Seasonal mesophotic coral bleaching of Stylophora pistillata in the Northern Red Sea

Abstract

Coral bleaching occurs when environmental stress induces breakdown of the coral-algae symbiosis and the host initiates algae expulsion. Two types of coral bleaching had been thoroughly discussed in the scientific literature; the first is primarily associated with mass coral bleaching events; the second is a seasonal loss of algae and/or pigments. Here, we describe a phenomenon that has been witnessed for repeated summers in the mesophotic zone (40-63 m) in the northern Red Sea: seasonal bleaching and recovery of several hermatypic coral species. In this study, we followed the recurring bleaching process of the common coral Stylophora pistillata. Bleaching occurred from April to September with a 66% decline in chlorophyll a concentration, while recovery began in October. Using aquarium and transplantation experiments, we explored environmental factors such as temperature, photon flux density and heterotrophic food availability. Our experiments and observations did not yield one single factor, alone, responsible for the seasonal bleaching. The dinoflagellate symbionts (of the genus Symbiodinium) in shallow (5 m) Stylophora pistillata were found to have a net photosynthetic rate of 56.98-92.19 µmol O2 cm(-2) day(-1). However, those from mesophotic depth (60 m) during months when they are not bleached are net consumers of oxygen having a net photosynthetic rate between -12.86 - (-10.24) µmol O2 cm(-2) day(-1). But during months when these mesophotic corals are partially-bleached, they yielded higher net production, between -2.83-0.76 µmol O2 cm(-2) day(-1). This study opens research questions as to why mesophotic zooxanthellae are more successfully meeting the corals metabolic requirements when Chl a concentration decreases by over 60% during summer and early fall.

Figure 1. Experiment setup.

Black squares represent the two water tables, holding the eight experiment tanks and experiment systems.

Figure 2. S. pistillata bleaching at 60 m depth.

(a) Unbleached fragment during March 2010. (b) “Chicken pox” bleaching pattern during October 2010. (c) On the left, S. pistillata, on the right Mycedium sp. both bleached and alive, during September 2011. (d) Percentage of bleaching measured for 10 colonies in 60 m, surveyed along 2010. Colors represent percentage of bleaching; lighter colors represent higher parentage of bleaching. Gray line indicates average monthly PFD at midday.

Figure 3. Algae numbers and chlorophyll concentration.

Algae numbers (black) and Chl a concentration per algae (gray) for Symbiodinium sp. within S. pistillata (a) Collected at 5 m depth during May and October (b) collected at 60 m, September and October represent bleached and partially bleached fragments; March and May are not bleached. Scales of both y-axes in figures (a) and (b) are similar for ease of comparison. n = 5 per each month in each depth, bars represent standard deviation.

Figure 4. Photosynthesis and respiration rates for S. pistillata from 60 m.

Chl a fluorescence yield (a) and O₂ production/consumption (b, c), plotted to light intensity (FI curve and PI curve, respectfully) for S. pistillata from 60 m. October and September represent bleached and partially bleached colonies, March and May are not bleached. A shallow corals plot is added for comparison. n = 5 fragments from different colonies per each line. Legend for the three plots is in Figure 4(b). Bars represent standard deviation. (b) O₂ production/consumption plotted to light intensity (PI curve). Red rectangle includes PFD that is ecologically relevant for 60 m colonies and is enlarged in Fig. 4(c). (C) An enlargement of the low PFD area in the PI curve, relevant for 60 m. For better appreciation of the results, only selected fragments are presented.

Figure 5. Environmental gradients in the north Red Sea water column.

The environmental parameters (a) temperature, (b) Chl a, (c) total organic nitrogen and (d) logarithm of PAR in the Gulf of Eilat. Parameters are plotted along a 140 m depth gradient, during March (red), May (green), August (blue) and November (brown). Dotted line highlights 60 m depth.

Figure 6. Results of tank experiment.

(a) Chl a concentration per algae (in black) and (b) algae numbers (in gray), of S. pistillata subjected to six different treatments (chilling the tank water to 21°C, hitting the tank water to 29–30°C, feeding once a day with Artemia nauplius, filtration of sea water via 0.2 µm mash creating conditions of starvation, High PFD of ∼135 µmol quanta cm⁻² s⁻¹ and Low PFD of ∼20 µmol quanta cm⁻² s⁻¹), T0 represent a measurement taken on the day the fragments were plucked, “Ctrl” is the control and “Trans” are the fraq1agments from transplantation experiment. n = 5 fragments per each treatment, control treatment includes 10 fragments. Bars represent standard deviation.