Potential effects of mechanically removing macrophytes on the phytoplankton community of a subtropical reservoir

Abstract

Intensive growth of aquatic macrophytes interferes with water quality and ecosystem dynamics worldwide. Although mechanically removing macrophytes is the most commonly used method for their eradication, it can also cause undesirable disturbances in aquatic reservoir communities. We performed laboratory incubations of phytoplankton sampled before and after macrophytes were mechanically removed from the Piraquara II reservoir, South Brazil. We analyzed changes in growth and composition of the main phytoplankton groups with respect to nutrient shifting. Prior to removing the macrophytes, the phytoplankton community was dominated by low cell abundances of diatoms and flagellates. In contrast, growth rates of cyanobacteria (mainly Cylindrospermopsis raciborskii, Pseudanabaena sp., and Geitlerinema sp.) and of colonial chlorophytes were favored after macrophyte removal, while the abundances of diatoms and flagellates decreased. Our results suggest that removing macrophytes causes dramatic changes in phytoplankton composition and biomass and selects for toxigenic species of cyanobacteria. These changes were probably associated with the disturbance caused by removing the macrophytes, which immediately created new environmental conditions prone to species competition. These findings indicate that the use of mechanical techniques to manage macrophytes should be carefully considered, along with monitoring of harmful species and changes of limnological parameters.

Short-term effects of macrophyte removal on aquatic biodiversity in rivers and lakes

“…Short-term increase of phytoplankton richness and functional richness could be explained by an overlap of remaining plant-associated species and the newly established open water-associated species directly after the removal. Former studies showed differences in the phytoplankton communities associated with macrophytes and open water sections (Gebrehiwot et al, 2017;Wojciechowski et al, 2018). In Hartbeespoort Dam, the only subtropical site in our study, we found a strong increase of cyanobacteria, aligning with studies with comparable results (Mangas-Ramírez and Elías-Gutiérrez, 2004;Wojciechowski et al, 2018).…”

“…Former studies showed differences in the phytoplankton communities associated with macrophytes and open water sections (Gebrehiwot et al, 2017;Wojciechowski et al, 2018). In Hartbeespoort Dam, the only subtropical site in our study, we found a strong increase of cyanobacteria, aligning with studies with comparable results (Mangas-Ramírez and Elías-Gutiérrez, 2004;Wojciechowski et al, 2018).…”

“…Opposite results for abundance were found in other studies in a Mexican lake (Mangas-Ramírez and Elías-Gutiérrez, 2004) and a river in the U.K. (Garner et al, 1996). After plant removal, several studies showed a clear increase in phytoplankton cell density (Wojciechowski et al, 2018) or in Chl-a concentration (Bicudo et al, 2007;James et al, 2002;Kuiper et al, 2017). However, other studies showed a short-term reduction in Chl-a concentration after plant removal (Alam et al, 1996;Morris et al, 2006).…”

“…Increased turbidity due to the removal likely causes this short-term decrease (Thiemer et al, 2021). Furthermore, the removal of macrophytes increased the abundance of cyanobacteria in tropical lakes (Mangas-Ramírez and Elías-Gutiérrez, 2004;Wojciechowski et al, 2018), while Morris et al (2006) did not find an effect on cyanobacteria in a shallow lake in Australia.…”

The relationship between water quality and riparian zone land-use variables and macrophyte removal permits issued in between 2013-2020 in the Rideau Canal Waterway

“…Submerged macrophytes in lakes and rivers play an important role in the ecosystem, providing habitat for invertebrates and fish species, influencing aquatic nutrient cycles, and contributing to the structure and functioning of the freshwater hydrologic regime (Chambers et al, 1999a;Journal et al, 2018). Macrophytes are used as an indicator of aquatic ecological health, as they reflect the quality of physical, chemical and biological habitat conditions (Ogdahl & Steinman, 2015;Wojciechowski et al, 2018).…”