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Coastal Eddies Promote Large-scale Algal Response to Urban Eutrophication

Published on: 05/13/2024

A NCCOS-funded study found that anthropogenic nutrients are promoting enhanced algal productivity in the Southern California Bight (SCB), that is predicted to result in an offshore zone of lowered subsurface oxygen and pH of around 278,40 km2 in size (about 30 percent of the SCB area). The SCB is an upwelling-dominated embayment (a bay with currents that bring water and nutrients up towards the surface) with a coastal population of 23 million. In this ecosystem, climate change is driving rates of pH and oxygen decreases that are among the highest in the world. 

Using a validated model, scientists found that anthropogenic sources of nitrogen discharged at the coast doubled algal productivity, causing eutrophication (excessive plant growth due to excess nutrients in the water). Persistent coastal eddies (circular water movement) trap anthropogenic nitrogen and algal organic matter, causing excessive algal growth that intensifies acidification and oxygen loss.

In a companion article that is in peer review, scientists have been working to translate how those changes in seawater chemistry translate to biological effects. In partnership with the management community, they have also begun to use the model to investigate to what degree management of the major sources of nitrogen can reverse the predicted changes. Finally, scientists are applying the model to determine how the magnitude of local anthropogenic change compares with that of global climate change to gauge the efficacy of this local management strategy to increase coastal ecosystem resilience.

This study is the result of multiple projects funded by NCCOS and the California Ocean Protection Council (under the Ecology and Oceanography of Harmful Algal Blooms Program, and the Coastal Hypoxia Research Program) and NOAA's Ocean Acidification Program (Ocean Acidification and Hypoxia).

Consequence of eutrophication on biogeochemical processes of (A) nitrogen uptake (i.e., Net Primary Production) in mmol N m-2d-1, (B) respiration in mmol O m-2d-1 and (C) remineralization of organic matter in mmol C m-2d-1. These are 5-year averaged maps, averaged from 0–200 m in water depth. Light contours on every map highlight 25 and 75 percentiles. Credit: Kessouri et al. 2024.

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