Continued from page 3

Index

• Abstract
• Case Study 1
• Case Study 2
• Discussion
• Conclusions
• Recommendations
• Acknowledgments
• References
• Appendix A - Fertilizer Recipe
• Comments on this Paper
• Later comments on this Paper

Discussion

The observations in the case studies are consistent with the following hypothesis: when light, CO₂, N, K, and all micronutrients and trace elements are present in slight excess relative to the amount of phosphate available for plant growth, certain higher plants in the aquaria are able to out-compete algae and cyanobacteria for the phosphate in the water column, starving them of this essential nutrient.

Exactly why higher plants should be able to outcompete algae for phosphate is unclear. Perhaps their roots give them some advantage, or they simply need much less phosphate than algae to thrive. Nor is it known which of the many plants in the test aquaria are responsible for stripping the water of phosphate, although the fast-growing duckweed and stem plants with roots growing above the substrate (notably Hygrophila spp.) are likely culprits. That phosphate is the factor limiting the plant and algae growth in the test aquaria has been reasonably well established; it is the only known plant nutrient not added to the 500L tank in any form other than fish food, and deliberately adding concentrated phosphate to this tank induced almost immediate algae growth (and a rapid duckweed explosion too). Since the plants continue to grow very well, they are clearly gaining preferential access to whatever phosphate is available. There may be some literature unknown to the authors that offers an explanation. If not, it should be fairly easy to conduct controlled experiments with a sensitive phosphate test kit and a few spare tanks containing only algae, one or two plant species, and nutrients. An experiment that shows that duckweed thrives at phosphate concentrations as low as X ppb, but green algae and cyanobacteria require significantly more than X, would offer strong support for the hypothesis.

According to the hypothesis, If the higher plants are unable to utilize all of the phosphate present in the water column because of a deficiency of some other nutrient, algae will thrive. The type of algae appears to depend on the availability of other nutrients. In the test aquaria, it was found that when nitrates were unmeasurable, cyanobacteria predominated. It is suspected that nitrogen deficiency favors the growth of cyanobacteria because these organisms can fix the atmospheric nitrogen dissolved in the aquarium water. When nitrates were available, green algae predominated. Some red algae was also observed in the 500L tank before CO₂ fertilization was introduced. Because others have observed that tanks with CO₂ fertilization have relatively little red algae [5], it tempting to speculate that at least some red algaes are able to utilize bicarbonate, giving them an advantage in aquaria where most of the available carbon is in this form (typically those with high carbonate hardness and high pH). The following paragraph summarizes the apparent relationship between nutrients, plants, and algaes:

If the aquarium is P limited, higher plants will outcompete algaes of all types for P, and the algae will disappear. If not, and N in the form of nitrates and ammonia is deficient, cyanobacteria will thrive, otherwise green or red algae will predominate. Red algae is favored over green algae if most of the available carbon is in the form of bicarbonates.

The factors that determine which species of algae will predominate in a given situation have obviously been greatly simplified. In [5], for example, nitrate concentrations in excess of 30ppm are claimed to be detrimental to the growth of green algae but not to cyanobacteria, so one would predict that cyanobacteria would predominate at high nitrate levels.

There is a tradition in the hobby of using fish food (usually processed by the fish first) as the source of all macronutrients for the plants in an aquarium. When this is done, it appears that first K and then N become the factors limiting plant growth (i.e. there is insufficient K and N in the food relative to the amount of P, at least for the fish foods the authors use). Thus, supplementary K and N must be added or free phosphate will be available to fuel algae growth (this contradicts the prevailing wisdom in the aquarium hobby that one of the ways to reduce algae growth is by reducing fertilization; in fact, additional nutrients are required). Other alternatives are to restrict feeding to the point where the growth of algae due to unused P is tolerable (another common piece of advice), an approach likely to result in poor plant growth due to nutrient starvation, or to use a phosphate-removing resin.

Some of the plant species in the 500L tank grow very slowly compared to the same species in the 160L tank (Echinodorus sp. in particular). The 160L tank has an enriched substrate with no deliberate water circulation, whereas the 500L tank has a relatively inert substrate with a 300gph UGF. It is highly unlikely that all plants are equally adept at extracting phosphates directly from the water column, and it appears that the fast-growing plants in the 500L tank are depriving the other plants of this nutrient which (thanks to the UGF) is distributed evenly throughout the tank. Slow-release phosphate tablets will be placed around the roots of these plants to see if growth improves. Both authors agree that the substrate design of the 160L tank (solid fertilizer at the bottom of an inert substrate) gives the better results, probably by making phosphate more-or-less equally available to all plants without allowing too much to leach into the water column where it is available to algae.

Conclusions

Despite the lack of controls on the various experiments, and the inability of the authors to directly measure phosphate in the aquaria, there is compelling evidence to support the hypothesis that all types of algae (including cyanobacteria) can be effectively controlled in planted aquaria by ensuring that phosphate is the factor limiting plant growth. In two aquariums with different volumes, substrates, lighting, and plant, algae, and fish populations, effective control of algae was achieved by enriching the tank water with CO₂, micronutrients, trace elements, N, and K. Despite high initial algae loads, these tanks are now almost free of visible algae and have remained so for several months. Furthermore, in the 500L tank it was shown that phosphate limiting was occuring by adding phosphate to the tank water and observing the almost immediate growth of green spot algae and cyanobacteria. It has also been shown in the 160L tank that disturbances to the phosphate-containing substrate result in algal growth if there is significant (more than approximately 1 ppm) nitrate in the water, and in growth of cyanobacteria if nitrate is not present at this level. It is important to note that plant growth in both tanks is excellent, so algae control has not been achieved at the expense of the plants.

NetPets® Main Page

contact information Main Library Fresh Water Plants/Algae

The Fish Center