Cowan includes small patches of the vulnerable species Posidonia australis. Management of these two priority habitats should consider whether steps could be taken to enhance their distribution and ensure human activities do not increase in the area where these habitats occur.

Recreational fishing and foreshore development posed an intolerable level of risk to habitats such as seagrasses and mudflats throughout the different sub-catchments and reaches. The most important issue arising from these human activities overall is to examine the extent of the interactions (intensity and location) between these human activities and the habitats (e.g., how many and frequently do recreational fishers fish in these habitats). Where the interactions are most intense, investigation of the condition of those habitats is needed to determine the most effective and efficient use of resources for their management.

The top three recommendations arising from this project were:

  1. That a spatial map of the human activities assessed be constructed and incorporated into the habitat maps for each sub-catchment and reach. This would enable the location of habitats to be overlaid with the location of their potential threats.
  2. That the condition of habitats that had intolerable levels of risk should be quantified and analysed for any signs of degradation. Quantifying both the condition of a habitat and the stressors potentially affecting it is needed to gain a more accurate assessment of the extent of habitat degradation and its possible causes.
  3. That priority habitats with acceptable levels of risk (e.g., saltmarsh) be appropriately managed to ensure stressors from human activities do not increase within the areas they occur.

A possible follow-on from this study could be detailed monitoring of habitats close to human activities that posed an intolerable level of risk and, where these activities are not occurring, to monitor rates of change in these contrasting environments. Helicopter imagery for a similar purpose is currently being developed by I&I NSW to map and monitor locations over time to determine impacts on coastal marine habitats. 

Affiliations
Researcher(s): 
K. Astles
Researcher(s): 
G. West
Researcher(s): 
R.G. Creese
Contact Address: 
Email contact form
Funding or Commissioning Agencies: 
Hornsby Shire Council
Funding or Commissioning Agencies: 
DECC Department of Environment and Climate Change, NSW
Funding or Commissioning Agencies: 
Natural Heritage Trust
Funding or Commissioning Agencies: 
NSW Department of Primary Industries

Locations

brooklyn, nsw 33° 33' 46.4112" S, 151° 14' 36.1392" E
33° 36' 36.846" S, 151° 18' 38.6964" E
wiseman ferry, nsw 33° 23' 9.1428" S, 150° 59' 34.8144" E
Triangle Island, Hawkesbury River, NSW 33° 27' 49.2084" S, 151° 9' 2.1204" E