Award: OCE-1642311

Sediments, nutrients and pollutants associated to coastal development and terrestrial runoff represent a current threat for about 50% of the world coral reefs. The negative effects of reducing the water quality on reef corals have been extensively documented. Varadero reef, located in one of the openings of Cartagena de Indias bay (Colombia), has a strong water column stratification characterized by a superficial layer of aprox. 1 m of sediments that results in high vertical light attenuation. However, below the murky waters a reef thrives, with a high coral cover (45.1% ± 3.6), and high diversity. Between 2.5 and 6.5 m depth the coral community dominated by the mountainous reef building star coral (Orbicella faveolata) with coral covers up to 80%. The current distribution of O. faveolata restricted to shallow depths in Varadero compared to other sites in the Caribbean and the consistent pattern of partial mortality of colonies with depth, evidence the serious threat that poor optical properties of water represents for the bioenergetics of symbiotic corals. Improving the optical properties of water may not only favor the survivorship and propagation of keystone reef-building coral species, but also enhance their resistance to other large-scale stressors related to global change. Coral sexual reproduction is vital to the persistence of coral populations, hence coral reef ecosystems. To succeed and in order to optimize fertilization, corals have adapted to environmental cycles to release eggs and sperm into the water column at the same time. In the Caribbean, synchronized spawning events occur per species, and its timings seems to be related to the moon cycle, water temperature and duration of the day. O. faveolata is a broadcast spawner that reproduces, in Southwestern Caribbean, in August or September, 6-7 days after full moon and between 21:30 and 22:30. Varadero?s O. faveolata fecundity and fertility data are similar to other Caribbean reef areas, however the spawning time differs for almost two hours even from nearby reefs (e.g., Baru), possibly as a result of the surface sediment layer and the effect on the light attenuation. Ecological implications of the early spawning event could be related to lower cross fertilization with nearby populations. Coral microbiomes show high specificity to their host but they are also affected by their surrounding environment. The microbiome interacts with the host as a first shield against pathogens but also assisting in nutrient cycling. Given the steep gradient in turbidity in the Bay of Cartagena, coral microbiomes are expected to change. Preliminary analyses suggest that O. faveolata in Varadero reef has a different microbial community profile relative to reefs from less turbid waters. The Varadero microbiome exhibits an increase in microbial taxa often associated with anthropogenic waste. Last Modified: 10/30/2019 Submitted by: Monica Medina