Impacts of a prolonged marine heatwave and chronic local human disturbance on juvenile coral assemblages

Coral reefs are threatened by climate change and chronic local human disturbances. Although some laboratory studies have investigated the effects of combined stressors, such as nutrient enrichment and heat stress, on growth and survival of early life stage corals, in situ studies remain limited. To assess the influence of multiple stressors on juvenile corals, we quantified densities of corals [≤]5 cm at 18 forereef sites with different exposure levels to underlying chronic local human disturbance before, during, and after the 2015-2016 El Nino. This marine heatwave caused prolonged heat stress and devastating losses of coral cover at our study site, Kiritimati, in the central equatorial Pacific Ocean. In total, we enumerated 7732 juvenile corals from 13 different families. Over 80% of corals were from four families: 70% from Agariciidae, Merulinidae, or Poritidae, which all have stress-tolerant life history strategies, and 11% from Acroporidae which has a competitive life-history strategy. Both local disturbance and heat stress were significantly negatively related to juvenile coral densities. Prior to the heatwave, juvenile densities were on average 72% lower at the most disturbed sites (7.2 {+/-} 1.9 m2) compared to the least disturbed ones (15.3 {+/-} 3.8 m2). Overall, juvenile corals had a lower bleaching prevalence and lower mortality during the heatwave when compared to their adult counterparts. Still, the heatwave resulted in the loss of half (50%) of all juvenile corals, with competitive and weedy life history strategy corals undergoing greater declines than stress-tolerant ones. Although juvenile coral densities increased slightly in the year following the heatwave, the effect was statistically non-significant. Our results highlight the influence of local chronic anthropogenic and climate change-driven marine heatwaves on juvenile coral densities.