Monitoring a eutrophic lake at Rotokawau - Virginia lake, Whanganui, New Zealand
Summer time Rotokawau-Virginia lake, viewed from the northern edge. The southern boardwalk where water sampling took place is to the top right of the lake.
Whanganui City College Science department completed a year long campaign to monitor the chemical and microbial content of the surface waters at Rotokawau-Virginia lake in September 2019. Getting out of school during the day is always difficult for a teacher and even more so accompanied by a student or two but amazingly Rotokawau-Virginia lake was visited every week, bar one, for an entire year! The purpose of this project was to try and better understand the processes that occur in a eutrophic lake. The Science programme at Whanganui City College is place-based and this means students learn about science in their own back yard. No one has spent this long studying our iconic lake so feel free to peruse the data and our observations, and understand that every one of us is capable of making science.
I guess if you go to a place often enough an attachment is bound to follow. Walking along the southern boardwalk, past the walkers and the odd jogger, to the same place week in and week out, you can find yourself making predictions about the state of the lake just from a few casual observations of the clarity of the water or the condition of the weather. You begin to hope that the lake is in good health for the animals that live there, both seen and unseen.
Quite what a healthy lake should look like even from a chemistry standpoint can be hard to define. The data we collected hinted at a myriad of processes that must be occurring in this nutrient enriched, eutrophic lake. Rotokawau- Virginia lake is a hugely dynamic environment even across the small number of variables that we measured. Nothing really stayed the same over the whole year.
What science we may have discovered will be used in the classroom and, I hope, it will add to what Whanganui already knows about Rotokawau-Virginia lake. If nothing else, this study has confirmed events that were seen in the lake in the past and reported by other authors. Some of the other interpretations of the data that are presented in this blog post are made with the best of intentions and may be not have been heard of before in the context of Rotokawau-Virginia lake. There is scientific evidence out there on the internet to support these assertions, nevertheless, they still may not be correct.
Later in October we will be returning to Rotokawau-Virginia lake with a group of Year 9 students to work with the national Lakes380 team who are reporting on the state of 10 percent of the nation's lakes. It will be exciting for the students to see how science is made and to record the events for our cross-curricular, lake environment day.
The data
A selection of data from Rotokawau-Virginia lake Sept. 2018 - Sept. 2019.
Anyone interested in the complete data set should contact the blog author.
The observations
Algae appear in the springtime
Late winter and spring appears to be the time algal blooms form at Rotokawau -Virginia lake. Although we did not have the means by which to quantify the amount of algae in the water, there was plenty of other information available to support this assertion. For example:
- Algae can be seen in the water column and on the lake surface during this time.
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| Surface algal bloom winter / spring 2019 Southern end Rotokawau-Virginia lake |
- The algae examined from water samples in spring 2019 revealed that we were possibly experiencing a bloom of cyanobacteria or blue-green algae in Rotokawau-Virginia lake. Cyanobacteria are related to bacteria, but are capable of photosynthesis and so can produce their own food from carbon dioxide and water using sunlight as an energy source. The Anabaena cyanobacteria identified can also fix dissolved nitrogen gas into the nitrate form which enables this algae to outcompete other species of algae if nitrate nutrient is in short supply. Cyanobacteria are one of the earliest life forms on this planet and they helped raise the levels of oxygen in the atmosphere of the early Earth. They also release toxins when they die that affect the nervous system and livers of vertebrate animals, including water fowl and fish.
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| Algae identified by a scientist from the Lakes380 team as possibly Dolichospermum (Anabaena) lemmermani (photos H Doney Whanganui City College) |
- Dissolved oxygen levels are at their highest level at this time of the year and supersaturation (the amount of oxygen in the water over and above that which can dissolve into the water directly from the atmosphere) is also at its highest level. The extra oxygen in the water is due to algal photosynthesis in the absence of weed beds in the sampling area. It is worth noting that the amount of supersaturation is lower than in data obtained by Whanganui District Council in 2007 which might be expected given that the algal bloom experienced in 2019 is not particularly extensive compared to some that have been seen at Rotokawau-Virginia lake.
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Rotokawau / Virginia lake in May 2018
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- Daphnia (water fleas) were clearly observed in water samples taken during springtime in both years. We did not start counting these freshwater invertebrates until the last eight weeks of the survey. They were not seen with the naked eye in water samples taken during the warmer months but a complete survey will be needed to confirm this. Our graphic shows that daphnia numbers rise and fall in these weeks, probably because of predation. Daphnia and Copepods are part of the lake zooplankton population and filter feed on algae, protozoa and bacteria. An increase in daphnia numbers must be linked to an abundance of food resources and this also coincides with the assumed increase in algae numbers and growing populations of coliform bacteria.
Anatomy of an algae-grazing daphnia (H.Doney)
Interestingly, we also captured images of daphnia and copepods in various stages of reproduction.
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| Copepod with eggs |
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| Daphnia with hatched eggs |
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| Daphnia with eggs |
Bacteria levels increase as the water temperature rises during the summer
Coliform bacteria are common in the soil and are also found in the intestines of animals. E.coli on the other hand are only found in the intestines of animals so they are used as the prime indicator of faecal contamination of freshwater. E.coli at water quality standard levels of 540/100mL of water present a risk of illness of 5%. Using this criteria, the lake would not have been judged swimmable from November through to May, that is, from late spring to late autumn. At Rotokawau-Virginia lake the source of faecal contamination is most likely the water fowl that reside there especially during the summer months when visitor-supplied food sources increase.
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| Rotokawau-Virginia lake coliform population changes 2018/19 |
The total coliform microbe population follows a three to four week cycle as the population climbs and then declines due to growth and predation. The population of coliform microbes in the surface water peaks during the warmer summer months. However, it will not have escaped notice that two very significant and large peaks were reported in mid May and late June of this year. The May event may be linked to bird fouling as E.coli numbers also spiked slightly at this time. On the other hand it was reported at the time that the lake was experiencing gale force winds and waves were forming on the lake surface. It is possible that, with water temperatures approaching their minimum, the turbulence could have caused some churning of the bottom sediments in this relatively shallow part of the lake. The June (winter) event will be considered separately, when lake turnover or 'flipping' is discussed.
Phosphate nutrient cycling may indicate bacteria-algae competition
Water nitrate levels were reasonably constant at 1.5ppm for most of the spring and summer period that we were able to take nitrate measurements. This figure is at least ten times higher than the nitrate concentration reported by NIWA in October 2007 (Report HAM2011-045). All living things require a source of nitrogen for building amino acid molecules, proteins and enzymes. As already mentioned, some species of cyanobacteria can fix nitrogen and form nitrates, so lower lake nitrate levels may not necessarily limit toxic algae growth, even if they do occur.
Phosphate levels are much lower than nitrate levels (typically 0.05 - 0.1ppm) and the figures obtained are comparable to NIWA data for the same period, eleven years earlier. Nevertheless, surface water phosphate levels fluctuate during the year. The highest phosphate readings obtained were recorded from late spring to early summer in the range 0.5 - 0.75ppm. The relatively high peak values of phosphate concentration coincided with the cyclical decrease in coliform bacteria numbers during this period. Thereafter the level of phosphate lowers to around the 0-0.15ppm level for about six months, that is, from January through to June and the start of winter.
Phosphate levels are much lower than nitrate levels (typically 0.05 - 0.1ppm) and the figures obtained are comparable to NIWA data for the same period, eleven years earlier. Nevertheless, surface water phosphate levels fluctuate during the year. The highest phosphate readings obtained were recorded from late spring to early summer in the range 0.5 - 0.75ppm. The relatively high peak values of phosphate concentration coincided with the cyclical decrease in coliform bacteria numbers during this period. Thereafter the level of phosphate lowers to around the 0-0.15ppm level for about six months, that is, from January through to June and the start of winter.
As stated, nutrient phosphate levels in the surface lake water become depleted during the summer months. Phosphate levels only seem to rise when coliform bacteria numbers are low during the winter period or, after coliform numbers decrease in the spring population cycles. As algal blooms were not observed during the summer, it is suggested that coliform bacteria outcompete algae for nutrient phosphate. Algae growth is only favoured when microbial phosphate demand is at its lowest, that is during the winter-early spring period. In winter from July onwards, water phosphate levels increase three-fold to around 0.2-0.3ppm. This is the phosphate that is not utilised by living organisms during the winter and the phosphate that probably fuels the algal growth later in spring.
Lake 'flipping' and turnover of nutrients during the winter months powers the algal bloom.
Nutrients arise from the microbial decomposition of biological carbon that enters the lake and from groundwater seepage from the surrounding area. A bore water sample from the Whanganui City College field gives a groundwater nitrate level of 2.0 ppm and a phosphate level of 0.5-0.75 ppm. It is not known if this is a typical of the groundwater entering Rotokawau-Virginia lake 2km away, but phosphate can be 'locked-up' by iron-rich sand in lake sediments thereby preventing its uptake by living organisms. This process of phosphate sequestration is however, pH sensitive and reverses when the water acidifies, which it does when water oxygen levels fall due to microbial activity in the lake bed.
The existence of a thermocline at a reported depth of 6m in Rotokawau-Virginia lake means that for much of the year, the colder, oxygen depleted, phosphate-rich lower lake water lies undisturbed. However, as the surface water cools to around 10.5⁰C during the winter, the now colder and denser surface water sinks through the thermocline and displaces the nutrient rich water from the lake bottom, bringing it to the surface. There is evidence that lake turnover or 'flipping' occurred at Rotokawau-Virginia lake around the end of June / first week of July which is very close to the date reported by Whanganui District Council in 2007.
Our results show phosphate increased from 0.1 to 0.2-0.3 ppm which at least matched the 2007 NIWA data for the water phosphate concentration at the maximum lake depth of 15-20m. Furthermore we recorded the lowest dissolved oxygen level measured in the lake surface water (3.8 mg/L) for the whole year and an 'unseasonal' spike in coliform numbers, higher than any other measured over the year. We can be reasonably certain that we had sampled water from the microbe-rich, phosphate-rich and oxygen depleted lower depths. The phosphate made available as a result of lake mixing is considered to power the ensuing algal and microbial growth in the spring.
We now move on to the Lakes380 day.
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