SCIENCE

A phenomenon explaining why fertile coral reefs form in the oceanic abyss has finally been proved correct. (Geographical)

Use our resources to better understand these oases in the ocean.

Teachers, scroll down for a quick list of key resources in our Teachers’ Toolkit.

Discussion Ideas

• The Geographical article suggests that long-held assumptions are correct and oceanographers have solved “Darwin’s Paradox.” What is Darwin’s Paradox?
  • Darwin’s Paradox describes an anomaly of the ocean—the dazzling diversity of coral reefs in the middle of stark abyssal plains. “While most of the open ocean is very low in nutrients, the isolated hotspots where coral reefs form are instead teeming with the energetic resources that stimulate marine ecosystems.” Paradox: How did they get that way?

 

• What is a coral reef?
  • Coral reefs are incredibly biodiverse ecosystems made up of multicolored limestone ridges built by tiny sea animals called corals. The corals’ hard exoskeletons are what make up coral reefs.

 

• Key to solving Darwin’s Paradox is an understanding of how “as a reef system starts to form, there are changes in the circulation of flow around that physical structure.” How does a coral reef form?
  • Charles Darwin, the all-star naturalist for whom the paradox is named, developed his theory about the formation of coral reefs in his imaginatively titled book The Structure and Distribution of Coral Reefs. (Read it here!)
  • Darwin thought the development of coral reefs was largely due to volcanic and geologic activity on the seafloor. Corals formed in the shallow water around volcanic islands and sites of tectonic uplift or subsidence. Shallow water is crucial: Most hard corals need clear water and sunlight to develop and flourish.

 

• The research paper examines coral reefs associated with two different ecosystems: islands and atolls. What is the difference?
  • Coral islands are lovely, low-lying islands whose land is made up primarily of organic material associated with coral. Coral islands are often associated with three types of coral reefs:
    • Atolls are ring-shaped coral reefs or strings of tiny coral islands that surround a central lagoon.
    • Barrier reefs are ridges of coral found parallel to the coast of an island or continent, but separated from it by a deep lagoon.
    • Fringing reefs extend from a coastline, with no real separation from the shore.

 

• The development of coral reefs largely dependent on the availability of phytoplankton. What are phytoplankton?
  • Phytoplankton are microscopic marine organisms that can convert light energy to chemical energy through photosynthesis, just like land plants.
  • Phytoplankton are the primary food of most corals, and are ultimately the basis of the entire ocean food web.

 

• What is the island mass effect (IME)?
  • The IME describes a variety of factors that reinforce each other to create ideal conditions for coral reef formation.

 

• IME largely solves Darwin’s Paradox by explaining how “differing factors work together in the near vicinity of oceanic islands, reinforcing each other and creating a positive feedback system that accumulates and retains the nutrients required to stimulate phytoplankton production and coral reef development.” These factors include geomorphology and bathymetry, upwelling and internal waves, and even the activities of organisms. Can you define each of these factors?
  • Geomorphology describes the geographic features of the seafloor and the forces that create them, while bathymetry describes the shape (heights and depths) of the seafloor.
  • Upwelling describes the process by which currents bring cold, nutrient-rich water to the ocean surface. A coral reef’s internal waves are generated from tidal currents interacting with underlying bathymetry.
  • Activities engaged in by reef organisms include birth, death, feeding, hunting, and reproducing.

 

• How do bathymetry and geomorphology contribute to a coral reef’s IME?
  • Geomorphology and bathymetry influence a region’s IME by determining the area of seafloor exposed to sunlight and clear water. Coral reefs forming on gentle slope gradients have much larger and more biodiverse ecosystems than those forming in regions with steep slopes.

 

• How do upwelling and internal wave production contribute to a coral reef’s IME?
  • Upwelling influences a region’s IME by delivering vital nutrients to the clear, usually nutrient-poor sunlight zone. Internal waves more readily reach shallower waters and fuel phytoplankton production where the underlying slope is more gradual. “If you have a very gradual slope, it facilitates the movement of water up into the shallows and with it cooler, nutrient-rich waters from the deep,” explains one of the study’s authors.

 

• How do animals such as birds and fish contribute to a coral reef’s IME?
  • Birds and fish create biologically rich nutrients—poop, shells, dead organisms. “Animal waste products, such as those derived from sea-bird guano, reef-associated fishes and mobile marine invertebrates also enhance nutrient concentrations in coral reef ecosystems.”

 

• Can human activity contribute to a coral reef’s IME?
  • Yes. Runoff from agricultural and industrial activities can initially support reef development by delivering nitrogen-rich nutrients to the reef. But “after a certain point it appears to do the opposite. ‘When and how humans become a negative effect is yet to be fully quantified,’” says one of the study’s authors.

 

TEACHERS’ TOOLKIT

Geographical: Solving ‘Darwin’s Paradox’

Nat Geo: Coral Reef Food Web illustrations

(extra credit!) Nature: Near-island biological hotspots in barren ocean basins