Early Insights into the hydrology of the Apsley Marshes (Tas)

Water level and salinity loggers installed at the Apsley Marshes Ramsar Site on Tasmania’s east coast, have now been out in the field collecting data for four months. Loggers were downloaded on 11-12th August and monitoring sites surveyed (measuring their precise location and elevation) to allow us to compare relative water levels across the site, over time.

Map of monitoring locations installed in the Apsley Marshes

A quick look at the data has provided us with some early insights into how both water levels and salinity vary across the site and in response to local rainfall, riverine inflows and tides.

Water level

  • There is a fall in water level above sea level of approximately 31cm from western drain site 1 to western drain site 5, with strong southerly flows to Moulting Lagoon witnessed in August 2022.
  • There is also a fall of 13 cm from the western wetland to the eastern wetland. Water depth in the eastern and western wetlands is the same (~30 cm) and in the middle wetland it was approximately half that at 16 cm. There was evidence of Melaleuca seedling establishment in this middle wetland too. Water depth was slightly lower in August than in April 2022.
  • Groundwater level has fluctuated over a range of 30cm since monitoring commenced in April 2022 and was slightly above ground level in August – indicating potential for seasonally sub-artesian conditions (i.e. when the standing water table or groundwater pressure rises above natural surface).
Melaleuca seedling establishment in the middle wetland. Water depth in this area is lower than that in the wetland cells to the east and west. This fits with our hypothesis that the main, parallel NW – SE convict drains were established in attempt to dry out the central Marshes.

Salinity

  • Salinity in the Apsley River ranges from fresh when there is high catchment inflow, to brackish (1.4 mS/cm) when there is not, which indicates significant groundwater dependence, consistent with historic data and anecdotal information (groundwater salinity has ranged from 1.4-1.7 mS/cm).
  • Western drain sites 5 and 4 display similar salinity levels to that of the Moulting Lagoon tidal logger site (up to 24 mS/cm). Western drain 3 indicates that tidal flows reach this site at least under very high tides, and particularly when river flow is low. Western drain sites 2 and 1 do not indicate any tidal influence but do experience salinities higher than the river inflows (up to 5 times). Two stone point salinity is fresher and consistent with the Apsley River salinity.
Western drain 3 salinity data, April – August 2022. Peaks in salinity are consistent with very high tides experienced in June and July 2002 (note 5-6 day time lag between high tides experienced at Spring Bay and those recorded in The Marshes).
  • The western wetland is slightly more saline than the middle and eastern wetlands (up to 3 mS/cm). Salinity at this location appears to accumulate over time indicating more direct links with tidal inflows, whereas the middle and eastern wetland salinities seem to increase and decrease in response to fresh inflows.
Western wetland salinity, April – August 2022. This site indicates accumulation of salt over time. This is consistent with anecdotal reports.

While we were out there, we also made the following observations:

  • Australasian bitterns were heard calling across the Apsley Marshes again in August 2022, as they were in October 2021.
  • A remnant black swan nest was uncovered deep within the rushes in the western wetland.
  • A wedge-tailed eagle was observed soaring over the western side of the Marshes.
Bec and Dave, our surveyor, wrapping up another adventure on the Apsley Marshes.

In summary, our trip to the Apsley Marshes was a success!

All of our loggers appear to be effectively recording data and are helping us to build our understanding – contributing to the story of the ecohydrology of this important Ramsar site. We look forward to continuing to analyse the data collected from this site and sharing our insights over the coming year.

This important work is supported by NRM South, with funding provided via the Australian Government.

Bec Sheldon


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