Diversity lost for ever


a water flea © Christian Rellstab, Eawag

Until the 1970s, the combination of agricultural methods and phosphate-based detergents caused the eutrophication of many Swiss lakes. Since then, the water quality has improved noticeably, but water fleas have not succeeded in attaining their former diversity. By Atlant Bieri

​Water fleas are just one to two millimetres in length and are thus among the smaller inhabitants of our lakes. And yet they are also among the most important. Billions of them swim through our lakes and constitute the main source of nutrition for young fish. Now researchers have discovered something disturbing about these water fleas. Their genetic diversity has altered fundamentally in the last hundred years. And it’s humans that are to blame.

Piet Spaak is the Head of the Department of Aquatic Ecology at the aquatic research institute Eawag and has been investigating the fate of water fleas for the past 25 years. In order to carry out his studies, he has had recourse to a unique biological archive: the bottoms of the lakes. A new layer of sediment is formed there every year. By drilling and removing cores from the sediment, he can read the layers to find out what happened in earlier times – rather like looking at the rings of a tree.

These layers include the dormant eggs of water fleas, which can survive undamaged for a long time in dry or cold conditions. Young animals can even emerge from them after several decades. “At the bottom of a lake, the water is always at four degrees and there is little oxygen”, says Spaak. “In such conditions, the eggs do not develop”. After some fifty years, most of them eventually die. But even after a hundred years, researchers can carry out genetic analyses of dead eggs and thereby determine the characteristics of the species alive at that time.

In order to access the past, the scientists have to drill up to a metre into the silt. Spaak and his doctoral students Nora Brede, Cristian Rellstab and Markus Möst have taken so many samples from numerous lakes in Switzerland and Italy that they are able today to document completely the development of the water flea over the past hundred years.

It transpires that the water flea most common in prealpine lakes in the first half of the 20th century was the species Daphnia longispina. It specialised in surviving in waters low in nutrients. The lakes back then were relatively clean, and it reproduced slowly. It was distinctive in that it tried to avoid fish by only coming up at night to feed on algae on the water surface. By day it hid in the lower levels of the lake.

In the 1930s its situation worsened because more and more liquid manure from farms began to seep into the streams and rivers. Untreated waste water also began to flow from factories and the growing number of private households. This all had a negative impact on Daphnia longispina. The sewage acted like a fertiliser and caused a rapid increase in the amount of algae in the lakes, resulting in so-called algal bloom. When the algae died, they sank to the bottom of the lakes where they were broken down by bacteria. These bacteria in turn used up the oxygen in the water, which suffocated most other living creatures in the lake. Biologists call this process eutrophication.

Not prudish when choosing mates

The climax of this phase came in the 1970s. Agriculture was becoming more intensive, and phosphate-based detergents were also popular. The phosphates acted like supercharged fertiliser and prompted yet more algae growth. With this increase in nutrients appeared a second species of water flea, Daphnia galeata. It had previously led a shadowy existence in the alpine regions and is native to nutrient-rich waters – unlike Daphnia longispina. This new water flea began to spread rapidly in the over-fertilised lakes.

These two species of water flea are related closely to each other. And because male water fleas are not particularly prudish when it comes to choosing their mates, a female of a different species is to them just as good as one of their own. “Male water fleas will try to copulate with anything. If you put a pipette into their container, they’ll latch onto it” says Spaak.

So eventually, Daphnia galeata and Daphnia longispina brought forth hybrids that were perfectly suited to conditions in the over-fertilised Swiss lakes. “These hybrids combined the advantageous features of both species”, says Spaak. He believes that this adjustment to the new conditions in the water took place over a period of just ten to twenty years. “Normally, evolution progresses by means of chance alterations in the genetic material. That can take thousands of years. But the hybridisation accelerated the process”, he says.

Improved water quality

These hybrids now reproduced and henceforth dominated in many of the eutrophic Swiss lakes. Daphnia galeata too was able to remain in high concentrations. Daphnia longispina, on the other hand, was marginalised. In the lakes that to this day have a large concentration of fertilisers, this distribution of the species has remained. One example is the Greifensee. While its phosphate content has decreased over the past fifty years from 500 micrograms per litre to 70, this is still roughly as much as was found in Lake Constance when it was at its most polluted. For this reason, the Greifensee remains eutrophic to this day. Fishes such as whitefish cannot reproduce on their own there because their eggs suffocate.

The ban on phosphate-based detergents introduced in the 1980s and the expansion of wastewater treatment plants improved the water quality of many lakes, however. As a result, the hybrids and Daphnia galeata became scarcer again. Lake Constance, Lake Lucerne and the Walensee are examples where this has happened: here, Daphnia longispina has once more gained the upper hand.

However, when analysing the genes of Daphnia longispina, the researchers discovered that this water flea is no longer the same one that was in our lakes a hundred years ago. By mating with Daphnia galeata, its genetic makeup has changed irreversibly. The new Daphnia longispina is in fact a hybrid of both species. The old water flea species are thus lost for good. Genetic diversity has been reduced, and biodiversity with it. “When man intervenes in the environment, the results are often impossible to foresee”, says Spaak. “Even when we are able to reverse our disruption of the environment, this does not mean that we can return things to their original state”.

Atlant Bieri is a freelance science journalist.

(From "Horizons" no. 103, December 2014)

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