Measuring the Impact of Thermal Stress on Coral Resilience in Hawai\u27i Using Large-Area Imagery

Coral reefs worldwide are declining due to several anthropogenic stressors, but rising ocean temperature is the most serious threat to coral reef persistence. Developing models that document changes in coral communities following thermal stress events and forecast trends in reef recovery is crucial in identifying resilient reefs. Traditional approaches to generating the coral vital rates necessary for demographic modeling are time consuming and field intensive; however, by leveraging Structure-from-Motion photogrammetry, we can accurately track populations over time at a large spatial scale. In this study, I assessed the population dynamics of the dominant coral species across the Hawaiian archipelago and investigated the impact of thermal stress on coral populations. The annual growth, survival and recruitment of 3,852 coral colonies (5,636 unique colony-level transitions) for 3 genera was recorded at 16 sites spanning the Hawaiian archipelago across 14 intervals from 2013 to 2019, including 3 bleaching events. These data were used to estimate vital rates (growth, survival, and recruitment) and build integral projection models to determine the impact of thermal stress on population growth. To overcome the inherent challenges in estimating coral reproduction, I modeled recruitment in four different ways and present a comparison of datarich to data-poor estimation methods. Degree Heating Week output from the NOAA Coral Reef Watch daily global 5km satellite was used to estimate thermal conditions at each site by calculating temperature stress severity (the mean of all maximum thermal anomalies) and frequency (number of thermal stress events per 10 years). I found that all three coral genera, which have different morphologies and life-history strategies, had negative population growth rates. As expected, smaller colonies experienced faster growth, but large colonies had a high probability of shrinking, due to partial mortality. Large, multi-fragmented colonies had high survivorship and it may be advantageous for larger colonies to fragment into smaller pieces to avoid total mortality. Population dynamics were primarily driven by coral growth and survival and should be targeted in future restoration and adaptation projects. Additionally, across all taxa, population growth rates (λ) varied spatiotemporally, but most sites exhibited a declining population growth rate (λ \u3c 1). While increased severity and frequency of thermal stress events negatively impacted the population growth rate of massive Porites corals, there was no signal of this effect on encrusting Montipora corals. I demonstrate that despite variations in the responses observed among taxa, there is an overall expected population decline across the Hawaiian archipelago. While most coral population growth rates are higher following bleaching events, signifying recovery, the projected increase in both the severity and frequency of thermal anomalies may overwhelm corals’ ability to recover and threaten coral population persistence

Coral Colony-Scale Rugosity Metrics and Applications for Assessing Temporal Trends in the Structural Complexity of Coral Reefs.

Globally, coral reefs are experiencing reductions in structural complexity, primarily due to a loss of key reef building taxa. Monitoring these changes is difficult due to the time-consuming nature of in-situ measurements and lack of data concerning coral genus-specific contributions to reef structure. This research aimed to develop a new technique that uses coral colony level data to quantify reef rugosity (a 3-dimensional measure of reef structure) from three sources of coral survey data: 2D video imagery, line intercept data and UAV imagery. A database of coral colony rugosity data, comparing coral colony planar and contour length for 40 coral genera, 14 morphotypes and 9 abiotic reef substrates, was created using measurements from the Great Barrier Reef and Natural History Museum. Mean genus rugosity was identified as a key trait related to coral life history strategy. Linear regression analyses (y = mx) revealed statistically significant (p < 0.05) relationships between coral colony size and rugosity for every coral genus, morphotype and substrate. The gradient governing these relationships was unique for each coral taxa, ranging from mean = 1.23, for (encrusting) Acanthastrea, to m = 3.84, for (vase-shape) Merulina. These gradients were used as conversion factors to calculate reef rugosity from linear distances measured in video transects of both artificial reefs, used as a control test, and in-situ natural coral reefs, using Kinovea software. This calculated, ‘virtual’ rugosity had a strong, positive relationship with in-situ microscale rugosity (r2 = 0.96) measured from the control transects, but not with that measured at the meso-scale in natural, highly heterogeneous reef environments (r2 < 0.2). This showed that the technique can provide accurate rugosity information when considered at the coral colony level. The conversion factors were also applied to historic line intercept data from the Seychelles, where temporal changes in calculated rugosity were consistent with changes in coral cover between 2008 and 2017. Finally, on application to 2,283 corals digitised from UAV imagery of the Maldives, the conversion factors enabled calculation of rugosity for three 100 m2 reef areas and prediction of how this rugosity will decrease during two future scenarios of coral reef degradation and community change. The study highlights that the application of genera-specific coral rugosity data to both new and existing coral reef survey datasets could be a valuable tool for monitoring reef structural complexity over large spatial scales