Light availability along depth gradients: a key driver of niche partitioning and biodiversity patterns in reef building corals
Open Access
- Author:
- Lopez Londono, Tomas
- Graduate Program:
- Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 22, 2022
- Committee Members:
- Roberto Iglesias-Prieto, Major Field Member & Dissertation Advisor
Jonathan Lynch, Outside Unit & Field Member
Todd Lajeunesse, Major Field Member
Monica Medina, Chair of Committee
Elizabeth Mcgraw, Program Head/Chair - Keywords:
- Coral reefs
physiology
ecophysiology
depth gradient
biodiversity patterns
species-energy
productivity-biodiversity
habitat specialization
turbidity
water optical properties
light gradient
Symbiodiniaceae - Abstract:
- The ecological success of corals in oligotrophic tropical environments since the Late Triassic is attributed to the nutritional advantages derived from the symbiosis with photosynthesizing dinoflagellates (Muscatine and Porter 1977) and their extraordinary efficiency in collecting and using solar energy for carbon fixation (Enríquez et al. 2017). The energetic gains conferred by the photosymbiosis significantly enhances coral calcification (Goreau and Goreau 1959; Colombo-Pallotta et al. 2010) which, ultimately, enabled the consolidation of one of the most biodiverse ecosystems on the planet: coral reefs (Perry et al. 2008). Due to the sunlight dependence to power metabolism, symbiotic corals are restricted to the photic zone of the world’s oceans where light is enough to allow photosynthesis (Kirk 2011). Importantly, because of the exponential extinction of light with depth, their vertical distribution in the photic zone occurs along steep gradients of irradiance in typically small spatial scales, such as dozens of meters, while other limiting resources and physical factors remain relatively constant (Stoddart 1969; Lesser et al. 2009). These traits, together with the great dispersal potential associated with a planktonic larval stage in the life cycle, make scleractinian corals a particularly interesting taxa to explore the effects of light availability on global patterns of biodiversity and the spatial organization of ecological communities, while avoiding potential confounding factors such as scale-dependent effects and dispersal boundaries of species (Roberts et al. 2019). The optical properties of the water column, correlated with the vertical attenuation coefficient for downwelling irradiance (Kd) (Kirk 2011), determine to a great extent the characteristics of the underwater light field and the depth of the photic zone. For example, the lower limit of the photic zone in turbid-waters with high Kd (e.g., Varadero with Kd of 0.193 m-2 (López-Londoño et al. 2021)) is estimated to be nearly 20 m, while in clear-waters with low Kd (e.g., the Red Sea with Kd of 0.047 m-2 (Kahng et al. 2010)) it can be almost 100 m. Moreover, the spectral composition of the underwater light field also changes according to the local Kd, typically because a wavelength-selective absorption towards the blue end of the spectrum in waters with high Kd (Maritorena and Guillocheau 1996; López-Londoño et al. 2021). This illustrates the importance of considering the local optical properties of the water column not only for determining the maximum depth that can sustain symbiotic corals, but also for exploring the association between depth, local light climate, photoacclimation status, and structure of symbiotic coral communities. The Orbicella spp. complex is a major reef builder in the Caribbean exhibiting a vertical distribution that spans the entire photic zone (Weil and Knowlton 1994; Pandolfi and Budd 2008). This species complex was originally regarded as one species with ecotypic variation, but recent research revealed the existence of three species partially segregated by depth (Fukami et al. 2004; Levitan et al. 2011). O. annularis (Ellis and Solander, 1786) forms disjunct columns with senescent edges, being consistently found in shallow-waters between 1 m and ~20 m. O. franksi (Gregory, 1895) forms irregular mounds and plates and is typically found deeper than its two sibling species (up to depths of 60 m). O. faveolata (Ellis and Solander, 1786) forms massive mounds and can partially overlap with both O. annularis and O. franksi habitats (Weil and Knowlton 1994; Pandolfi and Budd 2008). The symbiotic dinoflagellate communities (Rowan et al. 1997; LaJeunesse et al. 2009; Kemp et al. 2015) and the photobiology of this species complex have been extensively studied (Warner et al. 2006; Scheufen et al. 2017; Prada et al. 2021), enabling the identification of important differences determined by light availability within colonies and across depths. Therefore, these recently diverged coral species offer an ideal system to study how corals specialize to live in different habitats across depth-mediated light gradients and the importance of niche specialization for the maintenance of corals diversity. In my dissertation, I have studied the effects of the physical interaction between light, the water column and corals at multiple levels of biological organization, from individuals and populations to reef coral communities. I considered physical and physiological principles that govern the vertical distribution of light in the water column and the photosynthetic activity of primary producers, including the symbiotic algae of corals. Some of my research has been focused on Orbicella spp. because of the ecological, morphological and physiological traits that characterize this species complex. In the first chapter, I tested the predictions of the species-energy hypothesis for explaining biodiversity patterns in symbiotic coral communities across depth gradients. In the second chapter, I explored the physiological and ecological consequences of the water optical properties degradation associated with anthropogenic pollution on reef corals. And lastly, in the third chapter, I studied the physiological strategies and photoacclimation potential leading to niche partitioning driven by light gradients in sibling species of corals.