Irreplaceable water fleas help solve the mystery of browning lakes
When you wade into a lake in summer, it is a good idea to look for signs of water fleas in the waters along the shore. If it is not windy and the lakebed is light enough, fleas swarming in the water are easy to detect. Quite likely, there are tiny representatives of the genus Polyphemus swimming around at your feet.
Water flea swim with jumping movements, by which the translucent organism springs upwards. Described with scientific accuracy, the movements are jerks caused when the crustacean strikes downwards with its antennae. At the ends of the antennae, you can see thin rippling bristles.
In the forehead of the translucent swimmer, you can discern a large dark compound eye. In a swarm, the eyes can be discerned as black dots.
The figure has been familiar to Academy Research Fellow Minna Hiltunen for almost ten years. This crustacean (Daphnia magna), which is a favourite among researchers, was introduced to her by her dissertation supervisor.
The water flea is a really cute creature,” Hiltunen says. “It’s a funny view in the microscope when their hatched juveniles are still lurking under the mother’s carapace .”
In lake ecosystems, the few-millimetres-long water flea is a key stone species. Without it, the lake’s organisms would face hard times: Small fish could not feed on them and would starve to death. Microalgae would fill the lake if there were no water fleas and other zooplankton feeding on it.
Water flea resting eggs provide new information about the past of lakes
The life cycle of water fleas progresses rapidly. After hatching, the water flea starts reproducing at the age of less than ten days. Its asexual reproduction is efficient: during its life span one female produces hundreds of copies of itself.
If the water flea faces hard times – with decreasing food, for instance – it resorts to sexual reproduction. The water flea lays resting eggs, ephippia, which are protected by a hardened shell. They drift down to the lakebed mud, sediment. Most of the resting eggs hatch in the following spring, but a small share remain in the resting stage, preserving their potential for future favourable times.
The oldest ephippia I know have hatched after a resting period of nearly 700 years,” Hiltunen says.
Resting eggs, more specifically their shells, include information that Hiltunen with her research team will soon access. This can give novel information about the conditions of small Finnish lakes decades or even a century ago.
Before the analysis can begin, Hiltunen must collect the resting eggs by hand. She travels to collect eggs from more than thirty lakes – from Hanko to Kilpisjärvi.
Research provides new information on calcium levels in lakes
When the resting eggs settled on the sediment of the lake bed, the wave of acidification had not yet swept through Finland's lakes.
Calcium in lake water originates from the catchment soils and it was effectively leached from the soil when sulphur and nitrogen emissions were acidifying Finnish lakes in the 1970s and 1980s. Based on follow-up studies, it is known that in many lakes the calcium levels have continued to decline after the lake has already recovered from acidification. As yet, there have been very little information on the calcium levels before acidification.
Calcium is an important substance for the water flea: it helps it to build a strong shell for itself and for resting eggs.
“This will be new knowledge. At present there is no good method for determining historical calcium levels from sediment,” Hiltunen explains.
The research method is also unique among paleolimnological methods. It has been developed at the Ģֱ through the collaboration of biologists and chemists: Hiltunen pursues this research together with Senior Lecturer Siiri Perämäki from the Department of Chemistry and Postdoctoral Researcher Henriikka Kivilä from the Department of Biological and Environmental Science. The study is funded by Research Council of Finland.
“We use the ICP-MS method to measure the composition of elements by means of mass spectrometry.”
How does the browning of lakes affect the elemental cycle?
Information about calcium supplements the picture of elemental cycles in lakes, such as how the harmful browning of lakes has affected various elements.
Browning has been a clear and continuing trend in Finnish lakes and rivers since at least the 1990s, as evident from monitoring data. Before that, there is data only for few lakes, however. Brownning has been harmful for the lake ecosystems.
Browning of lakes refers to the water colour becoming darker due to increased loading of organic matter and iron from the catchment. Browning has been boosted by the recovery from acidification, climate change, and land use. For example, the drainage of peatland and clear-cutting of forests increase the load of organic matter to lakes.
Both climate change and the recovery from acidification cause browning of lakes. We aim to find out how the impacts of these factors differ from each other,” Hiltunen says.
“What kind of changes have they caused in the cycles of elements? In previous research, all browning has been handled as one, although the impacts may vary if the cause is different.”
This can be investigated by comparing resting eggs collected from different areas.
“Resting eggs collected from different places can tell us whether elemental cycles and water flea populations have changed in different ways, if browning has been caused by a different factor.”
Browning waters deteriorate conditions for water fleas
The browning of waters has had extensive impacts on lake organisms. Surface waters have become warmer and lakes more strongly stratified. When the water in lakes mixes less, oxygen levels in the bottom layers can decrease. Light gets attenuated in the dark water, and this decreases the amount of algae deeper in water.
The change has not been beneficial to water fleas.
Browning suits bacteria and microalgae of poor quality, which water fleas then feed on due to the lack of better nutrition."
"The quality of fatty acids of the water fleas declines, and thus the same happens to the fish that feed on the water fleas. For example, if a water flea feeds only on bacteria, it dies,” Hiltunen explains.
Like Hiltunen, many other researchers also wish that the browning of waters would be monitore more closely in Finland than at present. The so-called water framework directive that guides water protection in the European Union does not require this.
“Changes in ecosystems due to browning remain unnoticed because monitoring in line with the water framework directive concentrates on the impacts of eutrophication, and the effects of browning may even be opposite ones. The indicators should be developed so that the impacts of browning on the ecological state of lakes can be assessed,” Hiltunen says.
Resting eggs are collected manually from lakes
An important part of this study is to develop the method so that the analysis would require as few resting eggs as possible. Currently, one sample calls for about twenty eggs in order to get a reliable measurement of the quantities of elements. This number has been in constant decline.
That makes Hiltunen happy.
If it required a hundred, collecting would be quite a difficult job.”
In summer, there will be much at stake when Hiltunen travels to collect eggs from the lakes. In order to verify the reliability of the new method the resting eggs must be collected from many lakes.
It’s a big effort, yet feasible. It calls for several weeks of wading and rowing in an inflatable raft along lakefronts. Hiltunen screens resting eggs from sediments with sieves. Eventually, when the finest sediment material has been drained through, the resting eggs can be found on the bottom of the sieve.
If there are any extra eggs, there is a research topic for them as well. The eggs are placed in a fridge to await their revival, and the tolerance of these newly-hatched historical water fleas are then tested for different conditions in a laboratory.
“In this way, we could explore whether water fleas have adapted to the browning of lakes and, for example, to the decrease in calcium. Do water fleas that once lived in calcium-rich waters cope as well in dark waters with low calcium levels as do the present water flea populations?”
