Hyperspectral and physiological analyses of coral-algal interactions

Abstract

Space limitation leads to competition between benthic, sessile organisms on coral reefs. As a primary example, reef-building corals are in direct contact with each other and many different species and functional groups of algae. Here we characterize interactions between three coral genera and three algal functional groups using a combination of hyperspectral imaging and oxygen microprofiling. We also performed in situ interaction transects to quantify the relative occurrence of these interaction on coral reefs. These studies were conducted in the Southern Line Islands, home to some of the most remote and near-pristine reefs in the world. Our goal was to determine if different types of coral-coral and coral-algal interactions were characterized by unique fine-scale physiological signatures. This is the first report using hyperspectral imaging for characterization of marine benthic organisms at the micron scale and proved to be a valuable tool for discriminating among different photosynthetic organisms. Consistent patterns emerged in physiology across different types of competitive interactions. In cases where corals were in direct contact with turf or macroalgae, there was a zone of hypoxia and altered pigmentation on the coral. In contrast, interaction zones between corals and crustose coralline algae (CCA) were not hypoxic and the coral tissue was consistent across the colony. Our results suggest that at least two main characteristic coral interaction phenotypes exist: 1) hypoxia and coral tissue disruption, seen with interactions between corals and fleshy turf and/or some species of macroalgae, and 2) no hypoxia or tissue disruption, seen with interactions between corals and some species of CCA. Hyperspectral imaging in combination with oxygen profiling provided useful information on competitive interactions between benthic reef organisms, and demonstrated that some turf and fleshy macroalgae can be a constant source of stress for corals, while CCA are not.

Figure 1. Examples of boundaries between coral and algae.

A) Pocillopora sp. vs. Montipora sp., B) algae vs. Montipora sp., C) Montipora sp. and various algae, D) crustose coralline algae vs. crustose coralline algae, and E) diverse interactions including coral, fleshy algae, crustose coralline algae, and other invertebrates.

Figure 2. Summary of interactions between corals and algae from surveys of Millennium Atoll.

A) Total number of interactions between corals and algae, B) outcome of coral-algal interactions by algal group.

Figure 3. Comparison of spectra from different algal and coral groups.

A) Relative reflectance of different algae: CCA, red alga, Bryopsis sp., and turf algae. B) Relative reflectance of 4 different corals: Montipora sp. (2), Pocillopora sp., Porites sp., and Acropora sp. C) Slope of coral and algal reflectance spectra, including CCA, red alga, Bryopsis sp., Pocillopora sp., Porites sp., and turf algae.

Figure 4. Different renderings of hyperspectral images of the interaction zones between the coral Pocillopora sp.and red alga Gracilaria sp.

A) The location of algae (A), coral (C) and cyanobacteria (cy) are indicated in the True Color image. The color coding is determined for True Color using the three bands red (640 nm), green (550 nm), and blue (460 nm); GOC using the three bands green (515 nm), orange (575 nm), and chlorophyll (685 nm); Color IR using the three bands green (550 nm), red (650 nm), and infrared (IR; 860 nm); SR (Simple Ratio) using the ration between 800 nm and 680 nm; NVDI (Normalized Vegetative Density Index) using IR (800 nm) and red (680 nm) in the formula (IR − red)/(IR + red); and ARVI (Atmospherically Resistant Vegetative Index) using the bands IR (800 nm), red (680 nm), and blue (450 nm) entered in the formula (IR − 2*red + blue)/(IR+ 2*red − blue). Each rendering uses a 2% Stretch Contrast Enhancement. B) The slope of the average relative reflectance for the algae, coral, and cyanobacteria imaged in A. Areas used to determine the average reflectance are indicated by hexagons in the IR rendering.

Figure 5. Dissolved oxygen profiles of coral interaction zones.

Interactions measured were Pocillopora verrucosa vs. Gracilaria sp. (n = 6), Pocillopora verrucosa vs. red turf algae (n = 7), Montipora spp. vs. red turf algae (n = 4), Montipora sp. vs. white band (n = 2), coral vs. CCA (n = 6), and coral vs. coral (n = 1).