Reefs

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Reef Topography

Coral reefs have several different forms, depending on their age and level of development [1].

  • Fringing reefs are the first stage of a reef's development. They form on coastlines and grow away from the shore, getting wider and wider. Waters over the reef are shallow and not navigable except for shallow draft boats.
  • Barrier reefs develop when a navigable channel or lagoon forms inshore of a fringing reef. The reef itself can have small islands, or motus, made from coral sand and rubble.
  • Atolls form when an island with fringing and barrier reefs subsides beneath the sea surface. If the reefs continue to grow, the island will become nothing but a large barrier reef, possibly with navigable passes, surrounding a lagoon.

Within a reef, there are several distinct regions [2].

  • The reef crest is the shallowest part of the reef, absorbing the force of breaking waves.
  • Reef slopes are outside the reef crest, progressively dropping out into deeper and deeper water before coral growth peters out.
  • Reef flats are shallow areas inside the reef crest.
  • Lagoons are protected navigable waters inside barrier reefs and atolls.

Navigable Atoll Observations: Tuamotus

Tuamotus Lagoon and Reef

This section has observations from visits in 2017 and 2018 to areas in several of the major atolls in French Polynesia's Tuamotu archipelago: Makemo, Fakarava, Tahanea, Faaite, and Toau. These atolls are all fairly large and contain navigable passes and deep lagoons.

These observations are general impressions, not meant to be rigorous or comprehensive.

  • Reef slopes usually follow a shallow incline to 10-15 m before dropping off sharply. Coral in these areas is almost always really nice. Visibility is almost always excellent (40m+), except around ebbing passes.
  • Reef flats vary quite a bit. Along edges where there is no motu (mostly the southern edges of the atolls), wide reef flats separate the reef crest from the lagoon proper. These can have some coral, reef fish, sharks, etc., but can also have little visible life besides many, many sea cucumbers. Along edges with motus (mostly the northern edges of the atolls) there is generally a very shallow and relatively narrow reef flat.
  • Shallow lagoon areas inshore of motus or the reef flat also vary a lot. They can be very narrow or very wide, and have either sand or mixed sand and limestone with varying amounts of coral, though never anywhere near as nice as the coral on the reef slopes. Visibility can be good if they are near a reef flat that is flushing a lot of water into the lagoon.
  • Deep lagoon areas (depth 10m+) are dominant in the inshore areas of all observed atolls. These can have a sand bottom, though usually the bottom has scattered blocks of limestone with small amounts of coral and lots of fish.
  • Bommies pockmark many of the deep lagoons. These are coral heads growing up from the lagoon bottom to the surface or just below it. The top 3m or so usually has good coral, and the rest of the bommie usually doesn't have much, though fish life is good.
  • Passes usually have walls or sloping walls of really nice coral on either side, with sand and limestone without much coral on the bottom. Some passes have little coral on the walls, or lots of coral on the bottom.

The weather and tides have a large effect on the atoll.

The Tuamotus are hammered by swells year round: southerly swells in the winter and weaker northerly swells in the summer. Southeast trade winds are strongest in the winter, and winds are more variable at other times of the year. The combination of wind and swell pushes a lot of water into the lagoon, so that reef flats connected directly to the lagoon almost always have a strong inflow of water. This current extends to nearby shallow lagoon regions and (to a lesser extent) deep lagoon regions.

Tidal range is small, ~.3m, but the large area within the lagoons and small passes can make for very strong tidal currents in the passes, up to several knots. Passes can have a normal flood/ebb cycle, a reduced flood and extended ebb, or a continuous ebb, depending on the pass and on how much water is entering the atoll's edges from the swell and wind.

The easiest and safest places to install equipment or perform other environmental modifications here would be in the shallow or deep lagoon regions. The reef slope can often be accessed, but is more difficult and dangerous when winds are heavy (frequent during the winter) or in areas far from a pass. Some passes don't experience strong currents often and could have equipment installed, but other passes experience strong and unpredictable currents almost all the time and would be poor locations for doing any work.

Tropical cyclones occasionally hit the Tuamotus, though historically only during strong El Ninos. It seems like a cyclone could damage or destroy pretty much any installed equipment, though deeper lagoon waters may be relatively safe.

Lagoon areas seem like the most promising area to investigate here. They are easy to access, unaffected by surge or runoff, are predictably affected by currents, and can have very clear water. A big outstanding question is why the corals are so much less extensive here than on the reef slopes, however. It seems like there are several possible explanations, but it is not clear which is correct. It would be really neat to establish a reef inside the lagoon of similar quality to the reefs outside the lagoon.

Barrier Reef Observations: Society Islands

Moorea Lagoon and Reef

This section has observations from visits in 2018 to many of French Polynesia's Society Islands: Moorea, Raiatea, Tahaa, Bora Bora, and Maupiti. These are volcanic islands surrounded mostly by barrier reefs and fringing reefs. The reefs here are similar in many respects to those in the Tuamotus, with some differences noted below.

These observations are general impressions, not meant to be rigorous or comprehensive.

  • Reef slopes are usually pretty nice, with some comparable in quality to reef slopes in the Tuamotus. Visibility is not as good as in the Tuamotus.
  • Lagoons tend to be more turbid than in the Tuamotus, with less coral and more macroalgae. Some areas can still be very clear, if they are flushed with water coming over reef flats.
  • Passes have worse coral than passes in the Tuamotus.

Water flow patterns are different from the Tuamotus. The lagoons in the societies are much smaller than in the Tuamotus, on account of there still being large islands present. As a result, there is less water exchange due to the tides, and passes have weaker currents. Many passes seem to always ebb.

References

  1. Charles R. C. Sheppard, Simon K. Davy, Graham M. Pilling, Nicholas A. J. Graham, The Biology of Coral Reefs, Oxford University Press (2018), pp 20
  2. Charles R. C. Sheppard, Simon K. Davy, Graham M. Pilling, Nicholas A. J. Graham, The Biology of Coral Reefs, Oxford University Press (2018), pp 24