Intra-colony spatial variance of oxyregulation and hypoxic thresholds for key Acropora coral species.

Oxygen (O 2 ) availability is essential for healthy coral reef functioning, yet how continued loss of dissolved O 2 via ocean deoxygenation impacts performance of reef building corals remains unclear. Here, we examine how intra-colony spatial geometry of important Great Barrier Reef (GBR) coral species Acropora may influence variation in hypoxic thresholds for upregulation, to better understand capacity to tolerate future reductions in O 2 availability. We first evaluate the application of more streamlined models used to parameterise Hypoxia Response Curve data, models that have been used historically to identify variable oxyregulatory capacity. Using closed-system respirometry to analyse O 2 drawdown rate, we show that a two-parameter model returns similar outputs as previous 12th-order models for descriptive statistics such as the average oxyregulation capacity (T pos ) and the ambient O 2 level at which the coral exerts maximum regulation effort (P cmax ), for diverse Acropora species. Following an experiment to evaluate whether stress induced by coral fragmentation for respirometry affected O 2 drawdown rate, we subsequently identify differences in hypoxic response for the interior and exterior colony locations for the species Acropora abrotanoides , Acropora cf. microphthalma and Acropora elseyi . Average regulation capacity across species was greater (0.78-1.03 ± SE 0.08) at the colony interior compared with exterior (0.60-0.85 ± SE 0.08). Moreover, P cmax occurred at relatively low p O 2 of <30% (±1.24; SE) air saturation for all species, across the colony. When compared against ambient O 2 availability, these factors corresponded to differences in mean intra-colony oxyregulation, suggesting that lower variation in dissolved O 2 corresponds with higher capacity for oxyregulation. Collectively, our data show that intra-colony spatial variation affects coral oxyregulation hypoxic thresholds, potentially driving differences in Acropora oxyregulatory capacity.