UvA ecologist: “Climate change likely to increase blue-green algae”
Due to high temperatures in June, the Grote Plas (Great Lake) in Delft is once again turning green with blue-green algae. UvA ecologist Dedmer van de Waal, who also works at the Institute of Biodiversity and Ecosystem Dynamics (IBED), is using measuring buoys to investigate what else plays into blue-green algae growth. “We expect that due to climate change, the blue-green algae nuisance will only increase in the coming years.”
“Look, you can see blue-green algae floating there,” says Dedmer van de Waal. The UvA professor of aquatic functional ecology, who is also a senior researcher at the Netherlands Institute of Ecology, points to the side of the Grote Plas in the Delftse Hout, an urban area of Delft consisting mainly of a lake, forest, and other greenery. “You can recognize blue-green algae by the greenish-blue, oily layer floating on top of the water. It also washes up on the tide line. The wind blows the algae to the lee side, where it accumulates.”
Blue-green algae, contrary to what the name suggests, are not algae: they are bacteria. Blue-green algae convert carbon dioxide into new cells and oxygen with the help of sunlight and nutrients such as nitrogen and phosphate. The combination of nutrients and warm temperatures can cause a growth spurt (bloom) of blue-green algae. Not all blue-green algae are toxic, but it is impossible to detect this with the naked eye.
This year blue-green algae appeared early in Delft’s recreational lakes, as they have in recent warm years, whereas historically blue-green algae do not come on strong until July or August. During the swimming season from May 1 to October 1, the water board comes every two weeks or sometimes every week to measure the concentration of blue-green algae in the lake.
Based on this, the water board issues a swimming recommendation. Every year about a third or even half of the swimming locations in the Netherlands receive a warning sign for blue-green algae or a no-swimming recommendation. The number of reports of blue-green algae has also risen sharply over the past decade.
Code yellow
The sign at the Grote Plas warns of blue-green algae: “Code yellow. Chance of skin irritation and stomach/intestinal complaints.” Swimming is not prohibited in this lake and Van de Waal would still go in the water there, too. “But thick floating layers of blue-green algae should be avoided. And wash your hands after swimming and take a shower as soon as you can.”
Van de Waal not only investigates whether there are blue-green algae in the lake but also wants to understand what causes the growth and whether the species is toxic. He studies the entire ecosystem of recreational lakes to better understand the competing aquatic species, of which blue-green algae is one. He does this not only at the Grote Plas in the Delftse Hout, but also at the Veerplas in Haarlem, and the Braassemermeer near Roelofarendsveen. “My philosophy is that if we really understand it, we can also convert that understanding into prediction and better control.”
Measuring buoy
Because a weekly measurement is not very often in the life of a microbe, a measuring buoy in the Grote Plas has taken a measurement since the beginning of the swimming season every 10 minutes.
The measurement buoy measures not only how much blue-green algae is present, but also other factors such as oxygen, acidity, and water temperature. In addition, a UvA PhD student comes and takes a sample every week. Based on those samples, the lab will determine at the end of the swimming season whether the blue-green algae in the Grote Plas are also toxic.
Toxic species can congregate in a floating layer, which can wash up on the beach. This green, paint-like sludge can be a risk, especially to children and dogs who play with it or eat it. It mainly causes nausea and diarrhea, but more dangerous species of blue-green algae produce toxins that affect the nervous system.
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Climate change
Climate change increases the likelihood of blue-green algae problems in the coming years. Blue-green algae multiply rapidly in warm water high in nutrients such as nitrogen and phosphate. Agriculture is the largest source of nitrogen and phosphate in surface water. “So how we manage the land also affects blue-green algae in recreational lakes.”
Last June’s stable weather - dry, windless days with high temperatures - provided ideal conditions for blue-green algae, especially when interspersed occasionally with a violent summer storm, such as storm Poly in early July. Nutrients from surrounding farmland then wash out and enter surface water.
Control
According to Van de Waal, the best way to combat blue-green algae is to prevent nitrogen and phosphate from agriculture from leaching into the surface water. In addition, nutrients can be captured. “Then the question is how many nutrients we should capture and in what proportions.”
Van de Waal also hopes to discover why blue-green algae flourish in one pond, while they don’t grow in a similar pond. “If you only take measurements in ponds with blue-green algae, you can discover all kinds of trends, but ultimately I am interested in the conditions that cause blue-green algae to reach high concentrations in the first place. To that end, next year we are going to expand our fieldwork to include sample pools that are more or less free from blue-green algae.”
At some point in the future, Van de Waal wants to expand the research area to the rest of Europe. “That way we can look across climate zones at the influence of nitrogen, phosphate, and temperature on blue-green algae in European lakes. But that idea is still in its infancy.”
How will Delft residents benefit from his research? “Through smarter monitoring of blue-green algae and combining more frequent measurements with measuring buoys with measurements of toxins, we can provide better swimming advice. The expectation is that with smarter monitoring, swimming will be possible more often. We will know at the end of this year whether this is the case for the Grote Plas.”
