Anthony Coote, local Geo-scientist reports:
Is our water quality testing of Lake Hawea proactive enough, and are any measured tolerances of deemed good quality appropriate, given climate change.
If you thought that Lake Hawea hasn’t looked right for about a month now, you are not alone and probably not mistaken: pale blue green in colour rather than the normal pale to dark blue. The water is not clear around the edges.
A panorama of the lake last night shows the colour change quite clearly. Of special significance is the drop off in clarity from the, milky look (unusual) in the immediate foreground, going out a few meters to where obscurity now reigns…
This is how it used to look regrading colour and clarity, for years gone by…
The change appears timed with the end of the really hot weather during mid to late January with resulting increased lake water temperatures.
The poor water clarity can’t be explained by suspended mud (clay and mica mineralogy) as the lake has been low all summer with little in-flow. A sudden increase in water inflow, resulting in minimal lake level rise, in relation to the down-graded cyclone Fehi rain event took place after the change in lake water appearance. Furthermore, the lake water quality remained high all last 2016-2017 summer when high lake levels were sustained by north-westerly rain in tributary catchments.
Working on the suspicion that an algal bloom is the cause of the lake colour change, water samples were analysed under a microscope. As a possible cause to the change in lake water quality, a range of micro-organisms were identified including algae, dynoflaggellates and possible cyanobacteria. Whilst the preliminary study is only semi-quantitative, alarmingly the most abundant identified micro-organism is the possible cyanobacteria, the “blooming” of which is mostly likely to cause the current lake water discolouration.
The questions are:
Is the change in water quality only temporary and that it will revert to normal conditions once water temperature stratification is lost resulting in termination of the algal bloom (assuming it is the cause)?
If the lake water clarity does return to normal, will the deterioration be repeated with increased hot weather spells in future summers?
Are our current tolerances of nitrogen levels within the lake too high given the probable increase in mean summer water temperatures resulting in risk to further and perhaps more serious algal blooms.
Anthony Coote MSc (1st Class Hons) MBA, member AIG & SEG