New research has found a simple method that can improve wetland stability, biodiversity and ecosystem functioning. By simply increasing the flow rate of water around wetlands, the growth and impact of biofilms on underwater vegetation decreases. This approach can have significant benefits for constructed wetlands or the rejuvenation of degraded wetland ecosystems.
Biofilms are integrated communities of microorganisms such as bacteria and fungi. Biofilms are extremely adaptable and can grow wherever conditions are suitable. This includes marine environments, sewer pipes, and even on your teeth. Although many biofilms are benign, they can have negative impacts on other organisms and man-made structures if growth is unchecked.
Macrophytes are plants growing underwater. Macrophytes are important for biodiversity and ecosystem functioning by providing habitat for microorganisms and animals, biomass for grazing herbivores, and dissolved oxygen from photosynthesis. Macrophytes, though, are stationary structures, submerged in a continuously aqueous environment, which provides an ideal substrate for biofilms to proliferate. Under certain conditions, biofilm growth can overwhelm macrophytes which can have ramifications for those other organisms and biogeochemical processes reliant on the macrophytes.
Scientists from Hohai University, Nanjing, China, tested whether increasing the flow of water around macrophytes controlled the growth and proliferation of biofilm. The scientists were also interested to know how flow rate affected dissolved oxygen profiles in, and around, the macrophytes.
An experimental design of a static and high flow water treatment was imposed on two macrophyte species and an artificial control “macrophyte”. The two species were Vallisneria natans and Hydrilla verticillate (Figure 1):
The experiment found that flowing water inhibited the growth and decreased thickness of biofilm on the macrophytes (Figure 2). But, the scientists also found that high water flow increased the bacterial taxonomic diversity in the biofilms. The high flow treatment, therefore, reduced biofilm but increased species diversity.
To quantify the dissolved oxygen profiles at the leaf/water interface, the researchers used a “puncture test”, also known as a micro-profiling procedure, on the macrophytes. A Unisense MicroProfiling System was used to perform the puncture test (Figure 3). The puncture test involved using a 10 µm diameter oxygen microsensor, connected to a motor controller, that measured dissolved oxygen at 100 µm steps over a 3 cm transect across the leaf/water boundary (Figure 3). Measurements were taken in the light and dark to measure oxygen fluxes caused by photosynthesis and respiration of the macrophytes.
The results found that oxygen concentration in the water column was higher during the light than dark due to the process or photosynthesis generating oxygen and respiration consuming oxygen, respectively (Figure 4). Oxygen concentration, during the light, increased towards, and through, the macrophyte leaf; whereas the opposite was observed during the dark. Oxygen concentrations were greater under static water than high flowing water during the light but were lower during the dark. The researchers in the study suggested that “the reduced oxygen concentration in plant leaves of submerged macrophytes in the flow may be ascribed to the decreased net photosynthesis and increased respiration” (Han et al. 2018, page 8), which was also suggested by Madsen et al (1993).
The researchers concluded that by increasing the flow of water in wetlands, it is possible to decrease the negative impacts of biofilm on macrophytes.
Han et al (2018). Effects of water flow on submerged macrophyte-biofilm systems in constructed wetlands. Nature Scientific Reports, 8:2650, DOI: 10.1038/s41598-018-21080-y
Madsen et al. (1993). Effects of water velocity on photosynthesis and dark respiration in submerged stream macrophytes. Plant Cell Environ. 16, 317–322. Weblink.