The vast subtropical “gyres” – large systems of rotating currents in the middle of the oceans – cover 40 percent of the Earth’s surface and have long been considered biological deserts with stratified waters that contain very little nutrients to sustain life.
These regions also are thought to be remarkably stable, yet scientists have documented one anomaly in the North Pacific Subtropical Gyre ecosystem that has puzzled oceanographers for years: The region’s chemistry changes periodically, especially levels of phosphorous and iron, affecting the overall nutrient composition and ultimately its biological productivity.
Above- NASA image of Asian dust transport over the North Pacific. CREDIT Robert Simmon
In a new study published this week in Proceedings of the National Academy of Sciences, researchers document what induces these variations: changes in the amount of iron that is deposited into the ocean via dust from Asia.
“We now know that these areas that were thought to be barren and stable are actually quite dynamic,” said Ricardo Letelier, an Oregon State University biogeochemist and ecologist, who in collaboration with David Karl at the University of Hawaii led this study. “Since these areas cover so much of the Earth’s surface, we need to know more about how they work in order to better predict how the system will respond to climate variations in the future.”
Both phosphorous and iron are key components for life and the researchers noticed that the levels of those nutrients in North Pacific gyre surface waters changed significantly during the three decades of the study.
Letelier said the team was able to relate these changes to the iron input from Asian dust – a combination of the desertification of that continent, with combustion, especially wildfires and factory output and the wind patterns across the North Pacific ocean – that accounted for the variance and provided varying amounts of nutrients to sustain life.
And a key to that variance is the Pacific Decadal Oscillation, an ocean-atmosphere relationship that varies between weak and strong phases of atmospheric pressure in the northeast Pacific. In years when the low pressure weakens, winds from Asia become stronger, move more southward, and bring more dust, fertilizing the ocean surrounding Station ALOHA. When the pressure strengthens, the opposite takes place.
Strong winds can bring significant amounts of iron, allowing organisms to grow and utilize all the phosphorus in the upper layers of the ocean. However, because most of the iron is not soluble, deep waters are enriched in phosphorus relative to iron. Hence, when winds are weaker, there is little iron input to fertilize and remove any excess phosphorus in the upper layers that may be introduced through deep water mixing.
Characterizing the mechanisms driving spatial and temporal changes in the stoichiometry of nutrient supply is crucial to understand the controls of an ecosystem’s carrying capacity and productivity. In marine oligotrophic regions, small changes in the ocean and atmospheric nutrient input ratio can shift the nature of the limiting nutrient. The present study documents such a shift at interannual scales between periods of phosphorus limitation and sufficiency in the North Pacific Subtropical Gyre. These shifts appear to be driven by interannual variations in the transport of iron-rich Asian dust across the North Pacific resulting from basin-scale changes in atmospheric pressure gradients, as reflected by the Pacific Decadal Oscillation index, causing the ecosystem to oscillate between phosphorus and iron limitation.
The supply of nutrients is a fundamental regulator of ocean productivity and carbon sequestration. Nutrient sources, sinks, residence times, and elemental ratios vary over broad scales, including those resulting from climate-driven changes in upper water column stratification, advection, and the deposition of atmospheric dust. These changes can alter the proximate elemental control of ecosystem productivity with cascading ecological effects and impacts on carbon sequestration. Here, we report multidecadal observations revealing that the ecosystem in the eastern region of the North Pacific Subtropical Gyre (NPSG) oscillates on subdecadal scales between inorganic phosphorus (Pi) sufficiency and limitation, when Pi concentration in surface waters decreases below 50–60 nmol⋅per kilogram. In situ observations and model simulations suggest that sea-level pressure changes over the northwest Pacific may induce basin-scale variations in the atmospheric transport and deposition of Asian dust-associated iron (Fe), causing the eastern portion of the NPSG ecosystem to shift between states of Fe and Pi limitation. Our results highlight the critical need to include both atmospheric and ocean circulation variability when modeling the response of open ocean pelagic ecosystems under future climate change scenarios.