Computer simulations show that climate change driven by human activity will significantly alter crucial nutrient cycles in our oceans. In a study published in the Proceedings of the National Academy of Sciences, researchers from the University of California, Irvine have found compelling evidence that marine nutrient cycles—vital for maintaining ocean ecosystems—are experiencing unexpected changes as global temperatures rise.
Adam Martiny, a professor of Earth system science and ecology & evolutionary biology, and one of the lead authors of the study, stated, “Model studies have indicated that when ocean temperatures increase, the ocean becomes more stratified, which can deplete specific areas of surface ocean nutrients.” While previous models suggested a link between ocean temperatures and surface nutrient levels, this research marks the first confirmation of climate change’s impact on nutrient cycles.
The research team, led by graduate student Skylar Gerace, examined half a century’s worth of nutrient data gathered from the ocean as part of the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP). Their analysis revealed a significant reduction in phosphorus—a nutrient crucial for the health of marine food webs—in the southern hemisphere oceans over the past 50 years.
Gerace noted, “There can be cascading effects throughout the food web,” explaining how phytoplankton—tiny organisms foundational to many marine food webs—depend on phosphorus for sustenance. “When there is less phosphorus available for phytoplankton, they become less nutritious, which can hinder the growth of zooplankton and fish.”
Interestingly, despite anticipating a drop in nitrate levels, the concentrations of this nutrient have remained stable. Martiny pointed out that while nitrate is essential for the functioning of ecosystems and its stability is reassuring, there is still a possibility of future declines as climate change progresses. “However, it’s uncertain— this is purely speculative,” he added.
Martiny highlighted the significance of initiatives like GO-SHIP for conducting this type of research; without expeditions that gather real data on marine ecosystems, it would be impossible to verify the predictions made by climate models. For example, while models have suggested that there should already be decreasing nitrate levels in ocean waters, direct observations have contradicted that forecast.
“Demonstrating long-term climate impacts on the ocean is generally quite challenging due to high variability,” said Martiny. “Our study contributes to a limited number of research efforts that showcase these long-term changes.” There are only a handful of confirmed long-term trends in ocean chemistry, he noted.
Moving forward, the research team aims to assess how these shifting nutrient cycles affect marine ecosystems in both hemispheres as climate change continues to evolve.
Gerace remarked, “We plan to explore how this nutrient data relates to larger ecosystem dynamics in the ocean, such as primary productivity. This could help validate studies like ours as comprehensive indicators for monitoring marine ecosystems in the face of ongoing ocean warming and stratification.”
This research received funding from various organizations, including the National Science Foundation, the National Oceanic and Atmospheric Administration, and NASA for Martiny, as well as funding from the DOI Office of Biological and Environmental Research and NASA for Professor Keith Moore in the Department of Earth System Science.
