Eutrophication, glyphosate herbicides and heat waves interact with each other to tip the scales towards phytoplankton development and deplete aquatic plant growth.
Submerged aquatic plants are an essential part of shallow aquatic ecosystems as they perform a wide range of functions and keep shallow water bodies healthy. However, their abundance has declined globally due to multiple stressors caused mainly by human activities, like increasing global temperatures, nutrient loading, and herbicides.
Previous studies has generally ignored studying the interaction between numerous stressors, concentrating instead on the effects of a single or two combined stressors on this process.
In this new study, researchers from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences, under the direction of Professor Xu Jun, showed that eutrophication, glyphosate herbicides, and heat waves can all interact to cause the loss of submerged aquatic plants while promoting phytoplankton growth.
Here, the team used 48 climate change simulation mesocosms (2500 L each) to simulate shallow lakes. They applied a combination of nutrient loading, continuous warming, heat waves, and glyphosate-based herbicides to test how these stressors interactively affect the growth of the different primary producers, namely, the submerged aquatic plants, phytoplankton, and periphyton.
The effects of the various stressors on the growth of different primary producers were then examined by the researchers. They found that while herbicides alone had little effect on the primary producers, nutrient loading increased phytoplankton biomass and decreased the growth of submerged aquatic plants. Heat waves, as opposed to continuous warming, amplified the effects of herbicides and nutrient loading on submerged aquatic plants.
According to further analysis of structural equation models, the effects of numerous stressors on primary producers relied on the combination of stressors and the species.
These findings showed that a combination of stressors can eventually result in the depletion of submerged aquatic communities and shift toward phytoplankton dominance, raising the possibility of a regime shift and degrading shallow aquatic habitats.
This study provides the first mesocosm experimental evidence at its scale that the loss of submerged aquatic plants can be caused by a number of stressors working together. These findings highlight the need for multiple stressor studies as a foundation for the management of shallow lakes and ponds in agricultural environments.
Source: Zhang et al. (2022). Heat waves rather than continuous warming exacerbate impacts of nutrient loading and herbicides on aquatic ecosystems. Environment International, 168, 107478.