Trépanier, C., Parent, S., Comeau, Y. and J. Bouvrette. 2002. Phosphorous budget as a water quality management tool for closed aquatic mesocosms. Water Research 36:1007-1017.
The Montreal Biodome, was built inside of the old Olympic Velodrome back in the early 1990’s and contains a number of biomes from the Americas. One of these is a 1600 m2 reproduction of the Gulf of St. Lawrence. This exhibit contains large numbers of fish, invertebrates and seabirds, which, as you can imagine, create quite a bit of waste in a closed system. Over the course of six years (1992-1998) measurements were taken of dissolved reactive phosphate (DRP, which is mostly orthophosphate), organic phosphate, ammonia, nitrite and nitrate. These measurements allowed the staff to develop not only a nitrogen budget (covered in a 2000 paper in Water Research 34(6):1846-1856) but also a phosphorous budget (the topic of this paper). What this basically means is that they were able to establish what the inputs and outputs were for both nitrogen and phosphorous in the system and determine how these nutrients were being maintained over the course of six years. During this time the
phosphorous level peaked at about 18 mg/L and then stabilized. The reasons for this are many, but mainly regular cleaning of the filtration system (rapid sand filters) and gravel washing removes particulate phosphate (detritus) before it can be mineralized into DRP by bacteria. Also, water losses/changes were instrumental in preventing DRP levels from rapidly climbing. Prior to 1995 only 7-9% of the water was changed per year compared to 16-18% per year from 1995-1998. Nitrogen levels increased rapidly due the use of trickle filters and rapid sand filters that promoted rapid nitrification to nitrate, which was not being removed. In response to high nitrate levels (180 mg/L NO3-N) water changes were increased and denitrification filters were added.
The uptake ratio of C, N and P by marine algae in the natural environment is normally 106C:16N:1P by atom or 42C:7N:1P by weight, also known as the Redfield Ratio. When the N:P ratio is 7.0 or more, phosphorous is considered to be the limiting nutrient for algae growth in natural marine systems. Other studies referenced in this paper noted that blue-green algae and other potentially toxic algae become predominant when the ratio falls to 3 (by weight) or lower in freshwater lakes. Since they were operating at a ratio of 12 (180mg/L NO3-N:15 mg/L PO4–P) the fear was that lowering the nitrate- nitrogen to their target value of 20 mg/L without similarly lowering the PO4–P would result in a ratio of 1.2, a value they thought might result in an outbreak of nitrogen fixing blue-greens or some other toxic algae. Keep in mind that a value of 15 mg/L of PO4–P is extremely high and would hardly if ever be encountered
in a closed reef system. Obviously the inputs and biomass in this system are extreme.
By far the greatest input of P was feeding. Over 76% of the P in the exhibit came from feeding, while 26% came from bird guano (i.e. poop). Fully 85% of these inputs were removed by cleaning filters and detrital removal, leaving 15% to accumulate over the year. Also 22% of the water was lost in 1998 through water changes and spills, with a direct link between DRP levels and water loss: the higher the DRP the greater the roll of water loss in lowering DRP. For example, the net increase in P was 7.4 mg/L PO4– P when water changes were at 9% between August 1992 and August 1993, and only 1.3 mg/L PO4– P in 1998 when water changes were at 18% per year. Both particulate organic phosphate (POP) and dissolved organic phosphate (DOP) remained low; this was primarily due to removal by the filtration system and cleaning, or by mineralization to DRP.
In comparing both nitrogen and phosphorous budgets several conclusions can be drawn. Phosphorous outputs are much greater than nitrogen outputs. This is primarily due to the fact that most phosphorous occurs initially as particles that can be easily removed by filters, whereas most nitrogen is present in dissolved form (ammonia) and is rapidly converted by bacteria into nitrate. The food also contained much more nitrogen that phosphorous (5-10X more N than P). As a result, much less phosphorous than nitrogen is mineralized (32.5 g/yr vs. 71 g/yr in this system). Finally, water changes removed 68% of the mineralized phosphorous (orthophosphate) but only 15% of the mineralized nitrogen (nitrate).
This study reinforces what most home aquarists should already know. Filtration is very important in controlling the levels of phosphate but even more important is regular cleaning. This is why it is recommended to regularly clean any mechanical filters and not to let detritus accumulate in them. The use of protein skimmers also helps to reduce not only orthophosphate but also helps in the removal of particulate phosphate and particulate nitrogen. Finally, the notion that an unfavorable ratio of N:P (3 or less) can lead to cyanobacteria problems may help to explain why some experience outbreaks of slime algae in new setups or after several years of trouble-free operation. In today’s reef aquaria, the control of nitrogen, especially nitrate, has been greatly simplified through the use of live rock and live sand, which offer areas of denitrification. It is not unusual to have levels of nitrate close to natural seawater conditions. Therefore total nitrogen levels in reef aquaria are
rarely a problem. The question may then be more one of phosphorous. How much do you add and where does it go? Certainly, in a healthy system, protein skimming, sequestering in sand beds and regular cleaning of mechanical filters goes a long way towards dealing with phosphorous inputs. However, the increasing use of large amounts of fish food, liquid invertebrate foods and live phytoplankton cultures can lead to significant inputs of phosphorous. Without an adequate means of removal, levels may accumulate and create an unfavorable N:P ratio that could result in a cyanobacterial outbreak. Again this underscores the delicate balances that exists in our systems and if one pushes the envelope too much, without providing for adequate safeguards (e.g. increased skimming, water changes, mechanical filter cleaning, a healthy sand bed, etc. etc.), things can quickly go wrong, something worth considering when trouble-shooting both fish-only and reef systems.
Some references of interest from the scientific literature over the last few months.
- Brown, B.E., Clarke, K.R. and R.M. Warwick. 2002. Serial patterns of biodiversity change in corals across shallow reef flats in Ko Phuket, Thailand, due to the effects of local (sedimentation) and regional (climatic) perturbations. Marine Biology 141(1): 21-30.
- Fax, T.Y., Li, J.J., le S.X. and L.S. Feng. 2002. Lunar periodicity of larval release by pocilloporid corals in southern Taiwan. Zoological Studies 41(3):288-294.
- Harii, S., Kayanne, H. Takigawa, H. Hayashibara, T. and M. Yamamoto. 2002. Larval survivorship, competency periods and settlement of two brooding corals, Heliopora coerulea and Pocillopora damicornis. Marine Biology 141(1):39-46.
- Hayashibara, T. and K. Shimoike. 2002. Cryptic species of Acropora digitifera. Coral Reefs 21(2):224-234.
- Reddy, N.S., Goud, T.V. and Y. Venkateswarlu. 2002. Seco-sethukarailin, a novel, diterpenoid from the soft coral Sinularia dissecta. Journal of Natural Products 65(1):1059-1060.
- Wang, G.H., Ahmed, A.F., Kuo, Y.H. and J.H. Sheu. 2002. Two new subergane- based sesquiterpenes from a Taiwanese gorgonian coral Subergorgia suberosa. Journal of Natural Products 65(1):1033-1036.
- Warner, M.E., Chilcoat, G.C., McFarland, F.K. and W.K. Fitt. 2002. Seasonal fluctuations in the photosynthetic capacity of photosystem II in symbiotic dinoflagellates in the Caribbean reef-building coral Montastraea. Marine Biology 141(1):31-38.
- Tsuchiya, M. and S. Najima. 2002. Occurrence of Trapezia associated with Acropora: on the “wrong” host coral? Coral Reefs 21(2):160-190.
- Gordon, A.K. and T. Hecht. 2002. Histological studies on the development of the digestive system of the clownfish, Amphiprion percula and the time of weaning. Journal of Applied Ichthyology 18(2):113-117.
- Larval clownfish do not have the necessary digestive structures or enzymes to be able to digest artificial foods until 9 days after hatching
- Proceedings of the Third Annual William R. and Lenore Mote International Symposium in Fisheries Ecology, Oct. 31-Nov.2; 2000; Sarasota, FL. Targets, Thresholds, and the Burden of Proof. Bulletin of Marine Science March 2002. This special issue of the Bulletin of Marine Science deals with fisheries, their impacts and ways to develop sustainable use. There are several papers of interest to anyone who has ever wondered how difficult it will be to determine what exactly is “sustainable”. There is even a paper dealing with the collection of marine ornamentals.
- Randall, J.E. and D.G. Fautin. 2002. Fishes other than anemonefishes that associate with sea anemones. Coral Reefs 21(2):188-192.
- Weis, V.M., Verde, E.A., Pribyl, A. and J.A. Schwarz. 2002. Aspects of the larval biology of the sea anemones Anthopleura elegantissima and A. artemisia. Invertebrate Biology 121(3):190-201.
- Major, K.M. and K.H. Dunton. 2002. Variations in light-harvesting characteristics of the seagrass, Thallassia testudinum: evidence for photoacclimation. Journal of Experimental Marine Biology and Ecology 30(2):173-179.