Ocean Productivity Declines as Marine Heatwaves Intensify

The ocean acts as Earth’s temperature regulator and carbon sink, absorbing over 90% of the excess heat trapped by greenhouse gases. As global warming accelerates, the ocean experiences rising temperatures that are altering marine ecosystems in profound ways. A new study published in Nature reveals unprecedented declines in ocean net primary productivity (NPP) – the foundation of marine food webs and carbon sequestration – coinciding with marine heatwaves in 2023.

NPP refers to the rate at which phytoplankton and other autotrophs convert inorganic carbon into organic matter through photosynthesis. Derived from 21 years of satellite data on ocean color, temperature, and light availability, researchers from Louisiana Universities Marine Consortium, University of Georgia, and University of Bremen determined that April 2023 saw global NPP drop 22% below the mean, falling to its lowest level on record at 2.76 petamoles of carbon per year. Further analysis revealed the most prominent NPP declines occurred in September 2023, when sea surface temperatures (SSTs) peaked during an intense marine heatwave.

Spatial patterns revealed NPP reductions were especially pronounced across equatorial regions of the Atlantic and Pacific Oceans as well as the northern Atlantic – all areas coinciding with above-average SST anomalies. To explore this connection, the researchers compared records of optimal temperatures for photosynthesis (Topt) among dominant plankton communities to observed September SSTs. They found Topt was exceeded in 65% of ocean area during the period, suggesting high temperatures physiologically impaired NPP rates.

Breaking the data down by oceanographic zones provided more insight. Surface waters in the pelagic ocean – deep waters far from coasts – experienced the most marked NPP declines. Analyzing biogeochemical provinces defined patterns of atypical NPP drops beginning in April 2023 across the Equatorial Atlantic, Equatorial Pacific, and North Atlantic subtropical gyre. September 2023 saw NPP in these vital regions fall 23.8-30.2% below long-term averages while temperatures rose modestly by 2.5-4%.

The study draws comparisons to the powerful 2016 El Niño, finding greater NPP reductions in more regions during 2023. Peak SSTs were similar or higher in the Equatorial Pacific for both events yet declines in NPP were equal. This suggests 2023 brought an additional mechanism like weakened upwelling diminishing nutrient delivery. With SSTs now regularly exceeding Topt across broad ocean stretches, marine heatwaves pose serious risks to ocean productivity and carbon drawdown until plankton communities adapt.

At its lowest point in April 2023, global NPP was already 25% less than total annual fossil fuel emissions – a massive decrease for ocean systems critical to global climate regulation. Sustained heatwave impacts risk compounding nutrient limitation effects, shifting plankton composition in ways altering carbon export. With ocean warming locked in for centuries, continued declines in NPP will ripple through marine food webs and diminish the ocean’s CO2 absorption capacity, accelerating climate change. As marine heatwaves intensify, monitoring primary production within the context of rising atmospheric CO2 levels is imperative for predicting future changes in Earth’s habitability.

In sum, this comprehensive analysis of satellite data reveals unprecedented drops in ocean productivity driven by the anomalous warmth of the 2023 marine heatwave. The ocean absorbs over 90% of excess anthropogenic heat but its capacity to act as climate regulator depends on phytoplankton thriving. As hotter conditions encroach on optimal temperature ranges worldwide, declining NPP underscores urgent needs to rein in greenhouse gas emissions and safeguard the ocean’s role in global carbon cycling.

Reference(s)

1. Decline in ocean net primary production coincident with the 2023 heat wave Marshall W. Bowles, Samantha B. Joye, Michael Schulz, Matthias Zabel, and Kai-Uwe DOI: https://doi.org/10.21203/rs.3.rs-4014371/v1

Click TAGS to see related articles :

CARBON | CLIMATE CHANGE | ENVIROMENT | OCEAN | SOCIETY