Award: OCE-1617255

Following the unprecedented sponge bleaching event observed in October of 2015, a 16 month-long project examined the effects of environmental stressors on the symbiosis between Cliona varians and Symbiodinium. The focus of the project was on the integrated (holobiont) trophic ecology of the symbiosis. The first goal was to determine whether the symbiont populations recovered from the bleaching event observed in deep-water C. varians forma incrustans. No samples were taken between October and January, so the rate of recovery is unknown. However, the symbiont populations rebounded from the October bleaching event to typical densities by January, 2016. Full bleaching recovery was estimated to have occured within a month based on anecdotal field reports. Isotopic signatures were measured in deep and shallow sponges across seasons. No isotopic anomaly of the bleaching event was detected in C. varians forma incrustans, which showed little seasonal variability in ∂C or ∂N values. Cliona varians forma varians, on the otherhand, showed significant shifts in stable isotope signatures across seasons. Sponges in shallow-water habitats became significantly depleted in C13 as the water warmed from winter to early summer. One reason for this shift may be that the symbiont translocated greater amounts of photosynthate (depleted in C13) to the sponge as the water warmed and photoperiod increased. It is also possible that the sponge derived more carbon from heterotrophic pools enriched for cyanobacteria. We observed shifts in the relative abundances of members of the bacterioplankton during the warm versus the colder months. Presumably, the cyanobacteria would have more negative C isotopic signatures, which might explain the patterns we observed. More work is needed in this area. Finally, the sponge may shift its biomass building strategies across seasons. Teasing apart these hypotheses will be the focus of future work. Through this work, 5 undergraduates at the University of Richmond received valuable research experiences. They will also eventually earn authorship on manuscripts produced from this work (with two in preparation). One of the students is currently in a PhD program at the University of Virginia, and another plans to matriculate in graduate school in the fall of 2018. This work is significant in that it emphasizes the integrated behavior of host and symbiont as it related to sponge feeding, and the results led to greater understanding of the role sponges and their symbionts play in coral reef communities. Last Modified: 10/04/2017 Submitted by: Malcolm S Hill