Anthropogenic Impacts on Coral-Algal Interactions of the Subtropical Lagoonal Reef, Norfolk Island.

Reef building corals are important in subtropical marine ecoregions, shaping ecosystems and providing habitats for fish and benthic species. Algal communities contribute substantially to the benthic population structure across subtropical coral reefs, however increasing algal cover on subtropical reefs is also linked to degraded ecosystems as has been shown on tropical systems. As such, the dynamics of coral-algal interactions on subtropical reefs are also likely to be an indicator of ecosystem health on subtropical ecosystems. The subtropical lagoonal coral reef of Norfolk Island within the Norfolk Marine Park has been impacted by a regime of disturbance since 2020 including flooding, sedimentation, and heat stress events. Assessing the type and extent of algal interactions with the dominant coral within the reef sites of Emily Bay, Slaughter Bay, and Cemetery Bay has the potential to provide insight into drivers of ecosystem decline within the reef. Similarly, photochemical efficiency, as measured by yield (Fv/Fm) using pulse amplitude modulated fluorometry, can be used to provide a measure of the health of corals on reefs during degradation events. Here we assess the extent of coral-algal interactions and health of colonies of prior to the onset of summertime conditions (April 2023) and during summertime conditions (December 2023). Seasonal and within site dynamics of coral-algal interactions were evident by a significant bloom of red cyanobacteria ( < 0.0001, April 2023) and { < 0.01 [Slaughter Bay West (SBW)],  < 0.01 [Slaughter Bay East (SBE)], December 2023}. Within reef, variability of coral-algal interactions was most evident for , and on the Norfolk reef, interactions of with were found to be significantly higher at slaughter bay west (SBW 30.2% of interactions) and east (SBE 24.6% of interactions) in December 2023 than at neighboring Emily (11.6% of interactions) and Cemetery Bay (0.6% of interactions). Pulse Amplitude Modulated (PAM) fluorometry also highlighted the potential for algal interactions to influence the photochemical efficiency of . Benthic structure, as measured by coral-algal interactions, and coral health within the Norfolk lagoonal, both highlight the potential for anthropogenic drivers of reef decline to influence the health of the ecosystem. Further investigation is therefore necessary to elucidate the specific causes and consequences of algae linked to poor water quality, such as red cyanobacteria and , interacting with corals.