Climate change is draining nutrients from the ocean's surface

Climate change is reshaping our oceans in ways that often go unnoticed, yet its impact on marine ecosystems is profound.

It’s easy to feel disconnected from the hidden changes occurring beneath the waves, but recent research reveals how human-induced climate change is disrupting the delicate balance of ocean nutrient cycles.

The water we can’t drink makes up about 97% of the Earth’s water, serving as the planet’s largest heat sink and providing nearly half the oxygen we breathe. This makes the ocean an essential component of our planet’s health.

However, a recent study conducted by researchers at the University of California, Irvine highlights how our oceans’ health is threatened by global warming.

Fewer nutrients at the ocean’s surface

“Model studies have suggested that when the ocean warms it gets more stratified, which can drain certain parts of the surface ocean of nutrients,” said Adam Martiny, a professor of Earth system science and one of the study’s lead authors.

In simpler terms, the warmer our oceans get, the fewer nutrients are available at the surface. This may slow marine primary productivity, causing cascading effects throughout food webs, noted the researchers.

“However, observing changes in upper ocean nutrients is challenging because surface concentrations are often below detection limits.”

The research team, led by graduate student Skylar Gerace, analyzed 50 years of nutrient data taken from the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP).

Cascading effects in the food chain

The researchers identified a significant decline in phosphorus, a nutrient vital for the health of marine food webs, particularly in southern hemisphere oceans.

“There can be cascading effects up the food web. When phytoplankton have less phosphorus, they become less nutritious, which can impair zooplankton and fish growth rates,” said Gerace.

The researchers found that concentrations of nitrate, another nutrient vital for ecosystem functioning, appear to remain steady.

However, Professor Martiny noted that nitrate concentrations may still decline in the future as the climate continues to change.

Phosphorus limitation is expanding

“The temporal trends of these depths revealed that upper ocean phosphate is mostly declining, while nitrate is mostly stable,” wrote the study authors.

“Using Earth System Models, we demonstrate that this difference is likely due to decreasing iron stress for nitrogen fixation, which replenishes nitrate with increasing stratification.”

According to the researchers, the findings suggest that phosphorus limitation is expanding throughout the ocean, consequently bringing many important implications for marine ecosystems.

Future research directions

Professor Martiny highlights the importance of initiatives like GO-SHIP for conducting research of this kind.

Without the empirical data collected by such seafaring missions, there would be no way to verify the accuracy of climate model forecasts.

The team is excited about the next steps in their research. The goal is to measure the impact of changing nutrient cycles on marine ecosystems as climate change continues.

“We aim to investigate how this nutrient metric relates with broader ecosystem dynamics throughout the ocean, such as primary productivity,” said Gerace.

The researchers hope that their measurements can serve as a comprehensive indicator for monitoring marine ecosystems as the oceans continue to warm and stratify.

The findings may ultimately provide scientists and policymakers with much-needed data to address the often neglected underwater effects of climate change.

Climate change and global fisheries

The loss of nutrients in the ocean doesn’t just harm microscopic marine life; it has ripple effects on global fisheries and food security.

Some of the world’s most important fisheries for fish such as anchovies, sardines, and tuna rely on nutrient-rich waters to support the plankton populations they feed on.

A decrease in phosphorus could lead to decreases in ocean primary productivity, meaning smaller fish stocks and disruptions in the seafood industry.

Changing the nutrient dynamics could have significant impacts on fishing communities that heavily rely on fishing as a source of food and income.

Scientists are closely monitoring these shifts, which might reshape marine ecosystems in ways that are difficult to reverse.

The full study was published in the journal Proceedings of the National Academy of Sciences.

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