As ocean pH fell over 24 years, dissolved silicon and chlorophyll fell at a Pacific site

Why it matters

Diatoms generate about 40% of ocean primary production and feed many coastal food webs. If acidification strips silicon from surface waters, fisheries and carbon uptake could suffer. The authors suggest this may already be happening in productive regions and could add to a feedback that speeds up pH decline.

What they did

The team used water samples and half-hour probe readings at Tatoosh Island, Washington, over 24 years. They measured nutrients, pH, salinity and chlorophyll. They compared trends in pH, the silicon-to-nitrogen ratio and chlorophyll. They tested alternative explanations, including upwelling, salinity, river discharge, nutrient changes and shifting water sources, using regression, mixing models and structural equation modeling.

Key findings

• pH fell by 0.027 units per year, much faster than predicted from equilibrium with atmospheric CO2.
• The dissolved silicon to nitrogen ratio fell 56%, from 0.28 to 0.13, and chlorophyll a fell from 65.4 to 5.3 mg/L.
• Lower silicon-to-nitrogen ratios went with lower chlorophyll, and this held after controlling for upwelling, salinity and river discharge.
• Dissolved nitrogen and phosphate showed no consistent trend, and a mixing model with a silicon-depleted deep source fit better than one without.
• Structural equation modeling attributed the silicon decline mostly to falling pH rather than to upwelling, salinity or river inputs.

Limitations

• The data come from one site and use annual means, so the results show association rather than direct proof of cause.
• The authors lack data on diatom-grazing zooplankton, so the food-web explanation for chlorophyll falling faster than silicon is only indirectly supported by herring catch data.
• At the finer scale of individual samples, upwelling and salinity contributed more to silicon-to-nitrogen ratios than in the annual analysis.

Glossary

• Diatoms: Single-celled algae that build glass-like shells from silica and drive much ocean production.
• Frustule: The silica shell of a diatom.
• Bakun upwelling index: A wind-based estimate of how strongly deep water is pushed up to the surface along the coast.
• Piecewise structural equation modeling: A method that links several regressions in a causal network to test how variables jointly affect each other.