Seasonal patterns of tissue biomass and densities of symbiotic dinoflagellates in reef corals and relation to coral bleaching

TLDR

The study hypothesizes that tropical reef‑building corals worldwide exhibit predictable seasonal cycles in tissue biomass and symbiotic algae. The authors monitored tissue biomass and symbiont density and pigment in five coral species across three depths in the Bahamas over four years. Seasonal monitoring showed that tissue biomass and symbiont parameters were lowest in late summer–fall, peaked in winter (symbionts) or spring (biomass), shallow corals generally had higher values, and bleaching occurred annually across depths, indicating light and temperature drive these cycles.

Abstract

Tissue biomass (ash‐free dry weight) and symbiotic dinoflagellates (density, chlorophyll a cell −1 or cm −2 of coral surface area) of five species of reef‐building corals were monitored seasonally for up to 4 yr at three different depths in the Bahamas. The lowest values of all tissue biomass and algal symbiont parameters occurred during the late summer–fall sample periods. In contrast, the highest densities and pigment content of symbionts usually occurred during the winter, whereas tissue biomass peaked most often in the spring, the time lag implying a functional relationship between these variables. Corals living in shallow water often (but not always) had higher levels of all parameters measured compared to deeper corals, except chlorophyll a content, which usually displayed the opposite trend. The results show that corals from all depths exhibited bleaching (loss of symbiotic dinoflagellates and/or their pigments) every year, regardless of whether they appeared white, tan, or mottled to the human eye. We speculate that these patterns are driven by seasonal changes in light and temperature on algal and animal physiology. Furthermore, we hypothesize that all tropical reef‐building corals, world‐wide, exhibit similar predictable cycles in their tissue biomass and symbiotic algae.