Macrophyte removal in the Otra river
Large amounts of underwater plants (= aquatic macrophytes) are often perceived as a nuisance, because they interfere with human use (for example fishing, boating, or swimming). In many rivers and lakes around the world, aquatic macrophytes are therefore regularly removed. Within MadMacs, we are studying what happens in an ecosystem when large amounts of aquatic plants are removed. To find this out, we established six case study sites in five countries. One of these is the Otra river in Norway. The Otra has experienced mass development of bulbous rush (Juncus bulbosus) for several decades. We measured a range of parameters related to biodiversity and ecosystem functioning before and after macrophyte removal in a control and impact area, following a BACI design (Before-After-Control-Impact).
The fieldwork in Norway was performed during the summer of 2020. In the beginning of June 2020, we took water samples, collected phytoplankton, zooplankton, periphyton, macrophytes and macroinvertebrates, and measured methane gas emission. Between the 15th and 22nd of June, plants were removed from the impact area, but not the control area. The removal was done with special boats. The plants were first shortened with something that resembles a large underwater saw or knife. Then a molding cutter removed the plant roots from the sediment. After plant removal, all measurements described above were repeated in both control and impact area. By comparing data collected before and after removal, we will be able to determine the consequences of plant removal on the ecosystem. For some parameters, samples were also taken 6 weeks after the plants had been removed and macrophyte cover was also mapped 11 weeks after the plant removal to determine regrowth.
Preliminary results show that the mowing boat reduced plant cover by about 40% in the impact area (from 75% to 35% cover), and average canopy height was reduced from 35 cm to 10 cm. These numbers indicate that the mowing boat did not completely remove the Juncus vegetation, but that was to be expected based on limitations reported for different methods of macrophyte removal. This is simply due to the difficulty of “cleaning” the bottom of a river or lake. Standing biomass before and after removal is, however, rarely measured. Our results therefore provide valuable information for water managers. Our observations also showed that plants started to regrow immediately after the removal (illustrated by increases in plant cover and canopy height in the period from “1 week after” until “11 weeks after” the removal). This is more remarkable than one would think, since water temperatures at our sampling site rarely exceed 15 °C even in summer. This is due to the Otra receiving water from a reservoir located in the mountains.
Harvest of plant biomass showed that Juncus bulbous dry weight was about 125 g/m2 in the control area. The mowing boat managed to remove about 90 g of dry plant biomass per m2 from the impact area, since plant dry weight was reduced from 150 to 60 g/m2. Using our measurements of the plants’ carbon and nitrogen content, we could calculate that 35 g /m2 of carbon and 1.5 g /m2 of nitrogen were removed from the impact area through plant removal.
When plants are removed, the macroinvertebrates (insects and other small animals) that live in and among water plants may be removed as well. This may reduce overall biodiversity. In addition, since macroinvertebrates are important food for the trout that live in this part of the Otra, their disappearance may impact the river’s fish stock. Among the macroinvertebrate taxa we collected from Juncus bulbosus plants, were species within the genus Oxyethira. These are microcaddisflies that are typically found among water plants in Norway. They are much smaller than other caddisflies (the individuals shown in the picture below were much shorter than 5 mm), and they tolerate some acidification (many rivers in South Norway are acidic). Oxyethira is assumed to feed on green algae that grow on the plants, by breaking the cell walls of the algae with their “mouth” and sucking out the cell contents.
Further analysis will show how many macroinvertebrates disappeared during the macrophyte removal, and to which extent these small animals were able to recolonize the impact area afterwards. In collaboration with NORCE, we also tagged about 100 brown trout and installed 20 receivers in the control and impact area. The receivers can “hear” and localize the fish. With this data, we hope to find out whether fish prefer areas with or without water plants.
In addition to our ecological work, we ran a large survey among local inhabitants and tourists to find out which user groups perceive the water plants as problematic and how important the removal of the water plants is for the different users of the river. In the end, we aim to summarize all MadMacs results using the ecosystem services framework, so that we can compare the direct and indirect benefits and costs of plant removal in an integrated way.