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Researchers at Griffith University are trying to figure out how things like climate change, pollution, dissolved nitrogen, and sediment from runoff affect coastal ecosystems as a whole.
Two studies that were published in Ecology Letters and Proceedings of the Royal Society B show that when increasing ocean temperatures, pollution, or dissolved nitrogen are combined with less light in the water due to sediment, the effects on seagrass and algae growth can either get worse or get better.
Dr. Mischa Turschwell, a Research Fellow at the Australian Rivers Institute, said, “Most importantly, we show that the combined effects on seagrass and algal growth can vary greatly depending on the amount of the two stressors and the length of exposure.”
“With the start of climate change, stressors like rising ocean temperatures, poor water quality, and pollution threaten coastal and marine ecosystems on more than one front.
“In order to take care of these coastal ecosystems well, managers need to know how each of these human-made changes affects them on their own and how they affect each other.”
Associate Professor Chris Brown, who is in charge of the Seascape Models group at the Australian Rivers Institute and the Coastal and Marine Research Centre, said, “To date, most attempts to find out how these stressors interact have failed to find consistent predictions for their combined effects by using data from multiple studies.”
“Few generalisations have been made about the effects of all of these stressors together, and meta-studies that look at multiple past studies on the same stressors often come up with different results.”
There are far too many possible combinations of stressors for researchers to ever hope to measure them all. Instead, they need accurate models that can predict how stressors in the environment might interact.
Dr. Turschwell and his team made a model to predict how temperature and light affect photosynthesis and growth of seagrass. The model also had an animal that ate the seagrass.
“Using the model, we looked at how the temperature and light in the water worked together by changing how much of each was in the water and for how long,” Dr. Turschwell said.
“Surprisingly, our model showed that combining the same two stressors could either make their effects on seagrass growth worse or make them less bad.
“The combined effect on seagrass depended a lot on how much each stressor changed. For example, when the amount of light loss was high and the temperature was high, the effect of the two together got stronger.
“The effect of temperature and loss of light on seagrass growth changed again when organisms that eat seagrass were added to the model to make it more like the real world.”
Olivia King, who is working on her Ph.D., did experiments on the effects of a herbicide (diuron), dissolved inorganic nitrogen, and less light to see if these models are accurate representations of what happens when water quality stressors are combined (due to sediment).
Pollutants in coastal water, like herbicides in agricultural runoff and sediment from erosion, can stop important algae species from growing.
Like what the model showed, Ms. King’s study of multiple stressors like diuron and less light showed that their individual effects could be amplified or lessened, depending on how much of each pollutant they were exposed to, how long they were exposed to it, and the biological response that was being looked at.
“This research makes it clear why it has been so hard to get a clear picture of how more than one stressor interacts in the past. Their combined effect can change depending on things like how long they are used and how much they are used.”
“To find consistent patterns of these interactions, we need to learn how stressors change depending on the situation and then design experiments that run over long periods of time and have treatments for different levels of stressors.”
There is an urgent need to learn more about how the effects of multiple stressors happening at the same time affect the marine environment as a whole. This will help us predict which stressors are the most important for managers of marine ecosystems to deal with.
Associate Professor Brown said, “The water quality on the Great Barrier Reef, for example, is managed by rules that only look at one pollutant at a time.”
“Our research shows that the Great Barrier Reef and other important ecosystems need to have their water quality guidelines updated to take into account how the effects of multiple pollutants can be worsened by each other. It also shows how this can be done.
“Similarly, the effects of pollution and rising temperatures are a clear sign that water quality guidelines need to take into account how pollution and rising temperatures will interact.”
Further information: Mischa P. Turschwell et al, Interactive effects of multiple stressors vary with consumer interactions, stressor dynamics and magnitude, Ecology Letters (2022). DOI: 10.1111/ele.14013
Olivia C. King et al, Interactions among multiple stressors vary with exposure duration and biological response, Proceedings of the Royal Society B: Biological Sciences (2022). DOI: 10.1098/rspb.2022.0348
Journal information: Proceedings of the Royal Society B , Ecology Letters
Source: Griffith University