Low-latitude mesopelagic nutrient recycling controls productivity and export

Area of Science:

• Oceanography
• Marine Ecology
• Climate Science

Background:

• Low-latitude (LL) oceans are critical for global primary production and export.
• Previous assumptions suggested the Southern Ocean dominates LL production and export, impacting climate change responses.
• Understanding nutrient cycling in LL oceans is vital for predicting future productivity.

Purpose of the Study:

• To investigate the primary drivers of LL primary production and export.
• To analyze the role of mesopelagic macronutrient cycling and Southern Ocean influence.
• To project future LL production under different climate scenarios using Coupled Model Intercomparison Project Phase 6 (CMIP6) models.

Main Methods:

• Observational analyses and sensitivity studies using an individual model.
• Analysis of five CMIP6 models under high-emissions (SSP5-8.5) and low-emissions (SSP1-2.6) scenarios.
• Examination of temperature-dependent remineralization and its impact on nutrient retention.

Main Results:

• Local mesopelagic macronutrient cycling controls 72% of LL primary production and 55% of export.
• Southern Ocean supply sustains 34% of LL export via a deeper overturning cell; shallow northward supply sustains only 7%.
• CMIP6 models show significant disparities in LL production projections; warming may stabilize or increase LL production under high emissions due to enhanced nutrient retention.

Conclusions:

• Mesopelagic nutrient cycling, not Southern Ocean export, is the primary controller of LL ocean production.
• Future ocean warming may enhance mesopelagic nutrient retention, potentially stabilizing or increasing LL production.
• Mechanistic understanding of mesopelagic remineralization and its sensitivity to warming is crucial for predicting future marine ecosystem changes.