Both ocean hypoxia and rising temperatures due to climate change are hot topics in any discussion of current challenges facing our marine environment. Ecologist Chet Rakocinski, a professor at The University of Southern Mississippi’s (USM) Gulf Coast Research Lab (GCRL), is leading a team of graduate students and interns in an investigation of how benthic polychaetes, small organisms that live within bottom sediments, cope with these two stressors.
Master’s student Alyssa Bennett and Ph.D. candidate Kelsey Gillam have spent the last year and a half exposing four species of sediment-dwelling marine worms to varying levels of temperature and dissolved oxygen to get an idea of how they physiologically respond to stressful conditions.
Few studies to date look at the joint effects of the two stressors.
“It’s imperative to consider how effects of multiple stressors extend from individual responses to populations, as they are likely to be synergistic,” Rakocinski said.
The response of the four worm species used in the experiments — three native to the Gulf of Mexico, and one from the northern Atlantic — can be an indicator of how marine resources might fare under the conditions. For one, they dwell in estuaries, the part of the marine environment most subject to fluctuations. Plus, they are forced to endure whatever stressors move into their habitat.
“They’re worms,” said Bennett. “They can’t escape.”
To find out how one model species fared in low-oxygen, high-temperature conditions, Bennett and Gillam isolated them in specially designed chambers. After acclimating them, they were subjected to sea water ranging from 15⁰ to 35⁰ C (59⁰ to 95⁰ F) in temperature and from 20 percent to 100 percent in oxygen saturation over four days. After each day of exposure, they measured egestion, growth and mortality. In addition, they measured respiration of worms under the same combinations of temperature and oxygen.
“We want to know how much they are breathing, how much they are eating, how much they are growing, and how much they are dying,” Rakocinski said.
While the study is not complete, early data indicates that when the temperature is at its highest and oxygen at its lowest, a large number are, in fact, dying. Bennett’s Gulf worms experienced a 90-percent mortality rate under those conditions. Beyond that, other effects varied widely among species.
As each organism has different adaptations, not all reacted the same. The team saw two very different responses in terms of respiration in low-oxygen environments. The worms either hyper-regulated, expending extra energy trying to get what little oxygen was available, or oxy-conformed, breathing less as at lower oxygen levels. Generally, all had more tolerance to low oxygen at cooler temperatures than at higher temperatures. That is partly because high temperatures exacerbate the effects of low oxygen by physically impeding oxygen from dissolving in the water, while at the same time elevating the organism’s metabolism.
The team also measured defecation rates and changes in body size to find out how the stressors impacted the worms’ ability to eat and grow. Since worms can digest their own bodies for energy, many of them actually decreased in size during the experiments. Speaking of size, it definitely played a role in the worms’ ability to withstand the stressors — the larger the organism, the better it coped with the environmental conditions.
The team greatly streamlined their methodology throughout the course of the project, improving their lab setup and protocols through experience. With their current methodology in place, Rakocinski hopes they can continue doing multi-stressor experiments when this study is complete, perhaps adding in other factors, such as pH levels using different benthic species.
In addition to these early findings, further research is needed before the investigators can generalize. When complete, Rakocinski will refine the Hypoxia Mass Balance Model (HMBM) he devised based on the Peters Mass Balance Model (PMBM) to synthesize how these stressors affect the organisms energetically. The PMBM describes body-size-related trends in energetic costs and benefits. The HMBM will expand on the PMBM by incorporating specific effects of hypoxia and temperature in terms of energetic costs.
This spring, Rakocinski’s team plans to offer a benthic indicator workshop for regional resource management personnel, including representatives of state and federal agencies who must deal with effects of manmade stressors. They will explain potential connections between their findings and certain currently used indicators to demonstrate the HMBM approach as another potential assessment tool for resource management. They will also present their findings through various public outreach events at GCRL’s Marine Education Center.
By highlighting how the combined stressors of hypoxia and rising temperature elicit mortality as well as chronic effects on benthic organisms, this study is an important step in understanding how climate change could impact marine life in the future.
*Tara Skelton is a freelance writer.