The channelized streams in our landscape are slowly restored into the original meandering streams that are naturally present in the Netherlands. These restoration projects are important to keep the streams functional and healthy. However, through monitoring of these streams it appears that restoration projects are largely unsuccessful and that only a limited number of the original fauna returns to the restored stream. Judith Westveer investigated why it is that aquatic macroinvertebrates cannot find their way back. She will defend her PhD thesis on 4 December at the University of Amsterdam.
In the 20th century, many streams were transformed into channels to enhance irrigation of agricultural crops and drainage of wastewater. These anthropogenic alterations led to an overall deterioration of water quality and a degradation of habitats for stream inhabiting organisms.
Westveer emphasizes the importance of streams: ‘They supply us with drinking water, irrigate agricultural land and drain waste water. Research has shown that aquatic macroinvertebrates – small aquatic animals such as insect larvae, water beetles, water snails and mussels – do not, or hardly ever, return to the restored streams. This is a problem, because these animals are responsible for certain ecological processes in the stream; they filter water, extract important nutrients from the sediment and accelerate the decomposition of leaves. Additionally, aquatic macroinvertebrates are essential food sources for fish and birds.’
There are several reasons why the aquatic macroinvertebrates do not return to the restored streams. The distance between the birthplace and the newly restored area determines the speeds at which the settlers populate the restored trajectories. The smaller the distance to travel, the faster they will colonize the new area. In addition, the abundance of macroinvertebrate at the birthplace appears to be important.
‘During a three year long field experiment we studied which macroinvertebrates were able to find a new stream by air’, says Westveer. ‘We now know that only a few flying species (Simulidae and Chironomidae: mosquitos) can reach a stream that is only two kilometres away. The majority of all macroinvertebrates can be found within a few hundred meters of the place where they came from, simply because they don’t have the skills to find a new stream that is further away. As a result, the recovery of streams that are not well-connected to other streams takes a very long time.’
To ensure the return of aquatic invertebrates to restored streams, Westveer has a number of suggestions. She thinks that a reintroduction of suitable species offers a solution. In addition, more effort should be made to map existing source populations around the area where recovery is going to take place. The life-cycles of macroinvertebrates should also be taken into account in the planning of stream restoration to ensure that most invasive activities do not occur during the sessile stages (for example during pupation of aquatic insects).
Ms. J.J. Westveer: Go with the flow - Unravelling the ecological mechanisms of dispersal and colonization by aquatic macroinvertebrates in restored lowland streams. Promotor is Prof. P.F.M. Verdonschot and co-promotors are Dr H.G. van der Geest and Dr R.C.M. Verdonschot (Wageningen UR).
The promotion takes place on Tuesday, December 4 at 14:00h.
Location: Agnietenkapel, Oudezijds Voorburgwal 231, Amsterdam.