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Ascidians increase in abundance on tropicalized reefs and may enhance benthic nitrous oxide production
Journal article   Open access   Peer reviewed

Ascidians increase in abundance on tropicalized reefs and may enhance benthic nitrous oxide production

Zuhairah Dindar, Dirk V. Erler, Stephanie G. Gardner, Hamish A. Malcolm, Michael Sutherland and Adriana Vergés
Limnology and oceanography, Vol.First online, pp.1-14
02/08/2025
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Published (Version of record) Open Access CC BY-NC V4.0
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Abstract

Nitrous oxide (N 2 O) is an ozone‐depleting greenhouse gas that has a global warming potential nearly 300 times greater than carbon dioxide. Although the oceans contribute about 21% of atmospheric global N 2 O emissions, we know relatively little about N 2 O dynamics in nearshore systems. Here, we document the fourfold increase in the abundance of a large N 2 O‐producing invertebrate ( Herdmania grandis ) over two decades in eastern Australia. This increase occurred in warming subtropical reefs that were previously dominated by carbon‐capturing kelp but have since become tropicalized as warm‐affinity species expand their ranges poleward. Temperature manipulation experiments demonstrated that Herdmania produces higher levels of N 2 O under current summer conditions (25°C) compared to winter (18°C). However, further warming to 28°C resulted in intermediate N 2 O production rates, which did not differ significantly from either summer or winter values, indicating that a threshold for temperature‐driven N 2 O production may have been reached. By integrating N 2 O production rates with Herdmania field density data and coastal bathymetry, we estimate that subtropical reefs may already be producing more N 2 O than adjacent estuaries. Our findings suggest that warming reefs, transitioning from carbon‐capturing kelp forests to N 2 O‐producing ascidian‐dominated reefs, could significantly alter coastal greenhouse gas budgets.

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