Coral bleaching recovery dynamics and symbiodinium repopulation on indo-pacific reef systems

Tropical coral reefs are bleaching with increasing frequency, yet our understanding of what controls the pace at which expelled Symbiodinium re-establish within host tissue remains incomplete. This research tracked bleaching recovery on four Indo-Pacific reef systems off the coast of Zanzibar, Tanzania, over 24 months following the mass thermal event of early 2022. Symbiodinium cell density, photosynthetic efficiency (Fv/Fm), and live coral cover were measured at five reef zones on each site at quarterly intervals. Severely bleached colonies recovered to 78.3% of pre-bleaching symbiont density within 18 months, but recovery was strongly zone-dependent: upper-slope corals regained symbionts 2.4 times faster than reef-flat colonies exposed to higher irradiance and temperature variance. Clade D Symbiodinium dominated the initial recolonisation phase regardless of the original clade composition, being detected in 84.6% of recovering colonies at month 6. By month 18, however, clade C had reasserted dominance in 62.1% of upper-slope colonies. Live coral cover at the most damaged site (Mnemba) dropped from 53.7% to 11.8% and recovered to only 31.4% within the monitoring window. These findings highlight that reef position and symbiont-community reshuffling jointly regulate recovery speed and that a full return to pre-disturbance states may take well beyond two years.