Seaweed ecosystems may not mitigate CO2 emissions

<p>Global seaweed carbon sequestration estimates are currently taken as the fraction of the net primary production (<em>NPP</em>) exported to the deep ocean. However, this perspective does not account for CO₂ from the consumption of external subsidies. Here, we clarify: (i) the role of export relative to seaweed net ecosystem production (<em>NEP</em>) for a closed system and one more likely open to subsidies; (ii) the importance of subsidies by compiling published estimates of <em>NEP</em> from seaweed-dominated ecosystems; and (iii) discuss their impact on the global seaweed net carbon balance and other sequestration constraints as a mitigation service. Examples of seaweed <em>NEP</em> (<em>n</em> = 18) were sparse and variable. Nevertheless, the average <em>NEP</em> (−4.0 mmol C m^–2 d^–1 SE ± 12.2) suggested that seaweed ecosystems are a C source, becoming increasingly heterotrophic as their export is consumed. Critically, mitigation of greenhouse gas emissions was mixed relative to their replacement or baseline states, and where CO₂ is supplied independently of organic metabolism and atmospheric exchange, we caution a sole reliance on <em>NEP</em> or <em>NPP</em>. This will ensure a more accurate seaweed mitigation assessment, one that does exceed their capacity and is effective within a compliance and carbon trading scheme.</p>