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JPIWATER-Water challenges for a changing world

Mass development of aquatic macrophytes - causes and consequences of macrophyte removal for ecosystem structure, function, and services

Alternative title: Massevekst av vannplanter - årsaker og konsekvenser av fjerning for økosystemstruktur, funksjon og økosystemtjenester

Awarded: NOK 0.80 mill.

Mass development of aquatic weeds in rivers and lakes is a global problem, and the attempts to remove and control these annually consume substantial sums of money. In MadMacs we, in collaboration with colleagues from Brazil, South Africa, France and Germany, try to find common underlying causes of mass development, study what exactly happens when the plant masses are removed, and analyze what people think about the plants. Indeed, mass developments of water plants are generally perceived as problematic, while it is largely unknown that they also deliver several ecosystem services. First results show that the effects of macrophyte removal differ among sites, and that the growth form of the water plants plays an important role. For example, we saw an increase in methane ebullition after removal of water hyacinth in South Africa. This effect is most likely due to a reduction in methane oxidation. The big floating leaves of water hyacinth normally form a barrier between the water surface and the atmosphere. Bubbles of methane, produced in the sediment, can get trapped by these leaves instead of being immediately released into the atmosphere. This causes accumulation of methane bubbles underneath the plants, where the methane can be oxidized to carbon dioxide, which has a 30 times lower global warming potential. In systems with submerged aquatic macrophytes, however, we see the opposite effect. We generally observed lower methane ebullition rates immediately after plant removal. This may be the result of mowing boats disturbing the sediment, causing outgassing of methane bubbles. Our subsequent measurement of lower methane emission could therefore be an aftereffect, as new gas bubbles would have to be produced again before emission is back to ?normal? levels. In some sampling sites, we observed increases in cyanobacteria biomass, or in biomass of periphytic algae, after macrophyte removal. Such growths often are perceived as unwanted side effects of macrophyte removal.

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Mass development of aquatic macrophytes in rivers and lakes is a worldwide problem, and substantial resources are spent on macrophyte removal. However, macrophyte stands either quickly grow back, or the removal causes other problems to surface. Macrophyte mass developments have known negative effects, but well-developed macrophyte stands also provide many ecosystem services. These are often poorly known to the public or to water managers. The specific regional reasons for macrophyte mass development are still poorly understood, likely because there is typically a combination of factors which together cause nuisance growth (multiple pressures). Also, there is a lack of standardized before-after-control-impact studies on the direct and indirect costs of macrophyte removal. We aim to address the following questions: 1) Which combination of natural conditions and pressures leads to undesired mass development of macrophytes? 2) What are the direct and indirect consequences of macrophyte removal for ecosystem functions and services? Which consequences of macrophyte removal are site-specific, and which are general? In collaboration with key stakeholders, we will execute a set of "real-world experiments" in a harmonized design across five countries. Macrophytes will be removed from an area of at least 1000 m2 at each site, and the following parameters will be quantified before and after the removal at control and impact sites: phyto- and zooplankton, benthic algae, macrophytes, macroinvertebrates, fish, nutrient and carbon retention and removal, impoundment, shoreline erosion, as well as relevant ecosystem services related to recreation and water use. We will develop a risk assessment tool of macrophyte mass development and its ecological impacts, as well as of the effects of macrophyte removal. We will compare benefits and dis-benefits of macrophyte removal and formulate guidelines for the management of water courses with dense aquatic vegetation.

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JPIWATER-Water challenges for a changing world