The fear, often realized, of any coral reef keeper is that their corals and live rock become overgrown with algae. To prevent this, much time, effort and money is spent on ways to prevent algae from taking over the reef systems and, if algae becomes established, eliminating it. There are many different opinions on how best to control algae growth but there is little detailed data on just what causes algae to grow so thick that it smothers the reef organisms. Most articles (and products) on the subject discuss phosphorous concentrations and recommend keeping it below 1 mg/L. Phosphorous is seemly always deemed the cause of algae blooms in reef tanks, which is why hobbyists are told to use such things as phosphate-free activated carbon. While this is sound advice, it does not always solve the problem. There are several reasons for this. One, maybe phosphorous really isn’t the problem, and perhaps another nutrient such as nitrogen is the real culprit. Secondly, is the level of 1 mg/L really the “magic” number, whereby if you maintain phosphorous below this, algae can’t grow? Actually, there is little data to support this number.
The problem of algae taking over reefs is not limited to the home aquarium. Worldwide there are coral reefs which are being overgrown with algae. There are several possible reasons for this. One study (Hughes, T. P. 1994. Catastrophes, phase-shifts and large-scale degradation of a Caribbean coral reef. Science 265:1547-1551) found that rather than high nutrient levels, the lack of algae-consuming fish was the cause for a shift from what was a coral reef community to a macroalgae community. This month, we take a look at a recently published study which investigated whether nutrient enrichment was a casual factor in the increased growth of macroalgae on coral reefs off Jamaica and Florida. The findings of this study have important implications for reef aquarists and their attempts to control algae. First, a little background. Two terms that are frequently used in community ecology are “top-down” and “bottom-up.” These refer to how the structure of the food web is regulated. Bottom-up refers to the availability of nutrients or resources. Thus, in the context of a coral reef community if nutrients are scare such that algae can’t grow, it is said that there is a bottom-up control on the community. On the other hand, if the production or coverage of algae is limited by the grazing action of fish (consumers) rather than nutrients, then there is said to be a top down control on the community. The terms come from the fact that in food webs, nutrients are at the bottom (used by the primary producers) while consumers are at the top. Thus, any effect nutrients have must go up the web, while any effect consumers may have goes down the web. It is easily seen that coral reef hobbyists pursue both bottom-up and top-down control strategies when dealing with algae. Inputs of nutrient to the tank are minimized and filtration devices (protein skimming, denitrification, special resins, etc.) are used to limit the nutrients available to the algae. At the same time, certain species of fish are kept in the reef tank expressly to graze on any algae that might start to appear.
In the following discussion, the word algae is used frequently. Algae encompass a large group of organisms. In a reef tank, as in nature, algaes can be grouped into 3 broad categories. There are encrusting coralline algae which are rarely a problem in a reef tank. The next group is small microalgae that forms turfs–in some cases these can be a problem. The last group is the macroalgae which includes both fleshy and calcareous forms. Algaes in this group can quickly take over a tank (or a natural coral reef). While I have used the more generic term, algae, the results of the paper discussed below pertain to marcoalgaes. The paper for this month was authored by Brian Lapointe and titled: Nutrient thresholds for bottom-up control of macroalgal blooms on coral reefs in Jamaica and southeast Florida. It was published in a special issue of the journal Limnology and Oceanography regarding The Ecology and Oceanography of Harmful Algal Blooms. The formal citation would be: Lapointe, Brian E. 1997. Nutrient thresholds for bottom-up control of macroalgal blooms on coral reefs in Jamaica and southeast Florida. Limnol. Oceanogr. 45(5 part 2):1119-1131.
The study centered on the shift from a predominantly coral environment to one dominated by algae in two areas: Discovery Bay, Jamaica and Palm Beach, Florida. According to the author, the areas differ in several aspects. The Jamaican reef is carbonate rich and receives significant inputs of dissolved inorganic nitrogen (DIN) in the form of nitrate from groundwater. In this study area, the extent of algae coverage was measured in surveys over 9 years, which started near shore and extended out to the fore reef (a distance of about 300 meters). At the present time, the area which was dominated by corals prior to 1980 is covered with an algae community.
Off the Florida coast, surveys were taken over 2 years at 4 sites, which parallel the coast running north to south. In some areas, algae mats covered the entire areas and there were thick, unattached mats drifting in the current.
In each area, water samples were taken and analyzed for dissolved inorganic nitrogen (DIN: which is the sum of the ammonia, nitrite and nitrate in the water), soluble reactive phosphate (SRP: which is the form of phosphorus that algae utilize). Salinity was also measured. He also performed some nutrient enrichment experiments whereby he added different combinations of nutrients to seawater in aquaria containing macroalgae, and measured how the macroalgae responded to the additions. For example, to test the response of algae to additions of nitrogen and phosphorus, aquaria were set up, which received nitrogen only, phosphorus only, nitrogen and phosphorus, and control tank, which did not receive any nutrients.
Among other variables, he measured alkaline phosphatase, which is an enzyme that macroalgae which are deficient in phosphorous produce, allowing them to use dissolved organic phosphorous instead of SRP. He also studied the productivity of the algae under different light regimes.
The results of his testing showed that nutrient inputs “were major factors causing increased productivity and standing crops of macroalgae.” Thus there was a bottom-up control on the community. This is not really surprising and does, to some extent, parallel what hobbyists know: mainly minimize the amount of nutrients in the reef tank. The most useful information from this paper for the reef hobbyist is that the problem nutrient was not the same for both locations, and the concentration of the nutrient, above which there was a problem. On the Jamaican reef, the data supported a conclusion that phosphorous was the limiting nutrient. This area has significant inputs of nitrogen (mostly as nitrate from groundwater). However, on the Florida reefs, DIN was the limiting nutrient. What this means is that algae are opportunistic and can change their physiology to maximize their production, depending on the local conditions. Therefore, a reef hobbyist trying to eliminate just phosphorous may be unsuccessful in controlling algae blooms, as the algae can adapt to the changing environment conditions.
Also of particular interest was the concentration of nitrogen and phosphorus that were considered threshold levels. This means that the concentrations need to be above these levels to sustain a bloom of macroalgae. For phosphorus, the amount is 0.1 µM (SRP) which is 0.003 mg/L or about 3 parts per billion (ppb). For dissolved inorganic nitrogen (and remember this is the sum of ammonia, nitrite and nitrate) the value is about 1.0 µM which is 0.014 mg/L or about 14 ppb. Obviously, these levels are very low. They are much lower than the commonly quoted 1 mg/L for phosphorous. Further, this study shows that nitrogen can be just as important as phosphorous. It is rather unlikely that the typical hobbyist can maintain nutrient levels at the concentrations found in this study, so what should one do to keep the levels as low as possible? (It is also very unlikely that these concentrations can be measured with the equipment available to the majority of hobbyists).
The best advice is to minimize inputs and maximize outputs. Outputs are protein skimmer waste (so use one and keep it clean), particulate material which can be removed with a good mechanical filter which should be cleaned often, and limiting the fish population to those that are algae grazers so that little fish feed have to be provided on a regular basis. In short, good attentive husbandry is what is needed. There are no quick easy cures. If you are experiencing massive macroalgae blooms then consider all the different sources of nutrient inputs and work to minimize, if not eliminate, them.
Finally, realize that aquaria can go though phases. For most, there is an algae bloom after set up. This is probably caused by the large amounts of nutrients from the live rock. So rinse the rock well or use “cured live rock.” Algae blooms cannot be cured overnight; it takes time and diligence but it can be done.
©1998, Timothy A. Hovanec, Ph.D.
Originally published in Aquarium Frontiers, Jan. 1998