Abstract:
Marine heat waves (MHWs) are extended periods of excessively warm water<sup>1</sup> that are increasing in frequency, duration, intensity, and impact, and they likely represent a greater threat to marine ecosystems than the more gradual increases in sea surface temperature.<sup>2</sup><sup>,</sup><sup>3</sup><sup>,</sup><sup>4</sup> Sponges are major and important components of global benthic marine communities,<sup>5</sup><sup>,</sup><sup>6</sup><sup>,</sup><sup>7</sup> with earlier studies identifying tropical sponges as potential climate change "winners."<sup>8</sup><sup>,</sup><sup>9</sup><sup>,</sup><sup>10</sup><sup>,</sup><sup>11</sup> In contrast, cold-water sponges may be less tolerant to predicted ocean warming and concurrent MHWs. Here, we report how a series of unprecedented MHWs in New Zealand have impacted millions of sponges at a spatial scale far greater than previously reported anywhere in the world. We reported sponge tissue necrosis<sup>12</sup> and bleaching (symbiont loss/dysfunction),<sup>13</sup> which have been previously associated with temperature stress,<sup>6</sup><sup>,</sup><sup>12</sup><sup>,</sup><sup>14</sup> for three common sponge species across multiple biogeographical regions, with the severity of impact being correlated with MHW intensity. Given the ecological importance of sponges,<sup>15</sup> their loss from these rocky temperate reefs will likely have important ecosystem-level consequences.