Coral populations on the Great Barrier Reef (GBR) are experiencing long-term shifts in size structure, including steep declines in small colonies, driving major concerns for recovery through the supply of new recruits. Whilst coral restoration began on the GBR in 2018, the combined influence of natural recruitment and outplanting for coral population recovery has not been evaluated. Here, we assessed 2 sites (Rayban and Mojo) at Opal Reef that were subject to intensive outplant efforts over a 3 yr period (2018-2021). Coral cover did not change significantly, with a baseline of 15% in 2018 and a cover of 28 and 25% in Rayban outplant and control areas, respectively, in 2021, while Mojo exhibited a coral cover of 38% in 2018 and 52% (outplant area) and 29% (control area) in 2021. Natural recruitment in 2021 did not vary by site and was characterised by a settlement rate of 5.5 and 3.7 recruits tile-1 at Rayban and Mojo, respectively. Juvenile coral abundance and diversity were similar for control and outplant areas at each site. Over the 3 yr period, coral cover as a metric did not identify differences between control and outplant areas; however, size-frequency distributions of key coral taxa revealed a higher frequency of small to mid-sized colonies in outplant communities compared to controls. Given that no differences were observed in recruitment rates or juvenile abundances, variations in population structure appear to be driven by planting efforts rather than natural recovery. Our results demonstrate the need for combined monitoring of natural versus intervention-based rehabilitation to understand the impact of coral propagation efforts for local site recovery.

Thermal tolerance is variable in corals, yet intrinsic and extrinsic drivers of tolerance are not well understood. Understanding the distribution and abundance of heat tolerant corals across seascapes is imperative for predicting responses to climate change and to support novel management actions. Rapid and high-throughput methods to measure heat-induced coral bleaching sensitivity are increasingly required to understand current and predict future responses to climate change. Experimental evaluations of coral heat and bleaching tolerance typically involve ramp-and-hold experiments run across days to weeks within aquarium facilities with limits to colony replication. Field-based acute heat stress assays have emerged as an alternative experimental approach to rapidly quantify heat tolerance in a large number of samples yet the role of key methodological considerations on the stress response measured remains unresolved. Here, we quantify the effects of coral fragment size, sampling time point, and physiological measures on the acute heat stress response in adult corals. The effect of fragment size differed between species (Acropora tenuis and Pocillopora damicornis). Most physiological parameters measured here declined over time (tissue colour, chlorophyll-a and protein content) from the onset of heating, with the exception of maximum photosynthetic efficiency (Fv/Fm), which was stable up to 24h post heating. Based on our experiments, we identified photosynthetic efficiency, tissue colour change, and host-specific assays such as catalase activity as key physiological measures for rapid quantification of thermal tolerance. We recommend that future applications of acute heat stress assays include larger fragments (>9cm2) where possible and sample between 10 - 14h after the end of heat stress. A validated high-throughput experimental approach combined with cost-effective genomic and physiological measurements underpins the development of markers and maps of heat tolerance across seascapes and ocean warming scenarios.

No abstract

We employed Fast Repetition Rate fluorometry for high-resolution mapping of marine phytoplankton photophysiology and primary productivity in the Lancaster Sound and Barrow Strait regions of the Canadian Arctic Archipelago in the summer of 2019. Continuous ship-board analysis of chlorophyll a variable fluorescence demonstrated relatively low photochemical efficiency over most of the cruise-track, with the exception of localized regions within Barrow Strait where there was increased vertical mixing and proximity to land-based nutrient sources. Along the full transect, we observed strong non-photochemical quenching of chlorophyll fluorescence, with relaxation times longer than the 5-minute period used for dark acclimation. Such long-term quenching effects complicate continuous underway acquisition of fluorescence amplitude-based estimates of photosynthetic electron transport rates, which rely on dark acclimation of samples. As an alternative, we employed a new algorithm to derive electron transport rates based on analysis of fluorescence relaxation kinetics, which does not require dark acclimation. Direct comparison of kinetics- and amplitude-based electron transport rate measurements demonstrated kinetic-based estimates were, on average, 2-fold higher than amplitude-based values. The magnitude of decoupling between the two electron transport rate estimates increased in association with photophysiological diagnostics of nutrient stress. Discrepancies between electron transport rate estimates likely resulted from the use of different photophysiological parameters to derive the kinetics- and amplitude-based algorithms, and choice of numerical model used to fit variable fluorescence curves and analyze fluorescence kinetics under actinic light. Our results highlight environmental and methodological influences on fluorescence-based productivity estimates, and prompt discussion of best-practices for future underway fluorescence-based efforts to monitor phytoplankton photosynthesis.