High incidence of partial colony mortality constrains realized growth for three coral species in southeast Florida

Coral colony growth rates are a key demographic trait which can reveal fundamental changes in population health and resilience. With changing environmental conditions, assessing spatial, temporal and taxonomic variation in net coral growth is fundamental to understanding the changing structure of coral assemblages, particularly in high-latitude reefs which may provide refugia against climate change impacts. The high-latitude Southeast Florida Coral Reef Ecosystem Conservation Area (ECA) recently experienced mass coral mortality from heat stress and disease, and most remaining coral colonies are small. It is unclear whether the preponderance of small colonies is due to slow growth rates, high partial colony mortality or high colony turnover. To assess this, the net growth rates of 3 abundant species were quantified interannually between 2000 and 2020. Of 136 tracked colonies, 33% died and a further 61% had partial mortality. Small Montastraea cavernosa and Porites astreoides colonies generally grew faster than large colonies and had less partial mortality, but Siderastrea siderea consistently had high partial mortality. M. cavernosa and S. siderea net growth rates generally declined following excess heat stress or disease outbreaks, which caused partial and whole colony mortality, but growth rates in all species were negligible even during inter-disturbance periods. Maximum annual growth rates of each species aligned with those found in the Caribbean, but mean growth rates were low and confidence intervals included zero (linear extension, mean +/- SD: M. cavernosa = 0.03 +/- 0.6 cm yr-1; P. astreoides = 0.2 +/- 0.6 cm yr-1; S. siderea = 0.1 +/- 0.3 cm yr-1). As a result, coral growth and colony size is significantly constrained, preventing reef growth. Without profound changes in the environmental conditions, recovery capacity will continue to be severely limited.