Tracking water quality at Rotokawau / Virginia lake, Whanganui, New Zealand

Rotokawau / Virginia lake

This headline, taken from the Whanganui Chronicle a few years ago, should serve to remind us that we are now entering the algal bloom season in Rotokawau / Virginia lake, Whanganui.  Algal blooms are one of the major effects of a natural lake process called eutrophication and they appear to occur in Rotokawau / Virginia lake any time from spring through to late autumn.  
Eutrophication is caused by a build up of excessive amounts of nutrients in lake water and this leads to a proliferation of algal blooms that are sometimes toxic, but which eventually die and decay leaving a lake depleted in oxygen and unable to support much life.  Although eutrophication occurs naturally, it is exacerbated by human activity such as increased nutrient flows that result from surrounding fertiliser use and sewage discharge, and from  animal excrement and the removal of natural plant buffer zones around the lake edge.  Nitrogen (N), needed to build proteins and phosphorus (P) needed to build DNA and RNA, and the chemical that is used to transfer energy in living cells, are the two nutrients of most interest to lake scientists because their abundance is usually the limiting factor in algal and large plant growth in aquatic systems.

One of the things that struck this writer while researching background information for the post, is the multitude of changes that can be made to a eutrophic aquatic ecosystem for the purpose of ridding a lake of algae. However, reducing the effects of one factor is usually not done without creating at least one other problem.  Consider, for example, the public outcry that would follow if duck feeding was banned or worse still, if the birds were culled in order to reduce nutrient loading in the lake from bird waste.  The solutions, if there are any easy ones, are best left to the experts and policy-makers to make.

At the time of writing, the Whanganui City College monitoring programme has been operating for 17 weeks and has covered the period from early spring through to early summer in New Zealand.  The only other comprehensive water quality data that we could find on Rotokawau / Virginia lake was published in a 2012 report by NIWA (p131 Guidelines for Artificial Lakes) and this draws on data gathered by Wanganui District Council in 2007 and 2008.  We intend to use this for comparison purposes when we can.

Any organic matter that enters the lake contains nitrogen and this is oxidised by bacteria into the nitrate form and made available for uptake by plants.  For most of the period shown in our study, nitrate levels in the upper surface of the lake have been between 1.5 - 2.0 ppm (parts per million = mg/L =g/m3).  The nitrate level reported in the lake surface water in October 2007 was less than 0.1ppm, so it would seem there has been up to a twentyfold increase in nitrate concentration since then.  Ammonium ion levels have remained constant at a level 0.4-0.5ppm which is similar to the level measured in the  surface water in 2007.
Phosphate ion levels are typically much lower than nitrate levels in lake water.   The level we recorded initially at the beginning of the survey was very similar to the very low surface water levels (below 0.1ppm) reported by the Whanganui District Council for one October day in 2007.  However, what our results show is that the phosphate ion concentration in the surface water is not constant and seems to be following a cyclical process.  The change in phosphate concentration did not correlate with any rainfall data we used but it does seem to follow a cyclical change in the coliform microbe population.

There have been four or five peaks in the total coliform and e.coli populations (although it is harder to see the e.coli peaks in the graph here because of the scaling used) over the four month period and these populations appear to grow in size as the water temperature increases.  E.coli numbers peak at around one to two thousand units per 100mL of water in each cycle, although one measurement exceeded 10,000 units, and all of these values are above the recommended safe level for swimming.  Total coliform numbers range from a baseline of 2000 units/100mL to peak populations of up to 46,000 units/100mL in the middle of summer.
As the different coliform populations grow in size it is probable that they are subject to increased predation from microflagellates (protozoa) that graze on bacteria as a food source and from bacteriophages that reproduce in the host bacterial cell.  Hence the two coliform populations decrease in size before they recover and grow again on a three to five week cycle.

The increase in the phosphate ion (PO43-) water concentration, from a baseline level of 0.1ppm to a high of up to 0.75ppm, appears to coincide with periods when coliform bacteria numbers are at their lowest levels.  One possible explanation could be that coliform bacteria outcompete algae  cells and larger green plants for phosphate nutrient.  This nutrient is only available to algae and larger plants when the microbial competition is diminished and coliform bacteria numbers are relatively low.  In conditions more favourable to bacteria growth, such as higher water temperatures and higher availability of organic matter, it might be expected for more nutrients (including phosphate) to be mobilised as bacterial decomposition increases.  In the short term, however, a significant portion of the nutrients must remain unavailable for algae and larger plants and locked in the microbial and protozoan biomass.     

The amount of Dissolved Oxygen (DO) in the lake surface water is dependent on water temperature, plant photosynthesis and microbial respiration.  Since the monitoring programme began in spring 2018, DO levels have generally tracked downwards which repeats a trend found in 2007 but which means that increased water temperature and microbe activity are reducing the oxygen level more that oxygen produced during plant photosynthesis is increasing it.

One outcome that can be predicted from the decrease in oxygen levels is a decrease in water pH and this is supported by the graphics above and below.   If the water pH continues to fall as oxygen levels decrease, phosphate nutrients immobilised in lake sediments can be redissolved in inorganic form and increase the nutrients available for a potential algal bloom.  We will follow with interest and continue to record what happens in the next 17 week cycle of the Rotokawau / Virginia lake.