Shifting community dynamics of mesophotic marine benthos facing multiple climate stressors

Emma Donald¹, Christopher Battershill¹

¹University of Waikato, New Zealand

Environmental stressors exacerbated by climate change are threatening marine ecosystems worldwide. Aotearoa New Zealand (NZ), a South Pacific island nation, is an appropriate model site where the dynamic effects of a fast-changing marine climate can be monitored and understood. A slew of recent cyclonic and marine heatwave events provided an opportunity to examine responses of the marine benthos, an ecosystem of vital importance to trophic connectivity and carbon flux. As is typical globally for temperate regions, sponge communities dominate the mesophotic reefs of the Motiti archipelago in the Bay of Plenty (east coast, North Island NZ). Although recently classified as a marine reserve for its biogeographic importance, the reef system was largely unexplored prior to this research. Remotely operated vehicle video footage was analysed to establish the biodiversity and current state of Motiti’s mesophotic sponge communities. A rapid shift in community composition appeared to be taking place at the time of survey, likely towards species more resilient to sedimentation and elevated temperatures. This shift was observed through widespread sponge tissue necrosis, sediment smothering the reef benthos, and the sudden decline of the ‘tumbleweed sponge’, which was prolific in 2021 preliminary surveys and almost non-existent just a year and a half later. This research has shed light on a significant biodiversity hotspot and advanced understanding of the short and possibly long-term effects of multiple stressors on mesophotic benthic ecosystems. Results signal a warning for the stability of temperate mesophotic ecosystems around the world.