One filtration system that has received attention in the scientific community is the algal turf scrubber, part of the microcosm system developed by Dr. Walter Acley of the Smithsonian Institution s Natural Histoiy Museum in Washington D.C. Dr. Adey began his development of a coral reef aquarium system in 1974 (Miller, 1980). These systems have been installed at numerous public and private aquariums in the United States, Canada, and Australia. Algal scrubbers are basically shallow troughs with a plastic mesh screen illuminated by intense lighting. Water pumped to the troughs enters them by means of a dump bucket, generating a surge that helps the algae exchange gases and take up metabolites, while preventing over-illumination or over-shading. Various turf-forming algae are grown on these screens, and they remove ammonia, nitrate, phosphate, and heavy metals from the water (Adey and Lovelancl, 199 0. The screens are periodically "harvested" by removing them and scraping off the excess growth with a plastic wedge. The harvested algae is thus removed from the system. It may be discarded, analyzed for nutrient content, or returned to the aquarium to stimulate higher productivity when nutrient levels are very low. The harvested screens are reinstalled with the still living cropped algae adhering to them.
Algal Turf Scrubber
Modified after Adey & Loveland, 1991
Water inflow from Aquarium into wave surge bucket
Alga! turf screen
Alga! turf screen
_ Water outflow T to Aquarium
A dump bucket sends a wave of water into an algal turf scrubber. Photograph shown with permission of Space Biospheres Ventures. J. Sprung.
These aquaria differ not only by their use of algae to filter the water, but also by the philosophy of their design and operation. They are designed specifically to model whole ecosystems, by setting up complex food webs and encouraging high productivity. Microcosms are exhibits of less than 20,000 L (5000 gal.). Larger exhibits are called mesocosms. The emphasis is not on the husbandry of individual specimens, but on the whole ecosystem. They are truly different from typical aquarium systems, though not very different from the "natural system" (see Eng, 1961, 1976, and Emmens, 1986), or modern reef aquariums using live rock. The substrate and attached algae is full of beneficial worms and crustaceans, and the fish are fat and very healthy, with little or no food input. There are typically noticeable populations of amphipods, foraminiferans and mysid shrimp among the rocks and algae, a sign of high productivity, as in nature. Adey and Loveland (1991) list a high diversity of plant species, benthic protists, and invertebrates found in the Smithsonian coral reef. The philosophy of creating complex food wrebs and whole ecosystem models is good, and we wish to endorse it. When live rock is used, food webs naturally occur, as the rocks introduce a tremendous variety of microorganisms, plants and invertebrates to the system. Aquarists can enhance the complexity of the food webs w ith the use of "live sand" collected near reefs, and an inoculation of natural seawater or cultured planktonic organisms. The type and quantity of live rock and the depth or presence of other substrates such as sand or gravel affect the abundance of microorganisms and microcrustaceans that populate the aquarium, serving as food for fish and invertebrates. We will discuss this more in a moment under the topic of "refugia."
We have seen a number of systems employing algal turf scrubbing in operation, and have spoken with quite a few marine scientists and coral reef biologists who have used them, maintained them, or observed them. Our personal observations have been of five exhibits at the Smithsonian's Museum of Natural History, one at the Pittsburgh Aquazoo, one at the Ontario Science Centre, four at Biosphere 2 in Arizona, and the Great Barrier Reef Aquarium in Townsville Australia. It is our opinion that although algal turf scrubbers work quite well for mangrove and estuarine microcosms, producing model ecosystems that really look exactly like the natural environment, the results in the coral reef microcosms we have seen that rely exclusively on this type of filtration with little or no water change, are less spectacular.
In our opinion, the algae that are an essential component in the productivity of a reef ecosystem (see Adey, 1987; Acley and Goertemiller, 1987), should not be relied on for its filtration. Although algal turf scrubber systems work quite well for the mangrove and estuarine microcosms we saw, the reef microcosms were not very good. The stony corals did not appear to grow much, and in some instances we saw what we believed was unacceptable mortality (at the Smithsonian 1988-1990, at Townsville in 1990, and at the Pittsburgh Aquazoo). Unfortunately, little data has been published exhibiting the growth rates and survivability of coral in these systems (see Adey and Loveland, 1991). While some corals do grow and even reproduce, "others show little or no growth and eventually shrink marginally" (Adey, 1983). We are not certain why some stony corals don't thrive in these systems, but we can offer a few possible explanations.
In systems receiving no food input, fishes are more likely to pick on corals as a source of food. New7 corals and other sessile invertebrates added to the aquarium are likely to be perceived as rood, and may be attacked. We have witnessed this. Coral-smothering dense algal turfs cultivated within damselfish territories, and occasional predation by corallivores such as parrotfish, butterflyfish, and bristle worms are cited as possible causes of coral mortality (Adey, 1983), and we have seen evidence that this is true in these systems, in other systems, and in nature. Temperature fluctuation in the Natural History Building where the Smithsonian coral reef is maintained, and the affect of different husbandry techniques by different curators of the other aquariums we have observed are also cited as possible causes for less titan ideal success with stony corals, or whole exhibits (Wr. Adey, pers.
comm.). These influences certainly are significant. The lack of planktonic food for some large-polyped corals is another possibility that Dr. Adey has explored (W. Adey, pers. comm.), and he found that feeding them reversed the progress of tissue recession and renewed their growth. However, the result of this experiment does not indicate whether the corals recovered because they received extra protein (nitrogen), trace elements, or both from the added food. We believe that there are additional reasons why stony corals might not grow well when algal turf scrubbing is the sole form of filtration.
These systems were based on early research into coral reef nutrient dynamics that suggested nitrogen and phosphate are tightly cycled within the reef ecosystem. Recent work shows that while nutrients are cycled within the reef ecosystem, coral reefs also export large quantities of these nutrients (D'Elia,1988), and can have significant import of inorganic nutrients (Rougerie and Wauthy, 1993). Systems filtered by algae exhibit a paradox that is an important lesson in reef ecology. Turf scrubbers maintain "nutrient poor" conditions in the water with respect to inorganic nitrogen (ammonia, nitrite, nitrate) and inorganic phosphate. Exports of inorganic nitrogen and phosphorous in closed aquaria can be achieved very nicely with the harvest of algal turfs in turf scrubbers (Adey and Goertemiller, 1987; Adey et al., 1993). One may wonder then why this nutrient poor water supports such high productivity. Why should an extremely nutrient poor system have lush growth of algae? As an aquarist, one can't help but notice that the more algae you have in an aquarium, the more algae you will have. The answer to this snowball mystery is clear when one finds that systems with heavy algal growth are rich in organic forms of both nutrients, tied up in the sediments, in compounds in the water, and in living algae. In the absence of protein skimming, systems that rely on algae to filter the water are not nutrient poor. They are rich in organic forms of the nutrients.
Algae in the turf scrubbers compete with the zooxanthellae for the same inorganic nitrogen (especially ammonia), and trace elements. This may account for reduced growth in corals, and could explain the positive effect Dr. Adey noted when he feci corals that exhibited tissue recession. The use of algal turf scrubbers also results in a yellowing of the water by organic leachates from the algae. Some of the leachates may contain substances that are toxic to some corals, though leachates may also be nutritious to other corals or filter feeders. It is known that some macro and micro-
algae exude organic substances that are toxic to fish and invertebrates (see Trainor, 1978; Ohta, 1979; Larsen and Moestrup,
1989), so encouraging their extensive growth in a closed system could harm corals if no steps are taken to remove the organic compounds from the water. The colour imparted to the water alters the light spectrum thai the corals receive. If the water is yellow, much of the blue wavelengths are absorbed by it. The golden brown symbiotic zooxanthellae use blue light most efficiently for photosynthesis (Benson, 1984). Corals living in yellow water must adapt to the new light field, which is quite different from the light on most reefs. In shallow aquariums, the yellow colour is not especially noticeable, nor is it's affect on the light especially important. In deep aquariums it may be. Many species from turbid inshore waters are accustomed to greenish or even yellow-brown water, but the species found in the clear water of the outer reefs are accustomed to spectrally blue light if they are from anywhere below 3 meters depth, and full spectrum daylight with even more blue in it if they are in shallower water. The j alteration of the light spectrum is something that corals can adapt to, but the yellow colour is a symptom of accumulated organic substances that may be toxic to them. The use of protein skimming would help alleviate this problem of algal leachates, especially when GAC is employed as well. The use of ozone or partial water change are other options for clearing the water. We should point out that Dr. Acley claims that the water does not become yellow, that the apparent yellow colour is caused by the lighting, not dissolved organic compounds (Acley, 1993). Our own observations indicate otherwise. We should also point out that these systems do not become yel!owr simply because of the turf scrubbers. Leachates from algae, invertebrates, and fish in any reef system will turn the water yellow (or green in large aquaria), even when protein skimming is employed. The addition of protein skimming seems to be enough to restore the corals to good health if trace elements are not too depleted (J. Sprung, pers. obs.), even when the water is still noticeably coloured by some organic compounds. Activated carbon or ozone can be used to completely eliminate the colour, producing the clear blue of truly nutrient poor water. We are curious about Jaubert's description of a "mysterious process" that eliminates the yellow compounds from his systems (Jaubert, 1989, 1991). It is tempting to accept that a more natural process than the use of activated carbon or ozone could achieve blue water. W7e are willing to believe it when wre see it.
Recently, small dosage of ozone has been used to remove the organic tint from the water at the Great Barrier Reef Aquarium in Townsville. Despite natural sunlight, the exhibit always had a greenish cast. Now, the water is blue. We expect this should improve the results initially, up to the point that trace elements are depleted by natural processes and oxidized by the ozone. If trace elements are not replenished, the corals and other creatures will not thrive. This brings up another important issue.
Some of the public exhibits using algal turf scrubbers also employ other forms of water purification, including protein skimming or, as at the Great Barrier Reef Aquarium, large sand filters to maintain water clarity (Adey and Loveland, 1991). Adey (pers. comm.) believes that supplemental filtration of this kind compromises the resulting ecosystem, primarily because of the impact on plankton, and that poor results with corals in systems employing other means of filtration cannot be blamed on aleal turf scrubbers. While o we don't feel there is a connection between plankton availability and coral health, we do agree that mixing systems and varying techniques makes it impossible to determine to what extent husbandry, system design, or biology is responsible for the results seen. Furthermore, of the 12 exhibits we have seen that used turf scrubbers, only 8 were reef ecosystems, and each one was maintained differently. This is too small a sample number to really make any definitive conclusion, though we must point out that the results have been very consistent.
Certain major, minor, and trace elements are critical for certain invertebrates, and success with these species in our systems
depends on periodic replenishment. Teh (1974) reports that boron, bromine, strontium, phosphorous, manganese, molybdenum, lithium, flourine, rubidium, iodine, aluminum, zinc, vanadium, cobalt, iron, and copper are essential for most invertebrates. With algal filtration, the algae used to purify the water also remove trace elements, and as they are harvested from the system, the elements are removed with them. They must be replenished. Trace element depletion is a feature of many forms of filtration, not just algal filtration. Protein skimming and activated carbon also remove trace elements, and so do the invertebrates, plants, and microorganisms. In a closed system aquarium it does not take long for trace elements to become depleted. The loss of trace elements is not entirely an undesirable feature. In fact, it is a desirable feature of algal filtration because it can be employed to advantage in the removal of toxic heavy metals that could enter the system with make-up water, or from tank construction materials (Adey and Loveland, 1991). The disadvantage of trace element removal with any form of filtration is realized only when the philosophy of the aquarium keeper is to avoid replenishment via supplemental additions or water change.
The exact method of trace element replenishment in the different systems we have observed is not consistent. Some references suggest no supplements added and no water change performed (see Adey, 1993). Others describe micro-nutrient input via food, top-off water, and the return of some harvested turf algae to the system when nutrient levels near the lower values found in the natural ecosystem being modeled, or when no food inputs are made to the system ( Adey and Loveland, 1991; Adey 1993; W. Adey, pers. comm.). Adey and Loveland (1991) recommend 1 to 2% water changes per month to replace micronutrients. The splash of saltwater from numerous wave dump-buckets in large exhibits must result in some loss of salt from the system, and this should be
calculated in the total water exchange figure. The 3000 gallon Smithsonian Reef exhibit has water exchanges of 5 gallons per day (=5% per month), using water collected from the Gulf Stream, about 50 miles off the Maryland/Virginia coast (T. Goertemi Her, pers. comm.). The unfiltered water is stored in Nalgene containers.
At least some systems utilizing turf scrubbing also incorporate additions of prepared trace element solutions as a part of maintenance. At the Smithsonian, keepers of the reel exhibit began adding a trace element supplement around January 1993 (T. Goertemiller, pers. comm.). The Pittsburgh Aqua zoo does not use a
supplement, but is considering this option. The Townsville Aquarium also does not yet use a supplement; very unfortunate for the corals considering the recent addition of ozone to this aquarium.
In his own home reef aquarium, Dr. Adey does not use a trace element supplement, but replenishes trace elements with the top-off water. He prefers natural water collected from streams, but suggests tap water or well-water can be used if they are not contaminated (W. Adey, pers. comm.). When natural water is not obtainable, and the tap or well-water is contaminated, then a water purification system must be used for the top-off wrater. Sometimes the top-off water is first treated in a freshwater system using algal turf scrubbers, to remove excess plant nutrients or heavy metals, while in some urban localities, other methods of water purification may be used to treat polluted tap wrater (W. Adey, pers. comm.). Dr. Adey uses Instant Ocean™ salt mixed with filtered water to replace water lost due to splashing (T. Goertemiller, pers. comm.). In the Smithsonian coral reef system, make-up water (Washington D.C. tap water) is filtered through twro large ion-exchange cartridges and one activated carbon cartridge from Millipore™, then through a Milli-Q™ water filter, and finally, it pours through a column of dried Halimeda plates (calcium carbonate skeleton of Halimeda algae). This last step changes the pH of the filtered water from 6.0 coming out of the Milli-Q™, to 8.2 or higher before it is stored in a 120 gallon reservoir. This technique supplies additional calcium carbonate, and perhaps some strontium. The make-up water is added to the aquarium on demand by a pump and level sensing device.
To growr stony corals in captivity, the culture water should maintain a calcium level of about 400 mg/L, a carbonate hardness of about 8 dKH or greater, and a strontium level of about 8 mg/L. Without additions of calcium and strontium, these levels usually fall in closed systems, and this deficit is detrimental to stony corals and coralline algae. There is documented evidence that the calcium and strontium values in some of Dr. Adey's systems are NOT depleted (Meyer, 1991). Replenishment is possible with hard make-up water, and also via the gradual dissolving of the aragonite (coral or oolitic) sand substrate. In his home aquarium, Dr. Adey adds several hundred milligrams of aragonite sand every fewr weeks.The calcium and strontium apparently are supplied by acid secretions dissolving the sand in the deep substrate, and with the other minerals in the top-off water. Jaubert, (1991), manages the calcium level by the same
process. This process needs to be examined and described in a manner that can make it simple for all aquarists to reproduce. We are most interested in the results achieved because we are proponents of making aquariums simpler, not more complex.
In concentrating on the problems for stony corals with this system we are targeting it's weakest point. Actually, microcosm and mesocosm systems employing algal turf scrubbers exclusively with no other forms of filtration work beautifully for nutrient rich ecosystems such as salt marshes and seagrass meadows, and can be employed with success for the maintenance of highly productive tropical reef ecosystems, but since we have observed that stony corals grow better in systems employing protein skimming and trace element addition, we feel it is important to point out the shortcomings of algal turf scrubbing as the sole filtration for a model coral reef ecosvstem. When other forms of filtration are employed to remove the organic leachates from the water, and trace element supplements are added, corals do thrive in systems using algal turf scrubbers (J. Sprung and J.C. Delbeek, pers. obs.) The philosophy of creating complex food webs and whole ecosystem models is good, and we wish to promote it. However, in our opinion the use of algae as a filter does not create a reef ecosystem, it hinders it.
It must be pointed out that fish do extremely well in these systems, often with no feeding at all. If combined with protein skimming and GAC or ozone, algal scrubbers may yet prove to be very useful for large fish displays. We would like to see more fish exhibits either using algal turf scrubbers or encouraging more natural growth in the aquarium, instead of the current trend in public aquaria of maintaining nearly sterile rock-work. The benefits are many. The fish stay healthy, they hardly need to be fed, and since nitrate doesn't accumulate, less water needs to be changed. All of these benefits afford a significant cost savings on maintenance. Furthermore, less labor is involved in the cleaning of the aquarium, though the cleaning of algal turf screens is admittedly labor intensive on large systems. Live rock can provide most of the same benefits without the maintenance time. A system with living rock and a deep sand substrate does not require algal turf scrubbers to maintain low nitrates and phosphates, and when protein skimming and GAC are employed, accumulation of organic substances is avoided.
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