Can corals bleach every year without looking white?

Details

Researchers seasonally monitored five species of reef-building corals at three depths in the Bahamas for up to 4 years. They measured tissue biomass as ash-free dry weight and monitored symbiotic dinoflagellate density and chlorophyll a per cell or per cm² of coral surface area.

All tissue biomass and algal symbiont measurements reached their lowest values during the late summer–fall sampling periods. Symbiont density and pigment content were usually highest in winter, while tissue biomass most often peaked in spring; the researchers said this time lag implied a functional relationship between the variables.

Shallow-water corals often, but not always, had higher levels of the measured parameters than deeper corals, except for chlorophyll a content, which usually showed the opposite trend. Corals at all depths showed bleaching every year, defined as loss of symbiotic dinoflagellates and/or their pigments, regardless of whether they appeared white, tan, or mottled to the human eye.

The researchers speculated that seasonal changes in light and temperature affect algal and animal physiology and drive these patterns. They also hypothesized that tropical reef-building corals worldwide show similar predictable cycles in tissue biomass and symbiotic algae.