• European Green Cities/Site Development

'Monitoring Floating Life" - Academic study on the ecological effects of floating island

Updated: Apr 6

The municipality of Westland and the Delfland Water Board have started the "Floating Life" initiative. The aim is to improve the ecological water quality and spatial quality by constructing floatlands (floating banks) in places where normal nature-friendly banks cannot be realized, such as along quay walls. With the construction of well-designed floatlands, the municipality and the Water Board want to contribute to improving ecological water quality and increasing biodiversity in the water and on the border between water and land. It is likely that the construction of the floatlands will lead to an increase in biodiversity and an improvement of the ecological water quality, because floatlands ensure variation in currents, places with clear water and hiding, foraging and spawning possibilities for fish and other organisms. in the root systems among the floatlands. An improvement in the chemical water quality has not led to the launch of the Floating Life initiative, because in waters where there is a continuous supply of nutrients as a result of flow no improvement in the chemical water quality can be expected as a result of the purifying capacity of the floatlands (Keizer-Vlek et al., 2014).


In June 2014, the Municipality of Westland and the Water Board launched a pilot in the Holle Watering (Figure 1), where floatlands in the form of mats are attached to a quay wall. A total of 400 m of floatlands have been built. The mats are approximately 1 meter wide and have been installed by Nautilus Eco-Civiel, for example. They are implanted with two different plant mixtures (Table 1). These mixtures are implanted on mats of 50 meters in length; alternately a 50-meter mat with mixture 1, followed by a 50-meter mat with mixture 2. A total of 8 mats have been placed: 4 mats planted with mixture 1 and 4 mats with mixture 2 (Tables 1 and 2, Figure 1).



Target

The aim of the 'Monitoring Floating Life' study is to determine by means of monitoring what the ecological effects are for fish, macrofauna and aquatic plants of the floatlands built in the Holle Watering, also taking into account possible differences in the effects such as result of the plant mixture sown.


Conclusions and Future


Ecology

Based on the monitoring between 2014 and 2018 of the “Monitoring Floating Life” project, it can be established that there is an increase in the tax richness of the macro fauna due to the construction of the floating banks against the bare quay wall in the Holle Watering. In terms of biodiversity, the intervention therefore appears to be positive. Because the material is not determined by type, it is not possible to determine to what extent the assessment based on the WFD measure would improve after the construction of the floating banks. The standard net sample taken in 2016 from the location in the section (OW013B001) with the floating banks is assessed as insufficient with the WFD-standard. At the OW014-000 location, the KRW benchmark score appears to be improving from poor in 2010 to inadequate in 2013 and 2016. However, this "apparent" improvement of the ecological status may also have been due to the sampling method, for which there are indications. Also, an overall quality improvement of the body of water cannot be excluded.


The fishing community also seems to be benefiting from the floating banks. More species were found at the locations with floating banks than in the control sections. This effect did not occur until the mats were aged; the lack of effects in 2014, when the floating banks were already in the Holle Watering, probably had to do with the lack of structure in the form of a well-developed root zone at the time. As noted in section 3.3, however, based on the present study, it cannot be excluded that the increase in the number of fish species has (partly) a different cause than the construction of the floating banks.


The type of plant mixture was found to have no influence on the composition of the macrofauna and fish community. The construction of floating banks, regardless of the planted plant mixture, results in a significant increase in biodiversity. Given the development of an extensive root system under both the mats implanted with mixture 1 and 2, this is in line with expectations.


As long as there is a well-developed root system, the floating banks will have an attractive effect on macrofauna as a shelter and / or foraging site, whereby it is less relevant which plant species are on the banks. The development time and therefore the size of the root system seems to be a relevant parameter for the fauna. In order to gain more clarity about the development of the root zone under the root cloth, it may be considered to lift the mats with a crane in the future, to take photographs using an underwater camera (the question is whether the water is not too turbid for that), or to wait for a moment when the mats come off and lie in the water in reverse (see figure 10).


Experience value

In addition to the potential ecological effects of the construction of floatlands, experience has shown that experience value is an important aspect that must be taken into account when deciding whether or not to proceed with the construction of floatlands. During the various field visits, employees were approached by local residents about the floatlands. In almost all cases positive comments were made about how 'beautiful' the floatlands looked and how the 'ugly' quay wall was hidden from view. In 2016, a request was even made to the municipality by the directors of a school to proceed with the construction of floatlands for a bare quay wall. In one case, the PVC floats were perceived as disturbing in the "natural" image of the floatlands. It was suggested that the tubes be covered with a more natural material to "improve" the appearance of the floatlands.


Maintenance

In addition to the ecology and the experiential value, the maintenance of the floatlands and their costs is also an important aspect of the assessment framework regarding whether or not to construct floatlands. During the monitoring, litter was found on the floatlands at some places (Figure 14). Litter was considerably less noticeable in the later years than at the start in 2014 due to the exuberant plant growth. If the amount of litter increases in the future, it may be necessary to remove it.


Nautilus has indicated that floatlands from two years after installation must be mowed at least once a year. Nautilus currently offers maintenance of comparable floatlands for € 1500 per 50 m2 per year. For this, the floatlands are then mowed once and 1x a visit is made to the floatlands to check for possible imperfections.


The floatlands were mowed for the first time in December 2016. An alternating mowing schedule was chosen whereby half of the floatlands were mowed in 2016 and the other half in the autumn of 2017. This mowing schedule was continued in the autumn of 2018. The total costs of this amounted to € 714 per year. Maintenance was carried out with a mowing boat. The clippings are drawn into the water with a rake and then collected with the mowing boat and deposited in a hopper (floating container). The container was then unloaded and dumped in a container wagon. This approach to mowing has proven effective. During the work, the contractor has established that the PVC pipes are broken here and there (leak). In addition, he indicated that the overall condition of the floating banks should be left to be desired and checked.


The present large water navel has been manually removed from the floatlands a number of times. In practice, it proved difficult and cumbersome to remove the plant effectively from the mats without causing damage to the structure and / or the other vegetation. This is because the root system of the plants had penetrated deep into the mats. For this reason, prior to mowing, it was decided to only remove the water navel that grows outside the mats and did not have to explicitly take into account the present water navel when mowing the mats. However, the clippings had to be disposed of properly. The removal of water navel is done Delfland wide, so it is difficult to say how much the removal of water navel costs at one specific location. The estimate is that the removal of the water navel from the floatlands in the Holle watering cost approximately € 2,500. The removal of water navel occurs every year in a 3-week cycle. This usually starts in March up to and including the autumn, with the summer peaking. If it turns out in the autumn that the water is clean (free from water navel), a short stop applies during the winter months. In 2018 the large water navel appeared to have fallen in coverage. It is not clear to what extent this can be related to management and maintenance or to natural factors, such as the ever-increasing biomass of high-rising plants, which could compete with the road species. Because both maintenance with regard to mowing and the removal of exotic plants are a cost item, this aspect must be included in the decision-making about the possible construction of floatlands in the future.


If you would like to read the original report in Dutch, feel free to contact us: info@eugreencities.org.




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