Linking population size structure, heat stress and bleaching responses in a subtropical endemic coral

Abstract

Anthropocene coral reefs are faced with increasingly severe marine heatwaves and mass coral bleaching mortality events. The ensuing demographic changes to coral assemblages can have long-term impacts on reef community organisation. Thus, understanding the dynamics of subtropical scleractinian coral populations is essential to predict their recovery or extinction post-disturbance. Here we present a 10-yr demographic assessment of a subtropical endemic coral, Pocillopora aliciae (Schmidt-Roach et al. in Zootaxa 3626:576–582, 2013) from the Solitary Islands Marine Park, eastern Australia, paired with long-term temperature records. These coral populations are regularly affected by storms, undergo seasonal thermal variability, and are increasingly impacted by severe marine heatwaves. We examined the demographic processes governing the persistence of these populations using inference from size-frequency distributions based on log-transformed planar area measurements of 7196 coral colonies. Specifically, the size-frequency distribution mean, coefficient of variation, skewness, kurtosis, and coral density were applied to describe population dynamics. Generalised Linear Mixed Effects Models were used to determine temporal trends and test demographic responses to heat stress. Temporal variation in size-frequency distributions revealed various population processes, from recruitment pulses and cohort growth, to bleaching impacts and temperature dependencies. Sporadic recruitment pulses likely support population persistence, illustrated in 2010 by strong positively skewed size-frequency distributions and the highest density of juvenile corals measured during the study. Increasing mean colony size over the following 6 yr indicates further cohort growth of these recruits. Severe heat stress in 2016 resulted in mass bleaching mortality and a 51% decline in coral density. Moderate heat stress in the following years was associated with suppressed P. aliciae recruitment and a lack of early recovery, marked by an exponential decrease of juvenile density (i.e. recruitment) with increasing heat stress. Here, population reliance on sporadic recruitment and susceptibility to heat stress underpin the vulnerability of subtropical coral assemblages to climate change.

Size structure of the coral Stylophora pistillata across reef flat zones in the central Red Sea

“…For each colony we also assigned a color morph category: yellow, purple, or mixed colors, if the colony was partially both colors. We also calculated the proportion of juvenile colonies based on previous studies that consider colonies of S. pistillata and related species less than 10 cm 2 2 planar area likely to be juveniles 11 , 40 . Finally, summing the total area of all colonies allowed us to calculate the percent cover of S. pistillata for each reef flat zone to compare with colony density.…”

“…Therefore, simple metrics such as size structure can offer a more in-depth “snapshot” of the state of a population. Often, size structure is used to infer the state of coral populations under anthropogenic stressors, such as pollution 7 , climate change 8 – 11 , and fishing pressure 12 , 13 . Therefore, in an era of intense degradation as a result of local and global disturbances, a more comprehensive evaluation of the status of coral reefs, potential causes of decline, and projected trajectories is essential to guide mitigation and conservation measures.…”

Transient demographic approaches can drastically expand the toolbox of coral reef science

“…Kayal et al 2018;Cant et al 2021c). Accordingly, there is a growing appreciation for how the size structure of coral populations influences their dynamics and endurance Meesters 1998, 1999;Anderson and Pratchett 2014;Dietzel et al 2020;Lachs et al 2021), with recent appeals for efforts to be concentrated towards gauging the resilience of coral communities (Pisapia et al 2020). However, we argue here that the contemporary application of statestructured demographic tools in evaluating the viability of coral populations, and their communities, does not reflect our need to anticipate their resilience.Prevailing knowledge of coral population dynamics centres around long-term (i.e.…”

SizeExtractR: A workflow for rapid reproducible extraction of object size metrics from scaled images

“…These include published studies such as the examination of coral population size structure, heat stress and mass coral bleaching (Lachs et al, 2021), and the determination of size spectra and inferred growth of nursery-reared and field-planted corals (Humanes et al, 2021) ing and mortality (Figure 3a). In planning the image analysis and ROI labeling system for this study, it was important to consider which variables were to be recorded.…”

“…We conducted an image analysis time trial on a subset of benthic images (N = 40) from Lachs et al (2021) to the timesaving benefits of using SizeExtractR-macros, compared to a fully manual method using default ImageJ tools. Each image was analyzed using both methods, all corals per image were annotated, and the image analysis time per image was recorded.…”