Intertidal invertebrate monitoring: From treatment plant to bird's plate (Baan)

By Elizabeth Morris

Sint red neck

Red-necked stint feeding in intertidal mudflats. Credit: Doug Blood.

Long term monitoring programs initiated to support conservation objectives can be difficult to implement successfully. One barrier to an effective monitoring program can be a lack of clearly defined ‘trigger points’: quantitative indicators  suggesting that further investigation or additional management actions  are needed to  reverse  the decline of target populations or communities.n effective monitoring program can be a lack of clearly defined ‘trigger points.

One barrier to an effective monitoring program can be a lack of clearly defined ‘trigger points'.

Treated sewage effluent is discharged from three outfalls on to, or just beyond, the intertidal mudflats of the adjacent Port Phillip Bay. This input has contributed to creating an abundant and diverse intertidal fauna that includes large numbers of local and migratory shorebirds using the intertidal mudflats adjacent to the WTP as feeding grounds. According to previous studies on shorebird use of the intertidal at the WTP, it is important to monitor habitat quality through sampling of prey abundance, in addition to monitoring the birds themselves.Melbourne Water are committed to an adaptive management approach to conserving habitat values in and around the WTP and, in 2011, initiated an intertidal monitoring program to:

  • Detect changes in community structure and/or populations of intertidal organisms that may reduce prey abundance, biomass and diversity for the overwintering bird populations; and
  • Determine if any changes that occur can be linked to effluent discharges from the Western Treatment Plant and, if so, recommend appropriate management action relating to the quality and quantity of effluent discharges.

In order to provide information on the  link between changes in invertebrate communities and  discharges that originate from the WTP, we have used a monitoring design that compares sites adjacent to effluent or river discharges to control sites.  CAPIM staff, in collaboration with Melbourne Water and GHD colleagues, have also set  trigger points  for changes in invertebrate numbers, biomass and diversity designed to activate a management response if required.

Trigger points for changes in invertebrate numbers, biomass and diversity were set to activate a management response if required.

During spring 2014 we presented data showing that there had been a reduction in the abundance and biomass of invertebrate prey items across the WTP intertidal region and, for the first time in the monitoring program, ‘trigger points’ were reached.  Further analysis indicated that the reduction in intertidal invertebrate abundance and biomass were mainly at river discharge sites and likely to be associated with low rainfall (used as a proxy for flow data) rather than treatment operations at the WTP.  Therefore, after consideration, no additional management actions were recommended.  Abundance and biomass measures had returned to more normal levels at the river discharge sites by February 2015 which was attributed to a larger than average rainfall in January 2015.

Using quantitative trigger levels enabled us to identify changes in infaunal invertebrates and link them to nutrient inputs.

We have demonstrated that the intertidal invertebrate monitoring program is able to identify changes in infaunal assemblages through the use of quantitative trigger levels and link these changes to nutrient inputs from discharges on to the intertidal adjacent to the WTP (in this case river discharges). As a consequence appropriate management recommendations can be made.

For further information please contact: Will Steele (Melbourne Water), Liz Morris (CAPIM) or David Petch (GHD)