NICA Waterwatch Summary March – April 2021

March and April have again experienced heavy rainfalls which have resulted in all parameters varying considerably, although consistent trends have followed throughout the river system.  Salinity and pH of the waters always decrease following rain after a drier period (February 101 mm, March 353 mm, April 314 mm), but steadily increase again as fresh water flows out.

Turbidity also increased following the March rain, but generally trended downward in April, suggesting that easily eroded or transported silt was removed in the first rain but had not re-formed before the second downpours.

We have been asked if any long term trends can be concluded from data which we have accumulated (going back to 2005), and some interesting conclusions have been produced in relation to Lake Cootharaba.  Below are shown percentiles (statistical “averages”) of data from 2005 to 2016, and 2017 to 2021 for NICA water testing sites.  There appears to be a step change in parameters between late 2016 and early 2017.  This was apparent at all sites in Lake Cootharaba, including downstream at Tronsons Canal.  There were no similar changes in the Upper Noosa River.  This step change has not been attributed to instrument or operator error since the data at Tronsons were measured quite separately from the Lake data with different instruments and operators.  In the recent period the Lake Cootharaba water became more alkaline and more saline, while the water has become clearer.  (To convert conductivity to salinity % multiply by 0.61).

Since 2016 20% less water has been flowing through the river system, so the most expected result would be that salinity must rise along with pH, and turbidity would drop with less erosion occurring.  The same trends have actually occurred in the river downstream of Lake Coroibah, but they are much reduced because of the influx and dilution by tidal water, so consequently become much less apparent.  Such changes are under consideration for their effect on river biodiversity and populations.

I must compliment our team member Pete Dorney who has carefully maintained rainfall records as well as NICA data at Boreen Point for many years – in contrast to BOM who closed the official weather recording at Boreen Point in 2012.  Thanks Pete.

While temperature rise is a popular subject, we cannot make comparisons of water temperature over the same time periods.  Healthy Land & Water were responsible for data collection prior to 2018, but HLW did not request data to be collected in December and January.   After 2018 NICA assumed responsibility for data collection, now carried out every month of the year.  As a result, mean annual temperatures since 2018 will appear warmer than in previous years.

We have continued collecting water samples for nutrient testing by Noosa Council.  Phosphates have remained low, within detectable limits.  In Weyba Creek and Noosa Sound nitrogen as both nitrates and ammonia rose in March, presumably due to the heavy rainfall of 355 mm, and only decreased a little in April after a further 321 mm of rain.

Thanks to the Waterwatch team members who have contributed to all the above work.

Bruce McConkey