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

## TL;DR

This study examines how marine heatwaves and human activities affect young corals, finding that both stressors significantly reduce their numbers.

## Contribution

The study provides new in situ evidence of how combined chronic human disturbance and marine heatwaves impact juvenile coral assemblages.

## Key findings

- Juvenile coral densities were 72% lower at highly disturbed sites compared to less disturbed ones before the heatwave.
- The heatwave caused a 49% loss of juvenile corals, with competitive and weedy species declining more than stress-tolerant ones.
- Juvenile corals showed lower bleaching and mortality than adults during the heatwave but still faced significant losses.

## Abstract

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 Niño. This marine heatwave caused prolonged heat stress and devastating losses of coral cover on the shallow forereef’s of Kiritimati, in the central equatorial Pacific Ocean. Here, we enumerated a total of 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 m-2) compared to the least disturbed ones (15.3 ±  3.8 m-2). 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 (49%) of all juvenile corals, with those corals with competitive or weedy life history strategies 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 chronic local anthropogenic and marine heatwaves on juvenile coral densities.