Decadal changes in California's temperate mesophotic reef invertebrate community through the 2014–2016 northeast Pacific marine heatwave

The 2014-2016 northeast Pacific marine heatwave (MHW) disrupted marine ecosystems in California, with well-documented impacts on kelp reef ecosystems, and deep reef fish assemblages. However, no studies have assessed the direct or indirect impacts on sessile benthic reef invertebrates inhabiting mesophotic rocky reefs (>30 m deep), despite their critical role in biodiversity and ecosystem function. Given that much of California's rocky reefs lie within mesophotic depths, understanding their responses to climate-driven disturbances is a high priority. Using remote operated vehicles surveys conducted from 2005 to 2021 across marine protected areas (MPAs) and habitat-matched reference sites, we quantified changes in sessile invertebrate abundance, both independently and in relation to urchin presence, during and after the MHW. Community responses varied across a five-degree latitudinal gradient (33.87°- 38.97°N), with no consistent differences attributable to MPA designation. Sponges and California hydrocorals increased regionally, while red and purple gorgonians, elephant ear tunicates, and sea whips declined during the MHW and remained suppressed five years later. Although surface warming reached mesophotic depths at some sites, temperatures remained within typical ranges, suggesting the MHW event may not be the primary driver of the observed changes. Instead, broader ecosystem perturbations likely facilitated red sea urchin expansion into mesophotic depths from 2015 onwards. Their presence was associated with declines in several species, representing a potential indirect pathway through which this climate disturbance affects mesophotic assemblages. These findings highlight the need for depth-resolved monitoring to assess changes in this understudied ecosystem and to identify mesophotic refugia at thermally stable sites.