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

Nitrous oxide (N2O) 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 N2O emissions, we know relatively little about N2O dynamics in nearshore systems. Here, we document the fourfold increase in the abundance of a large N2O‐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 N2O under current summer conditions (25°C) compared to winter (18°C). However, further warming to 28°C resulted in intermediate N2O production rates, which did not differ significantly from either summer or winter values, indicating that a threshold for temperature‐driven N2O production may have been reached. By integrating N2O production rates with Herdmania field density data and coastal bathymetry, we estimate that subtropical reefs may already be producing more N2O than adjacent estuaries. Our findings suggest that warming reefs, transitioning from carbon‐capturing kelp forests to N2O‐producing ascidian‐dominated reefs, could significantly alter coastal greenhouse gas budgets.