Terra Nova – a shallow Dutch peat lake with an (environmental) history

– By Melanie Münch –

As part of the lake-crew within the P-Trap project, one of my jobs is to figure out the long-term effects of iron treatment on the internal phosphorus loading in lakes. This is important because iron has been added to lakes by water managers for a while already as it usually yields good results and has little effect on the lakes’ ecosystem, at least on the short term. But despite this practical knowledge, only little is known about how iron treatment affects the internal phosphorus loading and the lake system in general on the long term. Thus, we needed a lake that was treated with Fe a while ago, to be able to research this. Luckily, one of the P-Trap partners is Waternet, the Dutch water authority for the Amsterdam region. Gerard ter Heerdt, senior ecologist at Waternet had a nice scientific challenge for us: lake Terra Nova.

Terra Nova (photo, dark rectangle) and its position within the Loosdrechtse plassen (orange rectangle) and the Netherlands (bottom right).

Lake Terra Nova is part of the Loosdrechtse plassen lake district. It is an artificial lake that was formed due to peat excavation during the 17 and 18th century, as is the case for many Dutch lakes. Thus, its maximum depth is as deep as a Dutch man can dig without using machines -approximately 2.5 m – which makes it a shallow lake. Such a shallow lake is well mixed all year, except during warm periods without much wind. Terra Nova was groundwater fed in the past and used to have transparent water, that was so clean, that nothing grew because there were no nutrients. But with industrialization and the intensification of agriculture its water quality grew poorer. Several measures were taken. The river Vecht was no longer used to nourish the lakes, as its water quality was so bad. Water from the Amsterdam-Rhine Channel was used instead, after cleaning by adding Iron-Chloride. . However, these measures made the water quality only slightly better, still several species of blue-green microalgae were abundant. As they prevented the light from penetrating to the bottom of the lake, bigger plants rooted in the sediment couldn’t grow anymore. Also, big populations of bream lived in the lake. Bream are burrowing fish, they plough through the sediment in search of food and create even more turbidity. To make the situation better, the bream were removed. But still the water quality did not get better.

Bloom of the uni-cellular, filamentous blue-green algae Planctotrix in Terra Nova in Mai 2021 that form flocs of different size.

This is when Waternet realized, that the problem might be something else. The decades of high P input had led to the accumulation of P in the sediment, without Fe accumulation. This phenomenon is known from water bodies all over the world and the accumulated P is called “legacy P”. The legacy P can get released into the lake water again by processes in the sediment leading to internal P loading of the lake. As Terra Nova is shallow, it has a relatively small water volume compared to the sediment surface, which means internal P loading can be even more significant than in deeper lakes with a higher water volume to sediment surface ratio. As the ratio between P and Fe was disturbed, , Gerard ter Heerdt and his colleagues from Waternet, tested the treatment with FeCl3 in two bays in the east of the lake (location B in the map, figure 1). It worked, internal P loading stopped and thus in 2010, they treated the entire lake with FeCl3. The treatment was successful for a few years, but then seasonal peaks of surface water P concentrations started to appear, with higher and higher amplitude, that largely exceeded the P concentrations from before the treatment. The blooms of blue green algae returned. To solve this puzzle, in depth investigations of the effect of the Fe treatment on the sediment geochemistry were necessary. And this is what Gerard ter Heerdt asked me to do, as of course, this was a very good fit with the research questions of the P-Trap lake group.

Thus, in June 2021, 10 years after the Fe treatment, we went to Terra Nova to take sediment cores and surface water measurements. We took them to the lab, analyzed the sediment for Fe and P speciation among other things, tested the release of P from the sediment to the surface water in core incubation experiments and even took sediment samples to the synchrotron in Grenoble to do x-ray adsorption spectroscopy on them (see blog “XAS-spectra, mountains, and more XAS-spectra” on my secondment with at Eawag in Switzerland). We coupled these studies to the long-term monitoring data and knowledge of the system that Waternet collected on Terra Nova over decades. This collaboration yielded material for two open-access publications: one that reports on the field and lab studies of the sediment geochemistry (currently under review at a scientific journal), and one on the detailed study of the Fe and P speciation in the sediment using X-ray absorption spectroscopy (currently in preparation). For detailed results I thus kindly ask you for some patience until the papers are published. But just a little sneak peek: the results were not quite what we expected. Stay tuned!

Taking sediment cores on lake Terra Nova and the sediment core incubation set-up in the climate chamber at Utrecht University.