Mike Kirda

Reef Zones

June 21st, 1998 on #reefs

Hello, My name is Mike Kirda, and I am here to give a talk entitled

"Reef Zonation: Implications for the Reef Aquaria". This is going to be a

multimedia talk, so get ready and fire up those browsers- I'll be giving you the

URL's in the text of the talk. Also, below, I make reference to an illustration in

The Reef Aquarium, Vol. 1, so you may want to have that in a handy place as well. I have a

LOT of material to cover, so let's dive right in, shall we?

Introduction

First of all, let me take just a bit of time to explain where this idea

originally came from. I am pretty active on the Reefkeepers mailing list. A few months

back, there was a rather heated discussion on lighting, you know, the MH vs. VHO type of

discussion. I think it originally stemmed from a newbie on the list asking "Which are

better VHO or MH?".

Answer: It depends... Mainly upon the animals you want to keep.

Part of the blame goes to JC and Sanjay Mani, who both really got me thinking about

this Reef zonation concept. The more I thought about it, the more important it seemed- It

is so simple that, in many ways, it would seem to have been overlooked by the reef keeping

community. The essence would be to build a tank around a specific reef zone. The ultimate

purpose would be to better re-create these conditions in our reef tanks. Not as simple as

it sounds, as I would discover...

On the list, I so often see these ad hoc recommendations on lighting and water

movement. As I delved deeper into this, the more these crazy recommendations bothered me.

I kept thinking about Veron's book, Corals of Australia and the Indo-Pacific, and the

wealth of info it contains. I began to think in terms of the animals, mostly hard corals,

that we keep and how little is truly known about their captive care requirements. How many

times we see the same questions, over and over... My coral isn't doing well... etc. etc.

I began to look at Veron's book once again to look for clues- What is different about

our tanks vs. the natural environment? As I looked at the descriptions of the "Reef

zones", I began to pull out my aquarium books, The Captive Reef, The Modern Coral

Reef Aquarium, The Reef Aquarium Vol.1 and 2, to see what, if any, information they give

on "reef zonation" and compared it to CAIP. Really, not one of the aquarium

literature books seemed to cover the zonation concepts well. So I began to search the

scientific literature, looking for more information. All this to back up a feeling I had,

that, as aquarists, we are really missing something here.

So, this is my attempt to pull together the information from many disparate sources

into one coherent whole.

Note: Once in a while, you will see me add a "{1}" to the text. For general

information, I am adding the reference in the text, but for several specific citations, I

am adding the equivalent of footnotes. If you are interested, the footnotes/references can

be found at: http://homepage.interaccess.com/~mlkjr/RZreferences.htm.

Reef Zonation

The "reef zonation" concept is pretty widely accepted within the

scientific community. Corals often seem to have a preference for one area of the reef over

another and the zones are rather consistent throughout the world. This is not always true-

there are many species with show no such affiliation, however, you can often tell what

area of the reef they come from just by looking at their shape or morphology.

FACE="Verdana">

Reef types

When we talk about reefs, the first thing that we need to understand is that

"the reef" consists of many diverse habitats. The three most common types of

reefs are Fringing reefs, Barrier reefs and Atolls.

For good illustrations, please see Delbeek and Sprung's The Reef Aquarium, Volume 1,

pages 24 and 25. See also

href="http://homepage.interaccess.com/~mlkjr/Zones.htm">http://homepage.interaccess.com/~mlkjr/Zones.htm

FACE="Verdana" SIZE="3">.

Fringing reefs typically fringes a rather large landmass. Think about the

stereotypical tropical island. There is a beach, which leads to a sand flat. As you go

further out into the water, you reach a lagoonal area, which may or may not contain patch

reef. Soon, you'll encounter a reef flat. Corals can reach up to the surface at low tide.

Soon you will reach the reef crest. This is the part of the reef the waves will break on.

This area gets pounded by waves. Past the reef crest, typically there is a slope down, the

fore reef slope. Note that this area can be filled with spur and groove formations,

usually down to 10 or 20 meters. The reefs themselves have to often contend with

terrigenous silts and muds.

 Barrier reefs- Again we usually have a large land mass. Here, however, the

major difference is the larger and deeper lagoonal area behind the reef crest. Behind the

reef crest in a barrier reef, there usually is a back reef slope instead of a reef flat.

Nearer to land, it is not atypical for there to be a fringing reef, although the reef

crest may not be so pronounced there.

Atolls- If you are really interested in atolls, Ron Shimek suggested Herold J.

Wiens' "Atoll Environment and Ecology". It is a very interesting book, if only

to see the photos of the devastation hurricanes can bring to the atoll environment. You

can see chunks of what would be reef crest or reef flat "rock" 5 to 8 feet

square carried 200 or 300 feet from the place they were ripped from just by wave action.

Think of all the corals destroyed just by rolling this chunk across a reef flat…

Atolls are usually reefs that are well removed from landmasses, though they may boast

small islands or cays. Interestingly, they are typically a coral reef that has grown on

the top of a submerged mountain. As the sealevels have changed over time, the coral growth

has managed to keep up with the rise and has reached sealevel. As there is no more room to

grow up, they have grown out. The middle erodes and forms a lagoon.

To visualize an atoll, think of a bowl submerged in water so that its lip is just at

the water's surface. In some atolls, the lagoon in the center is cut off from the ocean

almost entirely- visualize raising the bowl slightly… In others, the atoll has no

landmass whatsoever, so the lagoon may approach 100% water exchange daily. More often than

not, the atoll has some landmasses around the edges, with channels cut between them. Some

of the lagoons are small, others are ten to twenty miles across. Most are not round, like

our bowl, and the varieties of shapes are astounding.

Descriptions of Reef zones

This is covered to a certain degree by D+S, TRA, Vol.1 Pg. 16-23. Their descriptions

are valuable because they do include several genus that will tend to colonize those zones.

In comparing their zonation to J.E.N. Veron's Corals of Australia and the Indo-Pacific, it

becomes clear they took most of their descriptions from Veron. What makes Veron's

reference so much more intriguing is that he lists a lot more information on coral

species- including abundance and the reef zone it typically is found in. Wiens book covers

this area as well, but differently. I'll get into that more later… I list here what

is probably considered the standard.

 Generalized Outer reef

The lower slope

Depth: 20 to over 100 meters

Not really an area which most of our corals come from. Some species are taken down to

about 90 feet, but this is not that typical of a collection zone for corals. For brevity

sake, this section will be mostly ignored. I did find a bit of info on lighting levels-

Maximum PAR readings at 120 meter depth was 2.55 micro-Einsteins/m2/s (0.15% of surface

levels). Water temperatures ranged from 22.6 to 30 degrees Celsius. {1} Interestingly,

Veron states that on flat substrates, bottlebrush forms of Acropora can be dominant.

(CAIP, pg. 31) I have often seen people say the bottlebrush forms of Acropora require high

light. More likely, they get their nutrition from feeding rather than zooxanthellae at

this depth. Often there are fairly large mono-specific stands.

Water motion information is difficult to come by at this depth, and seems to be mainly

due to prevailing currents.

Upper reef slope

Depth: 2 to 20 meters

One interesting feature is the angle of the slope- this can vary from a gentle slope to

a nearly vertical wall. The transect profiles from numerous references give all sorts of

slopes and curves- there is tremendous diversity here. On the windward side, the upper

slope often features Spur and Groove formations. These formations essentially channel the

wave forces into a groove, which often will continue all the way up to the reef crest.

Weins mentions that the reef crest can be a very treacherous place to explore as the tops

of the groove are often "paved" over by coralline algae. The channeled force

apparently can create numerous geyser-like water spouts at the reef crest.

Coral diversity is often highest at about 10 or 20 meters. Light levels are high, so do

not limit coral growth. As you move up, the monospecific stands are replaced by a high

diversity of species, typically not dominated by any one species. Veron notes that if the

slope is not too steep, that large stands of elkhorn Acropora may predominate at shallow

depths.

Reef fronts (Reef Crest)

Depth: 0 to 2 meters

This is the part of the reef that takes the brunt of the wave action. The corals that

survive and thrive here are pretty amazing. They survive exposure to tropical sunlight

levels that are very near surface level irradiance (2000 E/m2). They can survive wave

surge/swell, which can have speeds of over 365 cm/sec. For comparisons sake, Dana Riddle

measured the output velocity of the Maxijet 1000 powerhead at 75 cm/sec 0.5 inches from

the nozzle. {2} At low tides, the corals are often completely out of the water. In order

to protect themselves from such an extreme environment, the corals tend to have

"short and stubby" (digitate) or encrusting growth forms. This enables them to

withstand the waves. To survive exposure to the sun/rain, the corals tend to be heavy

slimers. Otherwise, the reef crest is dominated by crustose coralline algae.

For a couple of pictures of the wave action on a tropical island, see

href="http://homepage.interaccess.com/~mlkjr/Aruba.htm">http://homepage.interaccess.com/~mlkjr/Aruba.htm.

The waves are truly amazing in person…

Outer reef flat

Depth: 0 to 2 meters

This area, the portion of the reef flat just past the reef crest, is generally not

highly populated with corals. This area also gets hammered by waves and seems to have a

rocky substrate that is usually exposed.

Inner reef flat

Depth: 0.5 to 2 meters

Veron indicates that there is a clear demarcation typically between the inner and outer

reef flats as the substrate of the inner ones has lots of loose rubble and some partly

consolidated sandy areas. Where the rock is exposed, there is a good coverage of corals.

At low tide, the tops of the corals can be partly exposed. As this area is behind the reef

crest, it is rather well protected from wave action. Consequently, the water motion is not

as great. Currents can be highly variable, from as few as 3-4 cm/sec, on up to about 60

cm/sec. Typically, they may range about 20 cm/sec. As the water is so shallow, irradiance

levels are very near surface levels, as much as 80 to 95% of surface levels.

Lagoons

Depth: (Highly variable) 3 to 30 meters

Lagoons... When I think of a lagoon, I think of an atoll. The folks that first noticed

the reef zonation did not seem to see them as any different than the space behind the reef

flat and the shoreline- what I would consider a "moat". Personally, I see some

differences, but concede that they may not be all that important...

Atoll lagoons have been pretty well studied, especially those that were the sites of

the nuclear testing back in the Cold War era... For the period of a year or so, scientists

intensively scrutinized two locations, giving them a great snapshot of what was there.

Then, they blew them up...

Afterwards, the locations were studied again, to see what would happen. I understand

that they are allowing diving at those sites now, and that they have recovered fairly

well...

As far as lagoons go, we can think of typical depths of 30 meters, but there are some

that are far deeper, upwards of 400 feet. Often the lagoon is littered with patch reefs

that grow in pillar-like fashion from the floor, all the way to the water's surface. There

are some described as mushroom-shaped, with a "stem" 15 feet in diameter, and a

"cap" of upwards of 100 feet in diameter. In some of the larger lagoons, the

number of patch reefs can number in the thousands.

Depending on the atoll, the lagoon may be very open to the ocean, or may be completely

cut off. For an example, for an atoll with many cays and channels, a complete turnover of

water in the lagoon may take 30 to 40 days. Circulation within the lagoon would be

primarily tidal or wind driven. During the summer, the wind driven currents provide much

of the circulation, and have been measured at 15 to 25 cm/sec. During the winter doldrums,

this form of circulation can drop by 80%. In a large lagoon, the tidal currents in the

channels have been measured at up to 400 cm/sec. You can figure that the average current

level in a lagoonal setting is about 20 cm/sec.

An interesting lagoonal factoid from Weins: In a large lagoon, the water that enters

due to tidal forces mixes very little with the actual lagoonal water- for the most part,

what enters, exits when the tide falls. Also, tidal force currents have been measured

three miles in from channels when the tide was rising.

In the moat areas behind the reef flats, the rocky substrates are often completely

eroded, leaving a sandy bottom. Veron puts these types of lagoons at a few meters deep,

usually completely surrounded by reef. The bottom of the lagoon may be rocky, or, in

deeper areas, covered in sediment. Where there is a hard substrate, the coral growth,

protected from wave action, is usually elaborate. Sometimes there is little to no growth,

but sometimes the corals grow in or on the sands. Veron mentions huge stands of branching

Acropora growing in the sand. In larger lagoonal areas, there may be only patch reefs.

Water quality in the lagoons is usually mixed- sometimes the waters are quite turbid,

which reduces and alters the light that reaches the corals. Often the conditions within

the same lagoon can vary from location to location- one section may have very clear water,

while another may be very turbid

Back reef margins

Depth: 0 to 3 meters

Veron describes this as areas of reef flat divided by sandy-floored fissures. The area

typically receives a decent amount of current, but is protected from wave action. As such,

it is home to very active reef growth. Veron notes that the coral composition of this area

will change markedly from place to place on the reef.

Back reef slopes

Depth: 0 to 20 meters

As this area is usually protected from waves, it can support very lush coral growth.

However, it varies according to the slope- typically, the less slope, the higher the coral

growth. Near vertical walls may have next to zero growth however. Veron indicates that

this area can abound with caves and canyons, and that as you reach the deeper sections,

you can often find spectacular soft coral communities. Water motion is difficult to

quantify, as it will change drastically from reef to reef depending on the location of

local currents. It will be less that what you would find on the windward side.

Inner-reef sea floor

Depth: (Highly variable) 3 to 30 meters

Little, if any hard coral growth here, other than some free-living species. Not covered

here.

Inner intertidal mudflat

Depth: 0 to 1 meter

Veron explains that for the this area, there is often no clear demarcation between the

mangrove swamps and a true intertidal reef flat and goes on to say that you can even find

some corals growing on mangrove roots. This area will have to contend with mud and

freshwater runoff.

Outer intertidal mudflat

Depth: 1 to 3 meters

Corals begin to appear as the water gets deeper. One of the more interesting features

are "micro-atolls", consisting often of Porities sp. that grow up to the

water's surface, then outwards. Often the middle section dies off and erodes, leaving

a miniature "lagoon". The outer edges will continue to grow up and out. The

colonies can get quite massive at many meters across.

The outer slope

Depth: 3 to 30 meters

In clear waters, this area can be very similar to the off-shore Forereef slope. Often

there is a reef crest area as well. The upper portion is dominated by massive colonies,

often Porities. Unlike the Forereef slopes, Acropora are much less abundant typically, but

often richer in soft corals and other genera. Veron also mentions that species of Pavona,

Goniopora, Porites, Turbinaria and other species can form huge monospecific stands.

Reef Types revisited

Each of the reef types listed, the Fringing, Barrier and Atoll may have some or all of

the reef zones listed above. Seeing the diversity of reefs in nature, some areas can only

fit into one of the zonation categories by stretching the imagination a little. Let's

look at each in turn and see how they can be categorized.

Fringing Reef

See URL: http://homepage.interaccess.com/~mlkjr/ReefTalkRef1.htm.

Fringing reefs can be zoned in a number of different ways, depending on the locale.

Some have moats or lagoons, while others do not. There is an interesting paper by Pichon

{3} that covers reef flat evolution very well.

A typical Fringing reef may be zoned as: Shoreline, inner reef flat, outer reef flat,

reef crest, then upper reef slope, then lower slope. Sometimes they may go: shoreline,

lagoonal moat (with or without patch reef), reef flat, reef crest, then upper and lower

reef slope. Sometime the reef crest is exposed to a degree that a well-defined algal ridge

is formed, or the Caribbean equivalent, a Millepora zone. Where a lagoon is formed behind

the reef-flat, the flat is often broken into finger-like formations, as if canyons are cut

out of the reef flat. Immediately behind the reef crest there is often a boulder zone,

consisting of broken reef rock and dead coral skeletons. Where the reef crest is exposed

to ocean swells, there is often a spur and groove zone.

Barrier Reef

See URL: http://homepage.interaccess.com/~mlkjr/ReefTalkRef2.htm.

A bit I goofed here, as this is a transect of an Atoll with the GBR system- that's

what I get for doing graphics a week ahead of time…

Barrier reefs typically form on a continental shelf. In some cases, they can be quite

far away from the shoreline, i.e. The Great Barrier Reef. Other times they are nearby a

landmass. When they are more open water, the zonation goes like this (from windward side)

Deep forereef slope, upper forereef slope, reef front/reef crest, reef flat, back reef

margins, back reef slopes. At times, there is a lagoon- then Deep forereef slope, upper

forereef slope, reef front/crest, reef flat, Lagoon (with patch reefs), then often a back

reef margin and/or back reef slope. I have not really mentioned yet the fact that there

often are sand cays on top of the reef flats, forming small islands. Often the reefs may

have channels cut within them as well.

Atolls

See URL: http://homepage.interaccess.com/~mlkjr/Atoll.htm.

Atolls are a very fascinating habitat. Out of the very deep ocean, they rise straight

up. Where drilling has been done, the reef limestone thickness has been measured at over

1. meters thick! Two thirds of a mile of VERTICAL growth by coral! Obviously, this

growth is over a long period of time, which would also encompass the rising sea levels

after the last Ice Age.

Atolls themselves have grown upwards, then outwards, forming a vaguely circular lagoon.

Most are not circular, and the shape of the lagoon is apparently largely determined by the

underlying structure of the sea floor.

Zonation will often be as follows: Deep forereef slope, upper reef slope, reef crest,

often with algal ridge, reef flat, back reef margin, lagoon (with often numerous patch

reefs), again back reef margin or slope, reef flat, the upper reef slope, then lower reef

slope. The reef slope is often very pronounced. The windward side frequently has spur and

groove zones.

Hurricanes: Some of the literature I have read describes reef destruction after a

hurricane. Believe it or not, the deeper reef slopes often suffer much more than the reef

crest. Apparently, the slopes are so steep that there is a avalanche of debris that

literally will scour the slopes, stripping it of near 100% coral coverage. In contrast,

the areas getting pounded with waves may suffer 60% or less coral mortality. Comparatively

more branching species will die than massive growth forms.

Now we get to the juicy part:

Implications for the reef aquaria

So how do we take the basic information I have listed above and apply it to the home

aquarium? This gets to be difficult, as most home aquaria are designed for very low energy

ecosystems. They are just a glass box. Almost no aquariums are designed to take the kind

of pounding that a reef front does. There are, however, some things we can do to design a

system around a particular reef zone. Where indicated, I am assuming a typical 75 gallon

tank. In talks with DBW, he argued that it would be better to measure in cross-sectional

area… In principal, I agree, but use a typical 75 gallon tank (48L x 18W x 20D) as my

standard. If you use a tank that is wider and deeper, you would have to increase water

flow, maybe by as much as 20 or 30%. Or you could add an additional powerhead. As always,

your mileage may very…

Deep Forereef slope

Light: I'd stick with NO or HO FL with a mixture of about 3 actinics to one

daylight bulb. This area has lower light levels, predominating in the bluish range.

Water motion: Typically, water motion is around 10 to 15/cm per second. Back and forth

motion is not that prevalent. You could probably make do with one 300 gph powerhead every

1. feet of linear tank length. Mostly laminar flow is ok.

A sandbed would be a perfect addition.

Forereef slope

Light: Levels do change rather rapidly as you go up. I would suggest VHO's again

with 3:1 or 2:1 Actinic to daylight ratio. If you wish to use MH, I'd go with the 150

or 175 watt bulbs with Actinic supplementation. The deeper you want to try to emulate, the

higher Kelvin rating bulb you MAY wish to get.

Water motion: Back and forth motion is prevalent here. I'd really recommend a

wavemaker. One pump for laminar flow, with two or more cycling back and forth would be

ideal. As current levels seem to average around 15 to 20 cm/sec, I'd say about one

1. gph powerhead per two linear feet of tank, these on a wavemaker, with another, say 300

gph powerhead, providing laminar flow.

A sand bed would be ok, although with higher water motion, you may want to stick with

larger grain Aragonite based sand. This will tend to stay more in place.

Reef Front

Light: As water depth is minimal, light intensity tends to be near surface level

irradiation levels. As such, intense light is very desirable. I would say a minimum of 250

W MH with Actinic supplementation, or 400 W MH with Actinic supplementation. If you

believe in PAR, the 400W 6500K Iwasaki bulb is the way to go here, with, probably, some

supplemental blue, either in the form of VHO's or Power Compacts.

Water motion: Here the water motion is very much back and forth in motion. Ideally,

you'd want to design a tank around this habitat.

Possible ways to duplicate nature's water motion:

1) Carson Surge device

2) Eric's Toilet flush device.

3) Tank with paddle system to move water back and forth.

4) Experimental water propeller on reversible electric motor. The prop would have to be

enclosed, with entrances covered with something like "eggcrate". Motor to drive

prop would be outside tank, and would drive prop with flexible plastic shaft. Motor should

ideally speed up, then slow down and reverse itself, speed up, then slow down.

Reverse…

Surge devices necessitate a "reef ready" design tank, with built in corner

overflows. NEVER Trust a U-tube siphon overflow box to handle the surge!!!

Your floor WILL get wet.

As far as a sandbed, I would not have one. Water motion is a bit too violent.

Maybe add some crushed coral substrate just for looks. Be prepared to vacuum it

though… If you wanted to incorporate a sandbed, add a refugium.

As you can see, the Reef Front zone would be difficult to emulate. Very high energy. I

can envision a tank custom built, with a paddle system, say 4 feet long, 20 inches high

and 30 inches wide. From the back, there is a separate panel of glass/Plexiglas six inches

in from the back, leaving a 6 inch wide section, 20 inches high, but it would not touch

the sides of the tank. It would be, say 38 to 40 inches long. This would leave openings on

each side 4 to six inches wide, by the full 20 inches high. Behind this is the paddle

system. One paddle moves back and forth, moving a very large percentage of the tank's

water back and forth. Industrial-type motor would have to be mounted outside of tank. Back

section would not have to be lit. May need to add eggcrate to stop curious fish from

getting hurt…

Same kind of concept for the propeller based tank… Needs a specialized motor, but

should work quite well. Ideally, you'd have what looked like a miniature box fan with

remote motor. This you could hide behind rockwork, however…

Inner/Outer Reef flat.

Light: Again, intense. 250W MH minimum. 400W would be better. See lighting suggestions

above.

Water motion: This tends to be variable. Waves breaking will tend to leave this area

saturated in O2. Little back and forth motion… Often little motion at all, but this

area does get quite the tidal flow. You could get from as little as 0 to up to 20 cm/sec,

and in areas, more than that. Much of this will be laminar flow, but will come from

different directions over the course of the day. As such, a wavemaker might be handy.

I'd shoot for 500 gph per two linear feet.

Sandbeds could be used. I wouldn't go with any silt in it though.

Lagoons

Light: This is a bit difficult as lagoons can be quite turbid. Because of this, the

light transmission is way down from clear oceanic waters. I would say that VHO's, or

possibly PC's (If you want to emulate a near-top patch reef) will work just fine. MH

may be overkill (again, unless you want to simulate a shallow water patch reef)- stick

with 175W or less.

Water motion: On average, Lagoons seem to have 15 to 25 cm/sec. Back and forth motion

is minimal- most of the circulation is either wind driven or tidal driven. One 500 gph

powerhead every 2 linear feet should work well. The system would benefit from a wavemaker

on longer, pulse setting (i.e. 5 minutes on, five minutes off, back and forth…)

As far as sandbeds go, here is a perfect setup for the deeper, mixed grain sandbed. May

need to tone the powerheads back to 300 gph if you do this, however.

Back reef slope

This area is very much like the forereef slope, only with less back and forth motion.

For lighting, recommendations are the same.

For water motion, again, I'd recommend the same… We are not likely to over do

it with back and forth motion- In fact, I'd say that we probably will not come close.

Intertidal mudflats

While there are corals that inhabit this area, I really doubt anyone will try to

emulate this one. If you really would like to, try the reef flat suggestions, and use a

thick sandbed with silt.

 The last thing I want to mention is the fact that I have a web page that lists

corals found at particular zones, as listed by Veron.

See URL: http://homepage.interaccess.com/~mlkjr/CommonHabitat.html.

At this point, let's open it up to questions and answers…

Thanks Mike!

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