Long-term monitoring of algal symbiont communities in corals reveals stability is taxon dependent and driven by site-specific thermal regime

Survival trajectories for coral reefs under climate change may depend in part on shifts in the composition of their algal symbiont communities (Symbiodinium spp.). Shifts favoring thermotolerant symbionts have been recorded in response to mass bleaching events but rarely tracked through time. A 10 yr monitoring study of Symbiodinium in a variety of Kenyan corals assessed their variability through time, across coral taxa and between sites, and their relationship to environmental conditions. Coral genera varied significantly in their propensity to host thermotolerant symbionts of Symbiodinium clade D, with some genera becoming dominated by clade D at annual maximum temperatures of 32 degrees C but others showing clade D only rarely at 35 degrees C. High annual maximum temperatures, high standard deviation, positive skewness and positive kurtosis characterized sites where clade D was common. In corals whose symbiont communities were thermally labile (e. g. Pocillopora) an increase in maximum annual temperature from 30 to 35 degrees C resulted in 3- to 4-fold increases in dominance by clade D. There was no directional change in symbiont communities over the study period, but there was evidence for a similar to 6 yr decline in the incidence of mixed (C + D) communities following the 1998 bleaching event. These data illustrate how acute and chronic thermal stress caused by oceanographic and tidal oscillations interact to produce highly dynamic symbiotic communities. The clade D niche is a function of the environment and host taxon and, through a variety of mechanisms, is expected to expand with climate warming. Corals from warm and variable conditions represent conservation priorities because they establish a niche for these symbionts in contemporary reef environments.